# Is the Sun slightly blue in the center? - Wavelength-dependent limb darkening of the Sun

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I've slightly modified the title to try to attract some attention. If we call sunlight "white" and limb darkening is a result of seeing deeper at normal incidence and shallower at oblique incidence, then the center end edge of the solar disk viewed from earth should appear to have slightly different effective color temperatures, since the temperature is varying rapidly with depth.

I'd like to have an approximate expression for the wavelength-dependent limb darkening of the sun in the visible spectrum, either a relatively simple analytical expression that I can understand (with the appropriate coefficients for the sun) or just some linear images of the sun in various bands in the visible wavelengths so I can try to make one of my own.

This question received this helpful comment which links to here but honestly I can't make my way through that to a practical expression I can use. The Wikipedia article is not helping me much either, except for the image there. I plotted scans of RGB but by the time an image gets into the internet, things like sRGB and gamma mean it may not be linear.

update: At the Solar Dynamics Observatory (SDO) website, I just found the image sdo.gsfc.nasa.gov/assets/img/latest/latest_1024_HMIIC.jpg">http://www.solarham.net/latest_imagery/hmi1.htm

I appreciate (the existence of) the complexities and intricacies of photon transport, instrumental effects, and color perception, but I am just starting to do astronomically correct animations, so please for right now, something imperfect, or not absolutely correct is good enough for me.

20 pixel wide averages of horizontal (-) and vertical (--) "scans" across the center of this 600x600 pixel image from here

note: It seems this is a somewhat futile example, as the image is likely to be a monochrome continuum image with false color!

You might find this paper helpful:

http://www.physics.hmc.edu/faculty/esin/a101/limbdarkening.pdf

Look specifically at Eq. 5 for the wavelength dependence. To get the intensity, use Equation 1. The value of $u$ can be found in the caption to Figure 2.

## 4. What are hydrogen alpha and calcium filters?

This was a highly specialised area several years ago but with the introduction of small portable solar scopes this type of solar observing has now become very popular. With these types of filters we are viewing the Sun using a specific light by using a interference filter. This type of filter blocks all other wavelengths of light passing only the a tiny part of the solar spectrum. We can see the Sun in hydrogen-A (red light) or calcium-K (blue light). Filters that work in this way are often referred to as narrowband filters.

Remember that to get a complete picture of solar activity it is worth watching the Sun in white light using either projection of a full aperture solar filter so that you can see the sunspots clearly.

Hydrogen alpha (H-a or H-alpha) is in the red end of the visible solar spectrum (at 656.3nm). By using a specially-made combination of small telescope and interference filter we are able to see (and image) solar prominences, filaments, plages and occasionally flares on the Sun that otherwise would remain invisible. A more recent development is the “DayStar Quark” which can be used with a normal telescope with some safety precautions. More of this below.

Calcium-K (or CaK) telescopes allow you to image the Sun in the blue light of calcium (393.4nm) also by using a specially-made combination of small telescope and interference filter. Because the image is so near the UV region of the solar spectrum some people cannot see the image clearly but it can be imaged with a camera.

HYDROGEN ALPHA:

A Coronado PST (Personal Solar Telescope) for viewing the Sun in hydrogen-alpha light, taken by John Chapman-Smith A solar scope

The hydrogen-alpha filter (and scope) like those shown above will show you:
Prominences: These are clouds of luminous hot hydrogen gas seen projecting off of the edge (or limb) of the Sun. Prominences are bright because they are seen in emission against a dark sky background. As we are looking at the Sun through an interference filter that allowing us to see features that are emitting nearly all their light at the wavelength of 6563 Angstroms the prominences appear red.

A typical prominence

Prominences come in two main types: quiescent (quiet) or eruptive. Prominences can last days or appear and disappear in hours. You will often see a number of descriptions such as: “hedgerow-type” prominence, or “smoke-stack” prominence, “mound” or “spike” prominence. These are widely-used descriptive terms used by observers to convey the general shape of a prominence with reference to terrestrial objects.

Filaments: These are ribbon-like features seen against the solar disk. They are the same as prominences but are seen against the bright solar disk so they appear dark by contrast. Sometimes at the solar limb we can observe a prominence against the sky and a filament on the disk if that feature is large enough to stretch from the limb and onto the Sun’s disk.

Plages: Also seen in the image below are plages. These are the bright areas visible around sunspots while observing in H-alpha light.

Dark filaments and bright plages

Flares: These are bright, occasionally very bright, points of light or ribbons of bright light usually seen near sunspots on the solar disk.

Flares usually last for about 10-20 minutes depending on the flare strength. The strength of solar flares are usually reported as: A-B-class, C-class, M-class and X-class. A-B-class are not reported as they are very common and the weakest type of solar flare. C-class are slightly more powerful, M-class are stronger and X-class are the strongest. Often these classes are sub-divided by using a numbering system from 1 to 9 (so we might see the term: “M7-class solar flare” for example. The exception is X-class where the numbering can go beyond 9.

The bright area is a solar flare

DayStar Combo Quark (provided by Section member, Carl Bowron)

DayStar Quark Combo with Barlow, tilting unit and imaging camera. Image by Carl Bowron

There are two ways to observe the Sun at Hydrogen-Alpha frequencies, either using a designated solar telescope, or using a special solar filter attached to a “normal” telescope.

DayStar have developed a filter which can be attached to any telescope, with a few safety precautions. A designated solar telescope can only be used for solar work but any astronomical telescope with the DayStar filter can be used for both day (solar) and night (stars and planets) observations. The ideal telescope to use with the Quark stsyem is a refractor. Why is this? A reflector could be use but it would require special energy rejection filter attached to the front of the telescope and this could be quite expensive. A refractor, on the otherhand, up to a maximum of 150mm aperture requires only a normal sized energy rejection filter just in front of the Quark saving on cost.

DayStar produce two types of Quark filters, the standard Quark which has a built in 4x Barlow lens, and the Combo Quark which has no Barlow. The first gives a fixed amplification which can produce quite a restricted field of view, while the second does not.

DayStar produce two versions of Quark which work at different bandwidths. There first, a chromosphere version with a narrow bandwidth, and a second, a prominence version with a wider bandwidth. The former is designed for greater resolution of surface features, however, it will also show prominence features at increased camera gain settings.

The Combo Quark requires an optical system of focal ratio F15 (focal ratio = focal length/aperture) or greater to work effectively. If, for example, you have an 90mm F10 refractor with an aperture stop of 60mm it will give a F15 system with quite a large field of view. A normal Barlow can be attached to the front of the Quark to boost the amplification. The 60mm/F15 example with the addition of a 1.5x Barlow will transform this to a F36 system. If you now remove the aperture stop to regain the full aperture you now have a 90mm/F24 system. A 2x Barlow with the 90mm aperture results in a F37 system. The focal ratio can be adjusted to the size and resolution required for the solar feature to be observed. The increase in F number also improves the contrast in the observed features. For a fixed cost of the Quark and the appropriate energy rejection filter you can have a modestly priced 90mm aperture solar telescope.

For imaging the solar features a monochrome digital camera is needed with, possibly, a tilting attachment. Working at this specific H-alpha frequency can produce annoying interference fringes (called “Newton Rings”) at the image plane which can be removed by slightly tilting the camera, hence the tilting attachment. Tilting adapters can be purchased for most astronomical cameras for a few pounds.

Armed with a DayStar Combo Quark chromosphere filter, an energy rejection filter, a monochrome digital camera and a tilting attachment, you are now set-up to take on solar imaging in all its glory. For large expanses of the solar disk you can start off with a F15 arrangement and then for more detailed imaging the focal ratio can be adjusted by the use of various Barlow lenses.

Solar imaging is usually conducted in relatively unstable air so precise focusing can take quite a while to achieve. Take time over this process as it is important. Unlike the eye, which can rapidly adapt to subtle changes in focal length, the camera plane is fixed so the focal plane will move back and forth in front and behind the imaging chip. Take as many frames as possible, aim for at least 60 frames per second over a one minute run and be prepared to reject up to 95% of these when stacking to form the final composite image.

For imaging prominences the same technique applies but this time the camera gain will need to be increased substantially to show these fainter features around the solar rim. The rest of the solar disk will be completely white. It is possible with the brighter prominences to show these as well as surface features but only at the larger focal ratios where the contrast is better.

Having acquired the images they will need to be processed through RegiStax or a combination of Autosakkert!2 and RegiStax to produce final monochrome images. Be prepared to discard the majority of the images at the stacking phase of these programs. Finally, the resulting image can then be suitably colour enhanced using most imaging software packages.

We also now have calcium light filters (often referred to as “CaK”) but they can only really be used with an imaging camera as our eyes are not good at seeing light at the deep blue-end of the solar spectrum.

This image, taken in the blue light of calcium shows the region immediately above the solar photosphere (the lower chromosphere). The very bright areas seen here in the image are closely associated with the sunspots (just visible in the picture).

Should you need advice on choosing and using these filters please email me using the contact form.

## Life Under a Black Sun What would it really be like if our sun was a black hole?

Every living thing on our Earth has to thank the Sun for its ability to survive, as it warms the Earth to liveable temperatures. It is hard for us to imagine how a civilization could function without a star shining bright in the sky of its host planet.

Scientists in the Czech Republic have been researching if a planet could harness enough energy to run a civilization if the planet were orbiting a black hole instead of a star. This would work in a fundamentally different way than how our planet and civilization work. The sun emits light and UV-radiation, the type of sunshine that causes a sunburn. This light and radiation carry energy that gets used by things like plants and solar panels. Black holes don’t produce light1, hence the name black holes, but they have mind-boggling amounts of gravity, enough to attract light and radiation.

There are two phenomena that work together to allow a planet around a black hole to get power and heat. First, the universe has a bit of radiation in it already. This radiation keeps the void of space at 2.7 K, or -454 degrees Fahrenheit. This is cold by human standards, however, it is still warm enough to provide energy. A black hole can be massive enough to attract a large amount of this background radiation. Therefore, a planet orbiting the black hole would also be orbiting through radiation and would warm up the planet slightly.

The second phenomenon is due to relativity. When a loud ambulance rushes by, you hear a change in pitch of the siren due to the Doppler effect. This is actually caused by a change in wavelength of the sound that is caused by the movement of the ambulance. The same effect happens to light. The enormous gravity of the black hole causes the light to Doppler shift. In the instance of the planet in a black hole, the light is “blue-shifted” meaning the wavelength gets shorter, equivalent to the pitch of the ambulance getting higher. For light and radiation, blue-shifting increases the energy, and thus provides more heat to the planet.

These two phenomena, the blue-shifting of radiation and the concentration around the black hole, mean that a planet orbiting a black hole collects a large amount high-energy radiation.

The scientists researched in depth to see if these phenomenons would be enough to support a civilization on this planet. They first modeled an Earth-sized planet around a Sun-sized black hole. From this, they saw that this planet around this black hole would not gather enough power to sustain a civilization. This planet would collect 910 watts, which is only enough to power 15 light bulbs.

Researchers repeated the model around a much larger black hole, one that would appear to take up a third of the sky, but it still did not gather enough power. This Earth-sized planet gathered 19 megawatts of power, enough to power 4000 houses. By allowing this theoretical planet to orbit faster around the black hole, they increased the blueshift phenomenon and found the power to be about 200 megawatts, which is about 20% of the production of 1 coal power plant.

Next they had the planet orbit a “rotating black hole,” causing high gravity pockets to form around the black hole. This increases the blue-shift effect and brings in more power to the planet. Such a planet would receive 6.7 gigawatts of power! This is still much lower than our civilization, but is comparable to power consumption of a country, which could sustain a small civilization.

For their fourth experiment, these scientists were inspired by the movie Interstellar. In this movie, there is a planet that is so near the black hole, that the black hole takes up 40% of the sky on the planet! In this analogy, the amount of background radiation the planet would receive would be 411 terawatts. This is about 100 times less than what the Earth receives, but is starting to be enough to power a reasonably small civilization! This is an exciting result. However, the concentration of radiation on the surface of the planet would be 300 times as much uv-radiation as a standard Earth sunburn, and the surface of this unlivable planet would boil to hot 1630 degrees fahrenheit!

Last, the scientists imagined that a civilization built a Dyson sphere, a spherical arrangement of solar panels for capturing energy, around the black hole. Except, instead of collecting the light and radiation emitting from the star, it’d be collecting the background radiation being sucked in by the black hole. A Dyson sphere built around a black hole with the same mass as our sun would collect about a puny, 250 watts. However, if we assumed a younger universe, when the vacuum of space was room temperature, then we get an amazing result: such a Dyson sphere in this early universe would collect 3 times the power consumption of the Earth!

This study showed that it is possible from an energy & power standpoint to sustain a clever civilization. This is an important study for our search for intelligence in the universe. This is also interesting because our sun has a limited lifetime, so researching energy alternatives for sustaining life is an important part of keeping human civilization around beyond the lifetime of our star.

## Comparison of far-ultraviolet emission lines formed in coronal holes and the quiet Sun

We present an analysis of 26 far-ultraviolet emission lines belonging to 19 atoms and ions observed on both sides of the boundary of polar coronal holes as well as other quiet Sun areas along the limb. The observations were made with the SUMER instrument (Solar Ultraviolet Measurements of Emitted Radiation) onboard the Solar and Heliospheric Observatory (SOHO). We compare line intensities, shifts and widths in coronal holes with the corresponding values obtained in the quiet Sun. We find that with increasing formation temperature, spectral lines show on average an increasingly stronger blueshift in coronal holes relative to the quiet Sun at equal heliospheric angle, with the coolest lines in our sample (formation temperature ≈ 10 4 K) indicating a small relative redshift. With respect to the rest wavelength, however, only lines formed above 5 · 10 5 K show blueshifts in coronal holes, which is not very different from the quiet Sun. The width of the lines is generally larger (by a few kilometers per second) inside the coronal hole. Intensity measurements clearly show the presence of the coronal hole in Ne VIII lines as well as in Fe XII, and provide evidence for a slightly enhanced emission in polar coronal holes for lines formed below 10 5 K. This last result is, however, less certain than the rest due to relatively poor statistics. Intensity histograms also exhibit distinct differences between coronal hole and quiet-Sun data. For cooler chromospheric lines, such as Ni II, the coronal holes display a greater spread in intensities than the quiet Sun. Transition-region lines, e.g. O IV, do not reveal such differences, while Ne VIII shows characteristics of a coronal line with lower average intensity and lower intensity spread inside holes.

Original language English 1145-1154 10 Astronomy and Astrophysics 363 3 Published - 2000

## What if they had a JWST in the TRAPPIST-1 system? A simulated observation of the Earth and our Moon transiting the Sun.

Synthetic infrared data generated with the planetplanet C/Python library. Plot made with matplotlib.

James Webb Space Telescope

Ah this takes me back to my undergrad labs. :) Love it!

Your limb darkening coefficients are messed up.

The Sun typically only changes a few percent in brightness across its disc. In the IR (e.g. the wavelength of JWST) it's even less. So as the Earth crosses the Sun's disc, there should barely be any change in brightness making the transit more trapezoidal shaped than U-shaped.

You're right, this plot was actually a test for a subroutine where I accidentally set the limb darkening coefficients equal to the visible ones for every wavelength.

What’s a Trapezoid system? /s

Actually what does the TRAPPIST do and how does it function?

trappist-1 is the name of a solar system 39.6 lightyears away in the constellation of aquarius. This post is simulating what our own solar system would look like from theirs, if there was a space telescope over there pointed in our direction

The shallow dip, whose tail is visible on the right of the main transit event, is due to our Moon transiting on the Sun blocking a tiny fraction of its light. The Moon transit event begins roughly at 1/3 of the main transit event (which btw is caused by Earth), you can see it as a small "jump" in the modeled lightcurve (blue line).

## Contents

α Aurigae (Latinised to Alpha Aurigae) is the star system's Bayer designation. It also has the Flamsteed designation 13 Aurigae. It is listed in several multiple star catalogues as ADS 3841, CCDM J05168+4559, and WDS J05167+4600. As a relatively nearby star system, Capella is listed in the Gliese-Jahreiss Catalogue with designations GJ 194 for the bright pair of giants and GJ 195 for the faint pair of red dwarfs.

The traditional name Capella is Latin for (small) female goat the alternative name Capra was more commonly used in classical times. [27] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) [28] to catalogue and standardize proper names for stars. The WGSN's first bulletin of July 2016 [29] included a table of the first two batches of names approved by the WGSN which included Capella for this star. It is now so entered in the IAU Catalog of Star Names. [30] The catalogue of star names lists Capella as applying to the star α Aurigae Aa. [31]

Capella was the brightest star in the night sky from 210,000 years ago to 160,000 years ago, at about −1.8 in apparent magnitude. At −1.1, Aldebaran was brightest before this period it and Capella were situated rather close to each other in the sky and approximated boreal pole stars at the time. [32]

Capella is thought to be mentioned in an Akkadian inscription dating to the 20th century BC. [33] Its goat-associated symbolism dates back to Mesopotamia as a constellation called "GAM", "Gamlum" or "MUL.GAM" in the 7th-century BC document MUL.APIN. GAM represented a scimitar or crook and may have represented the star alone or the constellation of Auriga as a whole. Later, Bedouin astronomers created constellations that were groups of animals, where each star represented one animal. The stars of Auriga comprised a herd of goats, an association also present in Greek mythology. [34] It is sometimes called the Shepherd's Star in English literature. [35] Capella was seen as a portent of rain in classical times. [36]

Building J of the pre-Columbian site Monte Albán in Oaxaca state in Mexico was built around 275 BC, at a different orientation to other structures in the complex. Its steps are aligned perpendicular to the rising of Capella at that time, so that a person looking out a doorway on the building would have faced it directly. Capella is significant as its heliacal rising took place within a day of the Sun passing directly overhead over Monte Albán. [37]

### Multiple status Edit

Professor William Wallace Campbell of the Lick Observatory announced that Capella was binary in 1899, based on spectroscopic observations—he noted on photographic plates taken from August 1896 to February 1897 that a second spectrum appeared superimposed over the first, and that there was a doppler shift to violet in September and October and to red in November and February—showing that the components were moving toward and away from the Earth (and hence orbiting each other). [38] [39] Almost simultaneously, British astronomer Hugh Newall had observed its composite spectrum with a four prism spectroscope attached to a 25 inches (64 cm) telescope at Cambridge in July 1899, concluding that it was a binary star system. [40]

Many observers tried to discern the component stars without success. [41] Known as "The Interferometrist's Friend", it was first resolved interferometrically in 1919 by John Anderson and Francis Pease at Mount Wilson Observatory, who published an orbit in 1920 based on their observations. [42] [43] This was the first interferometric measurement of any object outside the Solar System. [44] A high-precision orbit was published in 1994 based on observations by the Mark III Stellar Interferometer, again at Mount Wilson Observatory. [45] Capella also became the first astronomical object to be imaged by a separate element optical interferometer when it was imaged by the Cambridge Optical Aperture Synthesis Telescope in September 1995. [46]

In 1914, Finnish astronomer Ragnar Furuhjelm observed that the spectroscopic binary had a faint companion star, which, as its proper motion was similar to that of the spectroscopic binary, was probably physically bound to it. [47] In February 1936, Carl L. Stearns observed that this companion appeared to be double itself [48] this was confirmed in September that year by Gerard Kuiper. This pair are designated Capella H and L. [49]

### X-ray source Edit

Two Aerobee-Hi rocket flights on September 20, 1962, and March 15, 1963, detected and confirmed an X-ray source in Auriga at RA 05 h 09 m Dec +45°, identified as Capella. [50] Stellar X-ray astronomy started on April 5, 1974, with the detection of X-rays from Capella. [51] A rocket flight on that date briefly calibrated its attitude control system when a star sensor pointed the payload axis at Capella. During this period, X-rays in the range 0.2–1.6 keV were detected by an X-ray reflector system co-aligned with the star sensor. [51] The X-ray luminosity (Lx) of

10 24 W (10 31 erg s −1 ) is four orders of magnitude above the Sun's X-ray luminosity. [51] Capella's X-rays are thought to be primarily from the corona of the most massive star. [52] Capella is ROSAT X-ray source 1RXS J051642.2+460001. The high temperature of Capella's corona as obtained from the first coronal X-ray spectrum of Capella using HEAO 1 would require magnetic confinement, unless it is a free-flowing coronal wind. [53]

With an average apparent magnitude of +0.08, Capella is the brightest object in the constellation Auriga, the sixth-brightest star in the night sky, the third-brightest in the northern celestial hemisphere (after Arcturus and Vega), and the fourth-brightest visible to the naked eye from the latitude 40°N. It appears to be a rich yellowish-white colour, although the yellow colour is more apparent during daylight observation with a telescope, due to the contrast against the blue sky. [54]

Capella is closer to the north celestial pole than any other first magnitude star. [55] [note 3] Its northern declination is such that it is actually invisible south of latitude 44°S—this includes southernmost New Zealand, Argentina and Chile as well as the Falkland Islands. Conversely it is circumpolar north of 44°N: for the whole of the United Kingdom and Canada (except for part of Southern Ontario), most of Europe, and the northernmost fringes of the contiguous United States, the star never sets. Capella and Vega are on opposite sides of the pole, at about the same distance from it, such that an imaginary line between the two stars will nearly pass through Polaris. [56] Visible halfway between Orion's Belt and Polaris, Capella is at its highest in the night sky at midnight in early December and is regarded as a prominent star of the northern winter sky. [57]

A few degrees to the southwest of Capella lie three stars, Epsilon Aurigae, Zeta Aurigae, and Eta Aurigae, the latter two of which are known as "The Kids", or Haedi. The four form a familiar pattern, or asterism, in the sky. [58]

Based on an annual parallax shift of 76.20 milliarcseconds (with a margin of error of 0.46 milliarcseconds) as measured by the Hipparcos satellite, this system is estimated to be 42.8 light-years (13.12 parsecs) from Earth, with a margin of error of 0.3 light-years (0.09 parsecs). [1] An alternative method to determine the distance is via the orbital parallax, which gives a distance of 42.92 light-years (13.159 parsecs) with a margin of error of only 0.1%. [16] Capella is estimated to have been a little closer to the Solar System in the past, passing within 29 light-years distant around 237,000 years ago. [59] At this range, it would have shone at apparent magnitude −0.82, comparable to Canopus today. [60]

In a 1960 paper, American astronomer Olin J. Eggen concluded that Capella was a member of the Hyades moving group, a group of stars moving in the same direction as the Hyades cluster, after analysing its proper motion and parallax. Members of the group are of a similar age, and those that are around 2.5 times as massive as the Sun have moved off the main sequence after exhausting their core hydrogen reserves and are expanding and cooling into red giants. [2] [61]

There are several stars within a few arc minutes of Capella and some have been listed as companions in various multiple star catalogues. The Washington Double Star Catalog lists components A, B, C, D, E, F, G, H, I, L, M, N, O, P, Q, and R, with A being the naked-eye star. Most are only line-of-sight companions, [62] but the close pair of red dwarfs H and L are at the same distance as the bright component A and moving through space along with it. [63] Capella A is itself a spectroscopic binary with components Aa and Ab, both giant stars. The pair of giants is separated from the pair of red dwarfs by 723". [16]

American astronomer Robert Burnham Jr. described a scale model of the system where Capella A was represented by spheres 13 and 7 inches across, separated by ten feet. The red dwarfs were then each 0.7 inches across and they were separated by 420 feet. At this scale, the two pairs are 21 miles apart. [64]

### Capella A Edit

Capella A consists of two yellow evolved stars that have been calculated to orbit each other every 104.02128 ± 0.00016 days, with a semimajor axis of 111.11 ± 0.10 million km (0.74272 ± 0.00069 au), roughly the distance between Venus and the Sun. The pair is not an eclipsing binary—that is, as seen from Earth, neither star passes in front of the other. The orbit is known extremely accurately and can be used to derive an orbital parallax with far better precision than the one measured directly. The stars are not near enough to each other for the Roche lobe of either star to have been filled and any significant mass transfer to have taken place, even during the red giant stage of the primary star. [16]

Modern convention designates the more luminous cooler star as component Aa and its spectral type has been usually measured between G2 and K0. The hotter secondary Ab has been given various spectral types of late (cooler) F or early (warmer) G. The MK spectral types of the two stars have been measured a number of times, and they are both consistently assigned a luminosity class of III indicating a giant star. [65] The composite spectrum appears to be dominated by the primary star due to its sharper absorption lines the lines from the secondary are broadened and blurred by its rapid rotation. [41] The composite spectral class is given as approximately G3III, but with a specific mention of features due to a cooler component. [8] The most recent specific published types are K0III and G1III, [11] although older values are still widely quoted such as G5IIIe + G0III from the Bright Star Catalogue [2] or G8III + G0III by Eggen. [61] Where the context is clear, these two components have been referred to as A and B. [66]

The individual apparent magnitudes of the two component stars cannot be directly measured, but their relative brightness has been measured at various wavelengths. They have very nearly equal brightness in the visible light spectrum, with the hotter secondary component generally being found to be a few tenths of a magnitude brighter. [16] A 2016 measurement gives the magnitude difference between the two stars at a wavelength of 700 nm as 0.00 ± 0.1. [67]

The physical properties of the two stars can be determined with high accuracy. The masses are derived directly from the orbital solution, with Aa being 2.5687 ± 0.0074 M and Ab being 2.4828 ± 0.0067 M . Their angular radii have been directly measured in combination with the very accurate distance, this gives 11.98 ± 0.57 R and 8.83 ± 0.33 R for Aa and Ab respectively. Their surface temperatures can be calculated by comparison of observed and synthetic spectra, direct measurement of their angular diameters and brightnesses, calibration against their observed colour indices, and disentangling of high resolution spectra. Weighted averages of these four methods give 4,970 ± 50 K for Aa and 5,730 ± 60 for Ab. Their bolometric luminosities are most accurately derived from their apparent magnitudes and bolometric corrections, but are confirmed by calculation from the temperatures and radii of the stars. Aa is 78.7 ± 4.2 times as luminous as the Sun and Ab 72.7 ± 3.6 times as luminous, so the star defined as the primary component is the more luminous when all wavelengths are considered but very slightly less bright at visual wavelengths. [16]

Estimated to be 590 to 650 million years old, [16] the stars were probably at the hot end of spectral class A during their main sequence lifetime, similar to Vega. They have now exhausted their core hydrogen and evolved off the main sequence, their outer layers expanding and cooling. [68] Despite the giant luminosity class, the secondary component is very clearly within the Hertzsprung gap on the Hertzsprung–Russell diagram, still expanding and cooling towards the red giant branch, making it a subgiant in evolutionary terms. The more massive primary has already passed through this stage, when it reached a maximum radius of 36 to 38 times that of the Sun. It is now a red clump star which is fusing helium to carbon and oxygen in its core, a process that has not yet begun for the less massive star. Detailed analysis shows that it is nearing the end of this stage and starting to expand again which will lead it to the asymptotic giant branch. Isotope abundances [note 4] and spin rates confirm this evolutionary difference between the two stars. Heavy element abundances are broadly comparable to those of the Sun and the overall metallicity is slightly less than the Sun's. [41]

The rotational period of each star can be measured by observing periodic variations in the doppler shifts of their spectral lines. The absolute rotational velocities of the two stars are known from their inclinations, rotation periods, and sizes, but the projected equatorial rotational velocities measured using doppler broadening of spectral lines are a standard measure and these are generally quoted. [41] Capella Aa has a projected rotational velocity of 4.1 ± 0.4 km per second, taking 104 ± 3 days to complete one rotation, while Capella Ab spins much more rapidly at 35.0±0.5 km per second, completing a full rotation in only 8.5 ± 0.2 days. Rotational braking occurs in all stars when they expand into giants, and binary stars are also tidally braked. Capella Aa has slowed until it is rotationally locked to the orbital period, although theory predicts that it should still be rotating more quickly from a starting point of a rapidly-spinning main sequence A star. [16]

Capella has long been suspected to be slightly variable. Its amplitude of about 0.1 magnitudes means that it may at times be brighter or fainter than Rigel, Betelgeuse, and Vega, which are also variable. The system has been classified as an RS Canum Venaticorum variable, [9] a class of binary stars with active chromospheres that cause huge starspots, but it is still only listed as a suspected variable in the General Catalogue of Variable Stars. [10] Unusually for RS CVn systems, the hotter star, Capella Ab, has the more active atmosphere because it is located in the Hertzsprung gap—a stage where it is changing its angular momentum and deepening its convection zone. [66]

The active atmospheres and closeness of these stars means that they are among the brightest X-ray sources in the sky. However the X-ray emission is due to stable coronal structures and not eruptive flaring activity. Coronal loops larger than the Sun and with temperatures of several million K are likely to be responsible for the majority of the X-rays. [69]

### Capella HL Edit

The seventh companion published for Capella, component H, is physically associated with the bright primary star. It is a red dwarf separated from the pair of G-type giants by a distance of around 10,000 AU . [63] It has its own close companion, an even fainter red dwarf that was 1.8″ away when it was discovered in 1935. It is component L in double star catalogues. In 2015 the separation had increased to 3.5″, which was sufficient to allow tentative orbital parameters to be derived, 80 years after its discovery. [16] [70] The Gliese-Jahreiss Catalogue of nearby stars designates the binary system as GJ 195. The two components are then referred to individually as GJ 195 A and B. [13]

The two stars are reported to have a 3.5 visual magnitude difference (2.3 mag in the passband of the Gaia spacecraft) although the difference is much smaller at infrared wavelengths. This is unexpected and may indicate further unseen companions. [16]

The mass of the stars can, in principle, be determined from the orbital motion, but uncertainties in the orbit have led to widely varying results. In 1975, an eccentric 388 year orbit gave masses of 0.65 M and 0.13 M . [70] A smaller near-circular orbit published in 2015 had a 300 year orbit, benefitting from mass constraints of 0.57 M and 0.53 M respectively for GJ 195 A and B, based on their infrared magnitudes. [16]

### Visual companions Edit

Six visual companions to Capella were discovered before Capella H and are generally known only as Capella B through G. None are thought to be physically associated with Capella, although all appear closer in the sky than the HL pair. [64]

Multiple/double star designation: WDS 05167+4600 [24]
Component Primary Right
ascension (α)
Equinox J2000.0
Declination (δ)
Equinox J2000.0
Epoch of
observed
separation
Angular
distance
from
primary
Position
angle
(relative
to primary)
Apparent
magnitude
(V)
Database
reference
B A 05 h 16 m 42.7 s +46° 00′ 55″ 1898 46.6″ 23° 17.1
C A 05 h 16 m 35.9 s +46° 01′ 12″ 1878 78.2″ 318° 15.1
D A 05 h 16 m 40.1 s +45° 58′ 07″ 1878 126.2″ 183° 13.6
E A 05 h 16.5 m +46° 02′ 1908 154.1″ 319° 12.1
F A 05 h 16 m 48.748 s +45° 58′ 30.84″ 1999 112.0″ 137° 10.21 SIMBAD
G A 05 h 16 m 31.852 s +46° 08′ 27.42″ 2003 522.4″ 349° 8.10 SIMBAD

Component F is also known as TYC 3358-3142-1. It is listed with a spectral type of K [71] although it is included in a catalogue of OB stars as a distant luminous star. [72]

Component G is BD+45 1076, with a spectral type of F0, [71] at a distance of 401 light-years (123 parsecs). [73] It is identified as a variable member of the Guide Star Catalogue from Chandra observations although it is not known what type of variability. [74] It is known to be an X-ray source with an active corona. [73]

Several other stars have also been catalogued as companions to Capella. [24] Components I, Q, and R are 13th magnitude stars at distances of 92″, 133″, and 134″. [75] V538 Aurigae and its close companion HD 233153 are red dwarfs ten degrees away from Capella they have very similar space motions but the small difference makes it possible that this is just a coincidence. [76] Two faint stars have been discovered by speckle imaging in the Capella HL field, around 10″ distant from that pair. These have been catalogued as Capella O and P. It is not known whether they are physically associated with the red dwarf binary. [77]

Capella traditionally marks the left shoulder of the constellation's eponymous charioteer, or, according to the 2nd century astronomer Ptolemy's Almagest, the goat that the charioteer is carrying. In Bayer's 1603 work Uranometria, Capella marks the charioteer's back. [78] The three Haedi had been identified as a separate constellation by Pliny the Elder and Manilius, and were called Capra, Caper, or Hircus, all of which relate to its status as the "goat star". [36] Ptolemy merged the Charioteer and the Goats in the 2nd century Almagest. [79]

In Greek mythology, the star represented the goat Amalthea that suckled Zeus. It was this goat whose horn, after accidentally being broken off by Zeus, was transformed into the Cornucopia, or "horn of plenty", which would be filled with whatever its owner desired. [33] Though most often associated with Amalthea, Capella has sometimes been associated with Amalthea's owner, a nymph. The myth of the nymph says that the goat's hideous appearance, resembling a Gorgon, was partially responsible for the Titans' defeat, after Zeus skinned the goat and wore it as his aegis. [80]

In medieval accounts, it bore the uncommon name Alhajoth (also spelled Alhaior, Althaiot, Alhaiset, Alhatod, Alhojet, Alanac, Alanat, Alioc), which (especially the last) may be a corruption of its Arabic name, العيوق , al- c ayyūq. [81] c Ayyūq has no clear significance in Arabic, [82] but may be an Arabized form of the Greek αίξ aiks "goat" cf. the modern Greek Αίγα Aiga, the feminine of goat. [81] To the Bedouin of the Negev and Sinai, Capella al-'Ayyūq ath-Thurayyā "Capella of the Pleiades", from its role as pointing out the position of that asterism. [83] Another name in Arabic was Al-Rākib "the driver", a translation of the Greek. [81]

To the ancient Balts, Capella was known as Perkūno Ožka "Thunder's Goat", or Tikutis. [84] Conversely in Slavic Macedonian folklore, Capella was Jastreb "the hawk", flying high above and ready to pounce on Mother Hen (the Pleiades) and the Rooster (Nath). [85]

Astrologically, Capella portends civic and military honors and wealth. [35] In the Middle Ages, it was considered a Behenian fixed star, with the stone sapphire and the plants horehound, mint, mugwort, and mandrake as attributes. Cornelius Agrippa listed its kabbalistic sign with the name Hircus (Latin for goat). [86] [87]

In Hindu mythology, Capella was seen as the heart of Brahma, Brahma Hṛdaya. [35] In traditional Chinese astronomy, Capella was part of the asterism 五車 (Wŭ chē English: Five Chariots), which consisted of Capella together with Beta Aurigae, Theta Aurigae, and Iota Aurigae, as well as Beta Tauri. [88] [89] Since it was the second star in this asterism, it has the Chinese name 五車二 (Wŭ chē èr English: Second of the Five Chariots). [90]

In Quechua it was known as Colça [35] the Incas held the star in high regard. [91] The Hawaiians saw Capella as part of an asterism Ke ka o Makali'i ("The canoe bailer of Makali'i") that helped them navigate at sea. Called Hoku-lei "star wreath", it formed this asterism with Procyon, Sirius, Castor and Pollux. [23] In Tahitian folklore, Capella was Tahi-ari'i, the wife of Fa'a-nui (Auriga) and mother of prince Ta'urua (Venus) who sails his canoe across the sky. [92] In Inuit astronomy, Capella, along with Menkalinan (Beta Aurigae), Pollux (Beta Geminorum) and Castor (Alpha Geminorum), formed a constellation Quturjuuk, "collar-bones", the two pairs of stars denoting a bone each. Used for navigation and time-keeping at night, the constellation was recognised from Alaska to western Greenland. [93] The Gwich'in saw Capella and Menkalinan has forming shreets'ą įį vidzee, the right ear of the large circumpolar constellation Yahdii, which covered much of the night sky, and whose orientation facilitated navigation and timekeeping. [94]

In Australian Aboriginal mythology for the Boorong people of Victoria, Capella was Purra, the kangaroo, pursued and killed by the nearby Gemini twins, Yurree (Castor) and Wanjel (Pollux). [95] The Wardaman people of northern Australia knew the star as Yagalal, a ceremonial fish scale, related to Guwamba the barramundi (Aldebaran). [96]

## Is the Sun slightly blue in the center? - Wavelength-dependent limb darkening of the Sun - Astronomy

Our solar system is composed of the Sun and all things which orbit around it: the Earth, the other eight planets, asteroids, and comets. The Sun is 150 million kilometers (93 million miles) away from the Earth (this distance varies slightly throughout the year, because the Earth's orbit is an ellipse and not a perfect circle).

The Sun is an average star - there are other stars which are much hotter or much cooler, and intrinsically much brighter or fainter. However, since it is by far the closest star to the Earth, it looks bigger and brighter in our sky than any other star. With a diameter of about 1.4 million kilometers (860,000 miles) it would take 110 Earths strung together to be as long as the diameter of the Sun. The Sun is mostly made up of hydrogen (about 92.1% of the number of atoms, 75% of the mass). Helium can also be found in the Sun (7.8% of the number of atoms and 25% of the mass). The other 0.1% is made up of heavier elements, mainly carbon, nitrogen, oxygen, neon, magnesium, silicon and iron. The Sun is neither a solid nor a gas but is actually plasma. This plasma is tenuous and gaseous near the surface, but gets denser down towards the Sun's fusion core.

The Sun, as shown by the illustration to the left, can be divided into six layers. From the center out, the layers of the Sun are as follows: the solar interior composed of the core (which occupies the innermost quarter or so of the Sun's radius), the radiative zone, and the the convective zone, then there is the visible surface known as the photosphere, the chromosphere, and finally the outermost layer, the corona.
The energy produced through fusion in the Sun's core powers the Sun and produces all of the heat and light that we receive here on Earth. The process by which energy escapes from the Sun is very complex. Since we can't see inside the Sun, most of what astronomers know about this subject comes from combining theoretical models of the Sun's interior with observational facts such as the Sun's mass, surface temperature, and luminosity (total amount of energy output from the surface).

All of the energy that we detect as light and heat originates from nuclear reactions deep inside the Sun's high-temperature "core." This core extends about one quarter of the way from the center of Sun (where the temperature is around 15.7 million kelvin (K), or 28 million degrees Fahrenheit) to its surface, which is only 5778 K "cool".

Above this core, we can think of the Sun's interior as being like two nested spherical shells that surround the core. In the innermost shell, right above the core, energy is carried outwards by radiation. This "radiative zone" extends about three quarters of the way to the surface. The radiation does not travel directly outwards - in this part of the Sun's interior, the plasma density is very high, and the radiation gets bounced around countless numbers of times, following a zig-zag path outward.

It takes several hundred thousand years for radiation to make its way from the core to the top of the radiative zone! In the outermost of the two shells, where the temperature drops below 2,000,000 K (3.5 million degrees F) the plasma in the Sun's interior is too cool and opaque to allow radiation to pass. Instead, huge convection currents form and large bubbles of hot plasma move up towards the surface (similar to a boiling pot of water that is heated at the bottom by a stove). Compared to the amount of time it takes to get through the radiative zone, energy is transported very quickly through the outer convective zone.

The Sun's visible surface the photosphere is "only" about 5,800 K (10,000 degrees F). Just above the photosphere is a thin layer called the chromosphere. The name chromosphere is derived from the word chromos, the Greek word for color. It can be detected in red hydrogen-alpha light meaning that it appears bright red. Above the surface is a region of hot plasma called the corona. The corona is about 2 million K (3.6 million degrees F), much hotter than the visible surface, and it is even hotter in a flare. Why the atmosphere gets so hot has been a mystery for decades SOHO's observations are helping to solve this mystery.

The Sun is not just a big bright ball. It has a complicated and changing magnetic field, which forms things like sunspots and active regions. The magnetic field sometimes changes explosively, spitting out clouds of plasma and energetic particles into space and sometimes even towards Earth. The solar magnetic field changes on an 11 year cycle. Every solar cycle, the number of sunspots, flares, and solar storms increases to a peak, which is known as the solar maximum. Then, after a few years of high activity, the Sun will ramp down to a few years of low activity, known as the solar minimum. This pattern is called the "sunspot cycle", the "solar cycle", or the "activity cycle".
Stars like the Sun shine for nine to ten billion years. The Sun is about 4.5 billion years old, judging by the age of moon rocks. Based on this information, current astrophysical theory predicts that the Sun will become a red giant in about five billion (5,000,000,000) years.

Astronomy is not simply the study of the night sky. There is a lot that can be seen in the daytime as well. The Sun is the power house of our solar system. This dynamic star changes every day and emits light in all wavelengths of the electro-magnetic spectrum. Because of this the Sun can be a great object to study from a hobbyist or scientific perspective. In this section we are going to cover the Sun and how to observe it safely.

The Sun is a massive nuclear furnace that runs via a process called nuclear fusion (the process of fusing smaller atoms into large, more complex atoms). With this process the Sun consumes nearly 550 million tons of Hydrogen per second. Nuclear fusion in the core of the Sun produces enough energy to run everything on Earth for eternity! The Sun is what is known as a main sequence star, not too big, not too small. The Sun contains 99.8% of the mass of the solar system, nearly 70% being Hydrogen gas. The Sun measures 864,948 miles in diameter at it’s equator. Over 1.3 million Earths could fit within the volume of the Sun’s sphere.

Being the correct distance away from the Sun is important, too close and we would be too hot. Too far and our planet would be too cold. Earth sits in a zone astronomers like to call the "Goldilocks Zone" which means its temperature is just right to support life, as we know it. The Earth orbits at an average distance of 93 million miles from the Sun, this distance is also called an astronomical unit (distance from the Earth to the Sun).

Image of the Sun showing its varying layers. Image Credit: NASA

Charlie Bates Solar Astronomy Project

The Charlie Bates Solar Astronomy Project (CBSAP is a 501c3 non-profit organization) is the largest astronomy outreach program in the world. Based out of Atlanta, Georgia, the CBSAP was founded and is directed by Stephen W. Ramsden, a master of the Sun and all things solar.

This program provides an advanced science based program to schools all over the world, reaching over 350,000 kids annualy with hands on solar observing and imaging, CBSAP affiliates can be found in all corners of the globe bringing like minded people together to share the wonders of our Sun.

Focus Astronomy is proud to be the official partner for the CBSAP here in Los Angeles. Our solar programs are based around the same concepts that are done in all other branches of CBSAP.

The most basic form of filter is known as a white light filter. These filters allow us see the "surface" (photosphere) of the Sun and displays sunpots (cooler regions of the Sun). These filters reduce all colors of visible light equally and display a yellowish/white solar disk.

For those looking for a simple way to view a planetary transit (like the May 9th, 2016 Mercury Transit) or solar eclipses (like the August 21, 2017 total eclipse) solar glasses can be a fun, affordable and easy way to observe the Sun safely without the need of expensive optics.

The easiest and most cost effective white light filter is black polymer or RG solar film. This film comes in sheets that can be cut to size to fit the optic you are using. These films are to be placed over the entire front of the optic being used. Solar film can be mounted by making a mounting cell out of cardboard or other materials or even rubber banded over the front of your optic of choice.

Pre made full aperture white light filters are also avaliable (as seen in the image to the left). These filters are available in RG Film of glass. RG film provides a more true color view of the Sun but can be easily damaged. Glass filters provide an orangish view of the Sun but are more durable. These filters are mounted within a sturdy aluminum cell that can be sized to fit camera lenses, binoculars or telescopes. Filters such as these come in a variety of sizes to fit any aperture optic.

Some larger telephoto lenses have filter drawers within them. This makes it easier when using filters instead of having to purchase a large front filter. These drawers DO NOT work for solar work. The light from the Sun will be partially focused before it gets to the filter drawer and that will damage the filter and lens. To ensure safety please use a full aperture filter over the entire front element of your lens of choice.

A more advanced White Light filter is known as a Hershel wedge. A Hershel wedge should only be used with a refracting telescope. A Hershel wedge is a specialized prism that is uncoated. No filter is used on the front of the telescope. All the light is focused onto an uncoated prism. Nearly 99.9% of the focused light is passed through the prism into a vented heat sync which dissipates the heat. The remaining light is reflected up into a neutral density filter and then through a polarizer and safely into the eyepiece or camera.

The image through a Hershel wedge provides the most accurate view of the color of the Sun’s photosphere (as seen in the image on the left) as no wavelengths are lost as is the case with many front mounted filters. A Hershel wedge is a highly specialized instrument so please make sure you are well versed before adding one to your line up. These types of filters are available from Lunt Solar Systems and Baader.

As with most objects in space, the Sun is composed of many different layers. The Sun’s has seven major layers. The inner layers are the core, the radiative zone and the convection zone. These inner layers are not visible to use as they are below the point where the Sun’s gasses are transparent to visible light. The outer layers of the Sun such as the photosphere, chromosphere, transition region and corona are visible using special equipment. In amateur astronomy the photosphere, chromosphere and corona are the only visible layers.

The photosphere is the deepest visible layer we can see in the visible light spectrum. This layer of the Sun is visible in White Light filters (we will cover solar equipment below) which display sunspots, faculae and granulation at times. The Photosphere is about 300 miles thick with a temperature of nearly 11,000 degree F at the bottom and 6700 degrees F at the top.

The chromosphere is the layer directly above the photosphere. Surprisingly this layer of the Sun, while further out, is nearly 14,000 degree F towards the upper edge. The chromosphere is most often observed by amateurs in H-alpha and Calcium-K/H wavelengths (we will get into these filters in the equipment section below). The chromosphere displays sunspots (cooler regions of the Sun), filaments, prominences, active regions, spicules, solar flares and many unclassified magnetic phenomena.

The corona is the outermost layer of our Sun with a temperature around 900,000 degrees F! The Corona is the Sun's outer atmosphere and can be difficult to see. The only time we can view this layer of the Sun from within the Earth’s atmosphere is during a total solar eclipse. Specialized scientific instruments called coronagraphs can also be used to observe this layer but are rarely available to the public.

Observation of the Sun can be an amazing activity especially when viewing eclipses or planetary transits. With the correct equipment it’s easy to understand why so many people become interested in this aspect of astronomy. Before we dig into observing the Sun and the equipment needed, there is one important thing we need to cover, safety.

Observing the Sun can be very dangerous! Due to the amount of light and energy, very specialized equipment is needed. DO NOT look at with Sun with your naked eye or with optical aid without the use of certified filters. Welders glass does not count as a safe filter, it does not block out harmful UV rays that are emitted by the Sun. Ensure all safety precautions have been met before attempting to view the Sun.

Now that we have covered the safety aspect lets look into the equipment that makes observation of the Sun possible. All the instruments listed in the section below have been proven to be safe. Focus does not recommend anything that would put one in harms way.

Aside from the popular white light filters above another method of observing the Sun is by using narrowband solar filters. Narrowband filters are, as the name suggests, capable of isolating very small wavelengths of the visible spectrum. Isolating these wavelengths allows us to study different layers of the Sun in greater detail. Narrowband filters bring a different dynamic to solar observing and can provide impressive and dynamic views as well as details about the Sun's chemistry. Narrowband filters are designed to very tight tolerances thus making them much more expensive than full spectrum white light filters. Below we will cover the varying types of narrow band solar filters and how to use them.

Hydrogen-alpha (H-alpha) solar filter is a specifically designed filter that allows us to view the glowing Hydrogen gas found in the Sun's chromosphere (top layer). With an H-alpha filter we are able to view

amazing solar events such as:

H-alpha filters are generally rated by their band pass (how narrow a band of light they allow to pass through). This band pass is usually measured in a unit called an Angstrom (one ten-billionth of a meter or 0.1 nanometer). The narrower the band pass, the more detail that can be see on the Sun's disk. The wider the band pass the better we can observe edge details like prominences or coronal loops.

In today's market, H-alpha filters generally range from 1 angstrom down to an amazing 0.2 angstrom. The narrower the band pass the more expensive the device due to the tolerances needed to obtain such a narrow filter.

There are two major types of H-alpha filters: solid etalons and air spaced etalons. An etalon is the heart of an H-alpha system, these filters allow light the bounce between to highly polished glass plates creting an interference pattern which allows the specific wavelength to pass through along with several other peaks on each side of centerline. The 2nd required stage of filtration in an etalon based system is called a blocking filter and it eliminates the other peaks on each side of the desired wavelength. In order for an etalon to work correctly light must enter the filter at right angles to the glass surfaces so additional collimation is required before and sometimes after most filters. These systems cannot be used without the blocking filter. Blocking filters are sold based on the size of the glass opening. Different sizes are of the same quality but have larger or smaller fields of view.

Solid etalons produced by companies like Daystar and Solar Spectrum offer a wide range of band passes. These filters are generally mounted to the focuser of a telescope and have to be used on long focal length (f/30) instruments in order to obtain a parallel beam of light. To achieve this, larger instruments must be stepped down in aperture or a barlow lens is needed to increase the focal ratio. These systems also require an energy rejection filter in front of the objective to prevent focused infrared radiation from striking the filter surface directly.

In order to tune a solid etalon to centerline, the temperature must be carefully controlled. These filters require built in heaters and external power sources to stay on band. Operating at such high focal ratios normally makes it impossible for a solid etalon system to display a full disk view of the Sun. These solid etalons are excellent choices for extreme close-ups and provide very even illumination.

Daystar recently introduced the “Quark”, a solid etalon, all in one “solar eyepiece” H-Alpha and Calcium H filter that can be used with smaller refracting telescopes to provide a full disk or close-up view.

Air spaced etalons work just like they sound two solid glass surfaces are separated by an air gap. Instead of using heat to tune these filters tilting, physical pressure or air pressure it used to tune them onto the correct band. Air spaced etalons are more difficult to produce due to small spacers that are needed to produce the small air gap. Air spaced etalons can be found from companies like Coronado, Lunt and SolarScope.

Unlike solid etalons an air spaced etalon can be mounted to the front of a telescope using machined adapter plates. Air spaced etalons work very well without any external heat and at the native focal ratios of most refractors. Most air spaced etalon systems are around 0.7 angstrom band pass. This is good for observing both prominences and some surface detail but not nearly as good as narrower filters.

Combining two or more etalons in the same system is called double or triple “stacking. Each additional etalon will reduce the bandpass of the filter at the expense of a 40% image brightness reduction per additional etalon. A Lunt Solar Systems single etalon filter normally has around a .7 Angstrom bandpass while a double stacked Lunt scope will normally be around .55 Angstroms. Triple stacking further reduces the bandpass but in almost all cases, the image is too dim to visually observe at a comfortable level for most people and the reflections created by all of the additional glass is usually problematic.

Dedicated Solar Telescopes

Now that we have covered H-alpha filters, lets discuss dedicated solar

Most H-alpha filters systems we have mentioned need to be used in tandem with another telescope. However another option is a telescope that has the filters built into it. A dedicated solar telescope can be an elegant answer to those looking to observe the Sun. Most solar telescopes today are based around an H-alpha filter.

A dedicated solar telescopes is an entire H-alpha filtration system contained within the telescope. A package such as this can make a great addition for educational organizations who need a simple set up with little to no removable parts. Dedicated solar telescopes are available from nearly all major solar manufacturers such as Coronado, Daystar, Lunt and SolarScope.

Sometimes a solar scope can provide a larger aperture for less money when compared to a filter set. This is mainly due to the size of the etalon used. Most solar scopes can use a smaller etalon inside the optical tube rather than a full aperture etalon. Solar telescopes can range in sizes from the smaller and affordable (under $1000) Coronado PST and Lunt LS50THa to the high level research class such as the Lunt LS152Tha ($6000) and LS230THa ($25,000). Another type of narrow band filter is Calcium-K (CaK) and H (CaH) line filters. Calcium filters such as this display a lower level of the chromosphere below what is visible in an H-alpha filter. Calcium filters display the Sun in a deep violet color in the near ultra-violet portion of the spectrum. Calcium filters are excellent for observing: Being on the very edge of the normal visible light sensitivity of most people (390nm-700nmn), it is difficult for most people to see CaK or CaH. Those with younger eyes are normally able to see the dark violet disk much easier than older eyes. This is due to our eyes yellowing as we age. Because of this, CaK filters are generally recommended for imaging purposes. CaK filters are avaliable through Daystar (which requires power) and Lunt Solar Systems (which does not require power). Coronado has manufactured dedicated CaK solar telescopes in the past but are no longer marketing them. Lunt CaK modules are available starting at around$600.

Another line of Calcium is the Calcium-H line (CaH). CaH filters see 397nm light from the solar spectrum. This wavelength is slightly more visible to the human eye and is equally good for imaging purposes compared to CaK. Daystar is currently making a CaH “Quark” which can be used, with external power, on most small refractors.

Over the years Focus has used CaK filters in our solar programs. As of February 2016 Focus has replaced our CaK filter with the Daystar CaH Quark which has expanded the ability for people to view the Calcium line of the Sun. For those teaching about the Sun we highly recommended looking into the Daystar CaH Quark to expand your outreach efforts across the spectrum.

## Contents

Notation: let the symbol dex represent the difference between powers of ten.

When any effort to acquire a system of laws or knowledge focusing on an astr, aster, or astro, that is, any natural body in the sky especially at night, [3] discovers an entity emitting, absorbing, transmitting, reflecting, or fluorescing ultraviolet, succeeds even in its smallest measurement, ultraviolet astronomy is the name of the effort and the result.

Ultraviolet astronomy consists of three fundamental parts:

1. logical laws with respect to incoming ultraviolet rays, or ultraviolet radiation,
2. natural ultraviolet sources, and
3. the sky and associated realms with respect to ultraviolet rays.

Def. "the spectral region bounded on the long wavelength side at about λ3000 by the onset of atmospheric ozone absorption and on the short wavelength side at λ912 by the photoionization of interstellar hydrogen" is called the ultraviolet. [4]

The draft ISO standard on determining solar irradiances (ISO-DIS-21348) 5 describes the following ranges:

Name Abbreviation Wavelength range in nanometers Energy per photon
Before UV spectrum Visible light above 400 nm below 3.10 eV
Ultraviolet A, long wave, or black light UVA 400 nm–315 nm 3.10–3.94 eV
Near NUV 400 nm–300 nm 3.10–4.13 eV
Ultraviolet B or medium wave UVB 315 nm–280 nm 3.94–4.43 eV
Middle MUV 300 nm–200 nm 4.13–6.20 eV
Ultraviolet C, short wave, or germicidal UVC 280 nm–100 nm 4.43–12.4 eV
Far FUV 200 nm–122 nm 6.20–10.2 eV
Vacuum VUV 200 nm–100 nm 6.20–12.4 eV
Low LUV 100 nm–88 nm 12.4–14.1 eV
Super SUV 150 nm–10 nm 8.28–124 eV
Extreme EUV 121 nm–10 nm 10.2–124 eV
Beyond UV range X-rays below 10 nm above 124 eV

"Vacuum UV" is so named because it is absorbed strongly by air and is, therefore, used in a vacuum. In the long-wave limit of this region, roughly 150–200 nm, the principal absorber is the oxygen in air.

Ultraviolet lamps are also used in analyzing minerals and gems. Materials may look the same under visible light, but fluoresce to different degrees under ultraviolet light, or may fluoresce differently under short wave ultraviolet versus long wave ultraviolet.

Ultraviolet lamps may cause certain minerals to fluoresce, and is a key tool in prospecting for tungsten mineralisation.

Many samples of fluorite exhibit fluorescence under ultraviolet light, a property that takes its name from fluorite. [5] Many minerals, as well as other substances, fluoresce. Fluorescence involves the elevation of electron energy levels by quanta of ultraviolet light, followed by the progressive falling back of the electrons into their previous energy state, releasing quanta of visible light in the process. In fluorite, the visible light emitted is most commonly blue, but red, purple, yellow, green and white also occur. The fluorescence of fluorite may be due to mineral impurities such as yttrium, ytterbium, or organic matter in the crystal lattice. In particular, the blue fluorescence seen in fluorites from certain parts of Great Britain responsible for the naming of the phenomenon of fluorescence itself, has been attributed to the presence of inclusions of divalent europium in the crystal. [6]

Between 190 and 1700 nm, the ordinary refractive index varies roughly between 1.9 and 1.5, while the extraordinary refractive index varies between 1.6 and 1.4. [7]

Under longwave (365 nm) ultraviolet light, diamond may fluoresce a blue, yellow, green, mauve, or red of varying intensity. The most common fluorescence is blue, and such stones may also phosphoresce yellow—this is thought to be a unique combination among gemstones. There is usually little if any response to shortwave ultraviolet.

The figure at right "is the first laser spectrum from the Chemistry and Camera (ChemCam) instrument on NASA's Curiosity rover, sent back from Mars on August 19, 2012. The plot shows emission lines from different elements present in the target, a rock near the rover's landing site dubbed "Coronation" (see inset)." [8]

"ChemCam's detectors observe light in the ultraviolet (UV), violet, visible and near-infrared ranges using three spectrometers, covering wavelengths from 240 to 850 nanometers. The light is produced when ChemCam's laser pulse strikes a target, generating ionized gases in the form of plasma, which is then analyzed by the spectrometers and their detectors for the presence of specific elements. The detectors can collect up to 16,000 counts produced by the light in any of its 6,144 channels for each laser shot." [8]

"The plot is a composite of spectra taken over 30 laser shots at a single 0.016-inch (0.4-millimeter) diameter spot on the target. An inset on the left shows detail for the minor elements titanium and manganese in the 398-to-404-nanometer range. An inset at the right shows the hydrogen and carbon peaks. The carbon peak was from the carbon dioxide in Mars' air. The hydrogen peak was only present on the first laser shot, indicating that the element was only on the very surface of the rock. Magnesium was also slightly enriched on the surface. The heights of the peaks do not directly indicate the relative abundances of the elements in the rock, as some emission lines are more easily excited than others." [8]

"A preliminarily analysis indicates the spectrum is consistent with basalt, a type of volcanic rock, which is known from previous missions to be abundant on Mars. Coronation is about three inches (7.6 centimeters) across, and located about 5 feet (1.5 meters) from the rover and about nine feet (2.7 meters) from ChemCam on the mast." [8]

"The Evershed effect . is the radial flow of gas across the photospheric surface of the penumbra of sunspots from the inner border with the umbra towards the outer edge. [9]

The speed varies from around 1 km/s at the border between the umbra and the penumbra to a maximum of around double this in the middle of the penumbra and falls off to zero at the outer edge of the penumbra.

Measurements of the spectral emission lines emitted in the ultraviolet wavelengths have shown a systematic red-shift. The Evershed effect is common to every spectral line formed at a temperature below 10 5 K this fact would imply a constant downflow from the transition region towards the chromosphere. The observed velocity is about 5 km/s. Of course, this is impossible, since if it were true, the corona would disappear in a short time instead of being suspended over the Sun at temperatures of million degrees over distances much larger than a solar radius.

Many theories have been proposed to explain this redshift in line profiles of the transition region, but the problem is still unsolved, since a coherent theory should take into account all the physical observations: UV line profiles are redshifted on average, but they show back and forth velocity oscillations at the same time.

In synthesis, the proposed mechanisms are:

• siphon flows in coronal loops driven by a pressure difference, [10]
• different cross-sections of the coronal loops footpoints, [11]
• the return of spicules, [12]
• multiple flows, [13] , [14] and
• thermal instabilities during chromospheric condensation. [15]

Charles Stuart Bowyer is generally given credit for starting this field.

UV light is found in sunlight (where it constitutes about 10% of the energy in vacuum) and is emitted by electric arcs and specialized lights such as mercury lamps and black lights.

In the diagram at right, the levels of ozone at various altitudes yellow line and blocking of different bands of ultraviolet radiation are shown. Essentially all UVC is blocked by dioxygen (from 100–200 nm) or by ozone (200–280 nm) in the atmosphere. The ozone layer then blocks most UVB. Meanwhile, UVA is hardly affected by ozone and most of it reaches the ground.

This diagram contains "a typical profile of ozone density versus altitude (yellow line) in the midlatitudes of the Northern Hemisphere (units=Dobson Units/kilometer). The stratosphere lies between the tropopause and stratopause (marked in red). Superimposed on the figure are plots of UV radiation as a function of altitude for UVa (320-400 nm, cyan), UVb (280-320 nm, green), and UVc (200-280 nm, magenta). The width of the bar indicates the amount of energy as a function of altitude. The UVc energy decreases dramatically as ozone increases because of the strong absorption in the 200-280 nm wavelength band. The UVb is also strongly absorbed, with a small fraction reaching the surface. The UVa is only weakly absorbed by ozone, with some scattering of radiation near the surface." [16]

Sunlight in space at the top of Earth's atmosphere, at a solar constant output of about 1366 watts/m 2 , is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total ultraviolet power of about 140 watts/m 2 in vacuum. [17] However, at ground level total sunlight power decreases to about 1000–1100 watts/m 2 , and by energy fractions, is composed of 44% visible light, 3% ultraviolet (with the Sun at its zenith), and the remainder infrared. [18]

A PG 1159 star, often also called a pre-degenerate, [19] is a star with a hydrogen-deficient atmosphere which is in transition between being the central star of a planetary nebula and being a hot white dwarf. These stars are hot, with surface temperatures between 75,000 K and 200,000 K, [20] and are characterized by atmospheres with little hydrogen and absorption lines for helium, carbon and oxygen. . The PG 1159 stars are named after their prototype, PG 1159-035. This star, found in the Palomar-Green survey of ultraviolet-excess stellar objects, [21] was the first PG 1159 star discovered.

Lyc photon or Ly continuum photon or Lyman continuum photon are a kind of photon emitted from stars. Hydrogen is ionized by absorption of Lyc photons. Lyc photons are in the ultraviolet portion of the electromagnetic spectrum of the hydrogen atom and immediately next to the limit of the Lyman series of the spectrum with wavelengths that are shorter than 91.1267 nanometres and with energy above 13.6 eV.

The Sun's emission in the lowest UV bands, the UVA, UVB, and UVC bands, are of interest, as these are the UV bands commonly encountered from artificial sources on Earth. The shorter bands of UVC, as well as even more energetic radiation as produced by the Sun, generate the ozone in the ozone layer when single oxygen atoms produced by UV photolysis of dioxygen react with more dioxygen. The ozone layer is especially important in blocking UVB and part of UVC, since the shortest wavelengths of UVC (and those even shorter) are blocked by ordinary air.

"In 1969, Chi Carinae was classified as chemically peculiar Ap star [22] because its absorption lines of silicon appeared unusually strong relative to the lines for helium. However, subsequent examination in the ultraviolet band showed the silicon bands were as expected and it was determined the spectra is normal for a star of its type.

In the figure at right, CUVOB stands for the cosmic ultraviolet and optical background.

"The first spectroscopic measurement of the diffuse cosmic ultraviolet background in the range 1700-2850 Å has resulted in the detection at high Galactic latitude of an intensity of 300 ± 100 photons (cm 2 s sr Å) -1 at 1800 Å without any correction necessary for starlight or airglow, a similar intensity over the range 1900-2500 Å after correction for measured airglow, and a similar intensity over the range 2500-2800 Å after correction for zodiacal light." [23]

The temperature for a lightning bolt channel is 28 kK or 28,000 K with a peak emittance wavelength of black-body radiation at approximately 100 nm (far ultraviolet light.

Optical astronomy includes those portions of ultraviolet, visual, and infrared astronomy that benefit from the use of quartz crystal or silica glass telescope components.

The color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. To measure the index, one observes the magnitude of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also: UBV system). The set of passbands or filters is called a photometric system. The difference in magnitudes found with these filters is called the U-B or B–V color index, respectively. The smaller the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is. This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller (more negative) magnitudes than dimmer ones.

"[T]he Lyman series is the series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1 (where n is the principal quantum number referring to the energy level of the electron).

The version of the Rydberg formula that generated the Lyman series was [24] :

Where n is a natural number greater than or equal to 2 (i.e. n = 2,3,4. ).

The wavelengths (nm) in the Lyman series are all ultraviolet:

In 1913, when Niels Bohr produced his Bohr model theory, the reason why hydrogen spectral lines fit Rydberg's formula was explained. Bohr found that the electron bound to the hydrogen atom must have quantized energy levels described by the following formula:

There is also a more comfortable notation when dealing with energy in units of electronvolts and wavelengths in units of angstroms:

Replacing the energy in the above formula with the expression for the energy in the hydrogen atom where the initial energy corresponds to energy level n and the final energy corresponds to energy level m:

where R_H is the same Rydberg constant for hydrogen of Rydberg's long known formula.

For the connection between Bohr, Rydberg, and Lyman, one must replace m by 1 to obtain:

which is Rydberg's formula for the Lyman series. Therefore, each wavelength of the emission lines corresponds to an electron dropping from a certain energy level (greater than 1) to the first energy level.

There is a He I (Is 21 S-Is2p 2 P) transition at 58.4 nm and another Is 21 S-Is3p 1 P) at 53.7 nm. [25] The He II lines are at 25.6, 30.4, and 164.0 nm. [25]

There is another He II line at 320.3 nm. [26]

"[T]he Be II 3130 Å region . contains the Be II resonance doublet with components at 3130.42 [313.042 nm] and 3131.07 Å [313.107 nm]." [27]

"High-resolution, high signal-to-noise ratio spectra, obtained at the Cerro Tololo Inter-American Observatory 4 m telescope, of the Be II 3131 Å [313.1 nm] region [are from] the metal-rich solar analog α Centauri A and its companion α Centauri B." [27]

"The photospheric abundances of the light elements Li, Be, and B provide important clues about stellar structure and evolution, as they are destroyed by (p,α)-reactions at temperatures exceeding a few million degrees." [27]

"For Cen A, . [Be/H] = +0.20 ± 0.15, where the error reflects random uncertainties at the 1σ confidence level systematic errors of 0.1 dex are also possible." [27]

Boron (B I) line is at 249.67 nm. [28]

Boron is detected in the Population II star HD 140283 by observing the "wavelength region around the resonance lines of B I at 2497 Å . with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope on September 5, 1992, . and continued on February 15, and 21, 1993" [28] "The resulting B/Be ratio is in the range 9-34 with 17 being the most probable value. This is in very good agreement with predictions for cosmic ray spallation." [28]

"[T]he solar system meteoritic NB/NBe ratio 28 ± 4 (Anders & Grevesse 1989) is within our limits of uncertainty, implying that the same process could in principle be responsible for the production of B and Be throughout the history of the Galaxy." [28]

There is a C III line at 97.7 nm. [25]

Carbon has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 229.687 nm from C III, 227.089, 227.727, and 227.792 nm from C V, 207.025, 208.216, 313.864, and 343.366 nm from C VI. [29]

Several emission lines occur in plasmas at 347.872, 348.30, and 348.493 nm from N IV and 252.255, 344.211, and 388.678 nm from N VII. [29]

Molecular nitrogen and nitrogen compounds have been detected in interstellar space by astronomers using the Far Ultraviolet Spectroscopic Explorer. [30]

Oxygen has three emission lines common in comets at 130.22, 130.49, and 130.60 nm from O I. [31]

Oxygen has several emission lines that occur in plasmas at 278.101, 278.699, and 278.985 nm from O V, and 253.04, 297.569, 348.767 nm from O VIII. [29]

Atomic oxygen "airglow emissions [have been] measured by using vertical-viewing photometers [on-board a sounding rocket for the] Herzberg I bands near 300 nm" [32] .

"[T]he presence of the [oxygen] green line can still be questioned, unless the 2972 Å trans-auroral line [ 1 S - 3 P] is detected (Herbig, 1976)." [33] "The transitions involved . in the spectrum of the oxygen atoms in a cometary atmosphere" include 295.8 and 297.2 nm, 98.9 nm (a triplet), and 1304 nm (a triplet), 102.7 nm (a triplet) and 1128.7 nm. [33]

Fluorine has several emission lines that occur in plasmas at 311.361, 311.57, 312.154, 312.478, 313.422, 314.278, 317.418, 317.476, and 321.397 nm from F III, 270.23, 270.717, 271.288, 272.106, 273.20, 273.691, and 275.62 nm from F V, 231.539, 232.335, and 232.728 nm from F VI, and 342.938 nm from F IX. [29]

Argon has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 333.613, 334.472, 335.211, 335.849, and 336.128 nm from Ar III. [29]

The calcium line Ca XII at 332.8 nm occurs in the solar corona. [34]

Chromium has several emission lines that occur in plasmas at 357.869, 359.349, and 360.533 nm from Cr I, and 222.67 and 223.59 nm from Cr III. [29]

Iron has several emission lines that occur in plasmas at 371.994 nm from Fe I, 234.35 nm from Fe II, and 273.9, 274.9, and 276.9 nm from Fe XV. [29]

Iron has two lines occurring in the solar corona in the near ultraviolet: 338.81 and 345.41 nm of Fe XIII. [34]

Nickel has lines occurring in the solar corona at 360.10 nm of Ni XVI and 364.29 nm of Ni XIII. [34]

Copper has two emission lines that occur in plasmas at 324.754 and 327.396 nm from Cu I. [29]

Ultraviolet light is found in sunlight. The sun emits ultraviolet radiation in the UVA, UVB, and UVC bands. The Earth's ozone layer blocks 97–99% of this UV radiation from penetrating through the atmosphere. [35]

The solar transition region is a region of the Sun's atmosphere, between the chromosphere and corona [36] . It is visible from space using telescopes that can sense ultraviolet.

The transition region is visible in far-ultraviolet (FUV) images from the TRACE spacecraft, as a faint nimbus above the dark (in FUV) surface of the Sun and the corona. The nimbus also surrounds FUV-dark features such as solar prominences, which consist of condensed material that is suspended at coronal altitudes by the magnetic field.

In the corona thermal conduction occurs from the external hotter atmosphere towards the inner cooler layers. Responsible for the diffusion process of the heat are the electrons, which are much lighter than ions and move faster.

Ultraviolet telescopes such as TRACE and SOHO/EIT can observe individual micro-flares as small brightenings in extreme ultraviolet light, [37] but there seem to be too few of these small events to account for the energy released into the corona.

The first direct observation of waves propagating into and through the solar corona was made in 1997 with the SOHO space-borne solar observatory, the first platform capable of observing the Sun in the extreme ultraviolet (EUV) for long periods of time with stable photometry. Those were magneto-acoustic waves with a frequency of about 1 millihertz (mHz, corresponding to a 1,000 second wave period), that carry only about 10% of the energy required to heat the corona. Many observations exist of localized wave phenomena, such as Alfvén waves launched by solar flares, but those events are transient and cannot explain the uniform coronal heat.

Ultraviolet irradiance (EUV) varies by approximately 1.5 percent from solar maxima to minima, for 200 to 300 nm UV. [38]

"1 percent of the sun's energy is emitted at ultraviolet wavelengths between 200 and 300 nanometers, the decrease in this radiation from 1 July 1981 to 30 June 1985 accounted for 19 percent of the decrease in the total irradiance". [38]

Energy changes in the UV wavelengths involved in production and loss of ozone have atmospheric effects.

The 30 hPa atmospheric pressure level has changed height in phase with solar activity during the last 4 solar cycles.

UV irradiance increase causes higher ozone production, leading to stratospheric heating and to poleward displacements in the stratospheric and tropospheric wind systems.

A proxy study estimates that UV has increased by 3.0% since the Maunder Minimum. [39]

"Solar satellite observatories such as ESA/NASA's Solar and Heliospheric Observatory (SOHO) have been studying the sun for over 10 years, and have created images of the entire solar surface using spectroscopic techniques. [The second image at right] shows a recent full-sun image created by the Extreme-ultraviolet Imaging Telescope (EIT) taken during sunspot-minimum conditions in 2008. [. ] By using the techniques of imaging spectroscopy, solar physicists can isolate gases heated to temperatures of 1,500,000 K and study their motions and evolution over time." [40]

The second image at right is "taken on December 16, 2008 during sunspot-minimum conditions, was created by isolating the light produced at a wavelength of 195 Angstroms (19.5 nanometers) by the ion Fe XII. By selecting the light from only one spectral line, a spectroheliograph works like a high-precision light filter and lets astronomers map, or image, a distant object in the light from a single spectral line. This information can be used to map the temperature and density changes in the gas." [40]

"[U]ltraviolet observations by Mariner 10 provided evidence for the presence of H and He in the atmosphere (

10 11 and 10 12 atoms cm -2 respectively 3 " [41] .

Aboard Mariner 10, "[t]he extreme ultraviolet spectrometer consisted of two instruments: an occultation spectrometer that was body-fixed to the spacecraft and an airglow spectrometer that was mounted on the scan platform. When the sun was obscured by the limbs of the planet, the occultation spectrometer measured the extinction properties of the atmosphere. The occultation spectrometer had a plane grating which operated at grazing incidence. The fluxes were measured at 47.0, 74.0, 81.0, and 89.0 nm using channel electron multipliers. Pinholes defined the effective field of view of the instrument which was 0.15 degree full width at half maximum (FWHM). Isolated spectral bands at approximately 75 nm (FWHM) were also measured. The objective grating airglow spectrometer was flown to measure airglow radiation from Venus and Mercury in the spectral range from 20.0--170.0 nm. With a spectral resolution of 2.0 nm, the instrument measured radiation at the following wavelengths: 30.4, 43.0, 58.4, 74.0, 86.9, 104.8, 121.6, 130.4, 148.0, and 165.7 nm. In addition, to provide a check on the total incident extreme UV flux to the spectrometer, two zero-order channels were flown. The effective field of view of the instrument was 0.13 by 3.6 degree. Data also include the interplanetary region." [42]

When imaged in the ultraviolet (right), Venus appears like a gas dwarf object rather than a rocky object.

"This unusual false-color image [at right] shows how the Earth glows in ultraviolet (UV) light. The Far UV Camera/Spectrograph deployed and left on the Moon by the crew of Apollo 16 captured this image. The part of the Earth facing the Sun reflects much UV light and bands of UV emission are also apparent on the side facing away from the Sun. These bands are the result of aurora caused by charged particles given off by the Sun. They spiral towards the Earth along Earth's magnetic field lines." [43]

In April 1972, the Apollo 16 mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph. [44]

Archaeological "[a]erial survey . employs ultraviolet, infrared, ground-penetrating radar wavelengths, LiDAR and thermography. [45]

In 1911, Professor Robert W. Wood used ultraviolet photography to take images of the crater area. He discovered the plateau had an anomalous appearance in the ultraviolet, and an area to the north appeared to give indications of a sulfur deposit. [46] This colorful area is sometimes referred to as "Wood's Spot", an alternate name for the Aristarchus Plateau.

Spectra taken of this crater during the Clementine mission were used to perform mineral mapping. [47] The data indicated that the central peak is a type of rock called anorthosite, which is a slow-cooling form of igneous rock composed of plagioclase feldspar. By contrast the outer wall is troctolite, a rock composed of equal parts plagioclase and olivine.

The Aristarchus region was part of a Hubble Space Telescope study in 2005 that was investigating the presence of oxygen-rich glassy soils in the form of the mineral ilmenite. Baseline measurements were made of the Apollo 15 and Apollo 17 landing sites, where the chemistry is known, and these were compared to Aristarchus. The Hubble Advanced Camera for Surveys was used to photograph the crater in visual and ultraviolet light. The crater was determined to have especially rich concentrations of ilmenite, a titanium oxide mineral that could potentially be used in the future by a lunar settlement for extracting oxygen. [48]

"Far-ultraviolet spectra . Mars in the range 820-1840 Å at

4 Å resolution were obtained on 1995 March 13 and 12, respectively, by the Hopkins Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the space shuttle Endeavour. Longward of 1250 Å, the spectra . are dominated by emission of the CO fourth positive (A1Π-X1Σ+) band system and strong O I and C I multiplets. . The Ar I λλ1048, 1066 doublet is detected only in the spectrum of Mars . CO fluorescence in both the B−X (0,0) and C−X (0,0) Birge-Hopfield bands is identified in . Mars . Below 2000 Å, the ultraviolet dayglow of . Mars is dominated by emissions of carbon monoxide and carbon (Durrance 1981 Fox 1992)." [49]

On the right is an ultraviolet photograph of Mars. It shows clouds and other aspects of the atmosphere.

High-resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres. [50] This was thought to be a crater. Later near-infrared images with a higher resolution taken over a whole rotation with the Keck telescope using adaptive optics showed several bright and dark features moving with the dwarf planet's rotation. [51] [52]

C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, [53] and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type. The proportion of C-types may actually be greater than this, because C-types are much darker than most other asteroid types except D-types and others common only at the extreme outer edge of the asteroid belt. . Their spectra contain moderately strong ultraviolet absorption at wavelengths below about 0.4 μm to 0.5 μm, while at longer wavelengths they are largely featureless but slightly reddish. The so-called "water" absorption feature around 3 μm, which can be an indication of water content in minerals is also present.

"F-type asteroids have spectra generally similar to those of the B-type asteroids, but lack the "water" absorption feature around 3 μm indicative of hydrated minerals, and differ in the low wavelength part of the ultraviolet spectrum below 0.4 μm.

G-type asteroids are a relatively uncommon type of carbonaceous asteroid. The most notable asteroid in this class is 1 Ceres. Generally similar to the C-type objects, but containing a strong ultraviolet absorption feature below 0.5 μm.

In astronomy, very hot objects preferentially emit UV radiation (see Wien's law). Because the ozone layer blocks many UV frequencies from reaching telescopes on the surface of the Earth, most UV observations are made from space.

At right is an ultraviolet image of Aurora at Jupiter's north pole by the Hubble Space Telescope.

"Experiments on the Voyager 1 and 2 spacecraft and observations made by the International Ultraviolet Explorer (IUE) have provided evidence for the existence of energetic particle precipitation into the upper atmosphere of Jupiter from the magnetosphere." [54]

The image at lower right "shows Jupiter's atmosphere at a wavelength of 2550 Angstroms after many impacts by fragments of comet Shoemaker-Levy 9. The most recent impactor is fragment R which is below the center of Jupiter (third dark spot from the right). This photo was taken 3:55 EDT on July 21, about 2.5 hours after R's impact. A large dark patch from the impact of fragment H is visible rising on the morning (left) side. Proceding to the right, other dark spots were caused by impacts of fragments Q1, R, D and G (now one large spot), and L, with L covering the largest area of any seen thus far. Small dark spots from B, N, and Q2 are visible with careful inspection of the image. The spots are very dark in the ultraviolet because a large quantity of dust is being deposited high in Jupiter's stratosphere, and the dust absorbs sunlight." [55]

"The Cassini ultraviolet images [of Io] . reveal two gigantic, actively erupting plumes of gas and dust. Near the equator, just the top of Pele's plume is visible where it projects into sunlight. None of it would be illuminated if it were less than 240 kilometers (150 miles) high. [The Cassini ultraviolet] images indicate a total height for Pele of 390 kilometers (242 miles). [One] Cassini image . shows a bright spot over Pele's vent. Although the Pele hot spot has a high temperature, silicate lava cannot be hot enough to explain a bright spot in the ultraviolet, so the origin of this bright spot is a mystery, but it may indicate that Pele was unusually active when the picture was taken." [56]

At right is a Hubble Space Telescope image in the ultraviolet of Io.

"Measurements at two ultraviolet wavelengths indicate that the ejecta consist of sulfur dioxide 'snow,' making the plume appear green in this false-color image. Astronomers increased the colour contrast and added false colours to the image to make the faint plume visible." [57]

"One of a series, this image [at right] of Saturn was taken when the planet's rings were at their maximum tilt of 27 degrees toward Earth. Saturn experiences seasonal tilts away from and toward the sun, much the same way Earth does. This happens over the course of its 29.5-year orbit. Every 30 years, Earth observers can catch their best glimpse of Saturn's south pole and the southern side of the planet's rings. . NASA's Hubble Space Telescope [captured detailed images of Saturn's Southern Hemisphere and the southern face of its rings." [58]

The movie at right records Saturn "when its rings were edge-on, resulting in a unique movie featuring the nearly symmetrical light show at both of the giant planet's poles. It takes Saturn almost thirty years to orbit the Sun, with the opportunity to image both of its poles occurring only twice during that time. The light shows, called aurorae, are produced when electrically charged particles race along the planet's magnetic field and into the upper atmosphere where they excite atmospheric gases, causing them to glow. Saturn's aurorae resemble the same phenomena that take place at the Earth's poles." [59]

Powered by the Saturnian equivalent of (filamentary) Birkeland currents, streams of charged particles from the interplanetary medium interact with the planet's magnetic field and funnel down to the poles. [60] Double layers are associated with (filamentary) currents, [61] [62] and their electric fields accelerate ions and electrons. [63]

"Towering more than 1,000 miles above the cloud tops, these Saturnian auroral displays are analogous to Earth's. . In this false color image, the dramatic red aurora identify emission from atomic hydrogen, while the more concentrated white areas are due to hydrogen molecules." [64]

"The best view of Saturn's rings in the ultraviolet indicates there is more ice toward the outer part of the rings, than in the inner part, hinting at the origins of the rings and their evolution." [65]

"Images taken during the Cassini spacecraft's orbital insertion on June 30 show compositional variation in the A, B and C rings. From the inside out, the "Cassini Division" in faint red at left is followed by the A ring in its entirety. The Cassini Division at left contains thinner, dirtier rings than the turquoise A ring, indicating a more icy composition. The red band roughly three-fourths of the way outward in the A ring is known as the Encke gap." [65]

"The ring system begins from the inside out with the D, C, B and A rings followed by the F, G and E rings. The red in the image indicates sparser ringlets likely made of "dirty," and possibly smaller, particles than in the icier turquoise ringlets." [65]

The image at right "was taken with the Ultraviolet Imaging Spectrograph instrument, which is capable of resolving the rings to show features up to 97 kilometers (60 miles) across, roughly 100 times the resolution of ultraviolet data obtained by the Voyager 2 spacecraft." [65]

The image at second left Saturn's northern UV auroras. These exhibit changes in shape over the course of the observing interval.

"Saturn’s magnetosphere -- the big magnetic bubble that surrounds the planet -- is compressed on the side facing the sun, and it streams out into a long “magnetotail” on the planet’s nightside. Just like with comets, the magnetotails of Earth and Saturn are made of electrified gas from the sun." [66]

"Now it appears that when strong bursts of particles from the sun hit Saturn, the magnetotail collapses and then reconfigures itself -- a disturbance of the magnetic field that’s reflected in the dynamics of auroras." [66]

“We have always suspected this was what also happens on Saturn. This evidence really strengthens the argument.” [67]

"The ultraviolet images were taken by Hubble’s Advanced Camera for Surveys during April and May of last year from the space telescope’s perspective in orbit around Earth. The images are able to provide the first detailed look at dynamics in the “choreography” of auroral glow because Hubble captured them right at that very moment when Saturn’s magnetic field is blasted by particles streaming from the sun." [66]

"Hubble managed to capture a particularly dynamic light show: Some bursts of light shooting around the polar regions traveled at least three times faster than the speed of Saturn’s rotation. (The planet has a 10-hour rotation period.)" [66]

“We can see that the magnetotail is undergoing huge turmoil and reconfiguration, caused by buffering from solar wind. It’s the smoking gun that shows us that the tail is collapsing.” [67]

"Ultraviolet spectra of comets show strong emission in the hydrogen Lyman-α line, the O I 130.2 nm resonance lines, and the OH Α 2 Π-Χ 2 Σ + bands." [31]

"Discovery of the S2 molecule in comets came from UV spectroscopy of the comet IRAS – Araki – Alcock ( 1983d) which passed close to the Earth [59]. . Emission from S2 was shown to be confined to a small region ( < 100 km) around the nucleus. Outside this region, S2 is destroyed. . its presence in comet Hyakutake [60] suggests it is ubiquitous and only its narrow survival zone close to the nucleus inhibits regular detection." [31]

"The transitions involved (allowed and forbidden) in the spectrum of the oxygen atoms in a cometary atmosphere" are 557.7 nm, 630.0 and 636.4 nm, 295.8 and 297.2 nm, 98.9 nm (a triplet), 799.0 nm, 844.7 nm, and 1304 nm (a triplet), 102.7 nm (a triplet) and 1128.7 nm. [33]

"Measurements of the spatial distribution of the hydroxyl radical in cometary atmospheres [may be] made by observations of ultraviolet emission at 309 nm . The distribution depends upon the velocities of the parent water molecules from which OH is produced by photodissociation and on the lifetime of OH . The ultraviolet data . yield a lifetime of OH at 1 AU from the Sun for Comet Bennett (1970 LI) of 2(+1,-1)10 5 sec (Keller and Lillie, 1974), for Comet Kobayashi-Berger-Milon (1975 IX) a lifetime of 2.3(+1.5,-1.3)10 5 sec (Festou, 1981b), for Comet Kohoutek (1973 XII) a lifetime of 2(+2,-0.7)10 5 sec (Blamont and Festou, 1974 Festou, 1981b), and for Comet Bradfield (1979X) a lifetime between 5 x 10 4 and 1.6 x 10 5 sec (Weaver et al., 1981 a)." [68]

"Measurements of hydrogen Lyman alpha emission from comets indicate the presence of two populations of hydrogen atoms, one with a velocity of about 20 km sec -1 , the second with a velocity of about 8 km sec -1 . the high-velocity component [may arise] from photodissociation of H2O and the low-velocity component from photodissociation of OH" [68] .

The Fermi glow are ultraviolet-glowing [69] particles, mostly hydrogen, [70] originating from the Solar System's Bow shock, created when light from stars and the Sun enter the region between the heliopause and the interstellar medium and undergo Fermi acceleration [70] , bouncing around the transition area several times, gaining energy via collisions with atoms of the interstellar medium. The first evidence of the Fermi glow, and hence the bow shock, was obtained with the help from Voyager 1 [69] and the Hubble Space Telescope [69] .

"Carbon monoxide is the second most abundant molecule, after H2, in interstellar clouds. In diffuse clouds, the amount of CO is mainly derived from measurements of absorption at UV wavelengths." [31]

The subdwarf B star is a kind of subdwarf star with spectral type B. They differ from the typical subdwarf star by being much hotter and brighter. [71] They are from the "extreme horizontal branch stars" of the Hertzsprung–Russell diagram.

Subdwarf B stars, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as globular clusters, spiral galaxy bulges and elliptical galaxies. [72] They are prominent on ultraviolet images. The hot subdwarfs are proposed to be the cause of the UV-upturn in the light output of elliptical galaxies. [71]

"The yellow/red "image" or "photo" of Betelgeuse usually seen is actually not a picture of the red giant but rather a mathematically generated image based on the photograph. The photograph was actually of much lower resolution: The entire Betelgeuse image fit entirely within a 10x10 pixel area on the Hubble Space Telescopes Faint Object Camera. The actual images were oversampled by a factor of 5 with bicubic spline interpolation, then deconvolved." [73]

The image at lower right is "of the supergiant star Betelgeuse obtained with the NACO adaptive optics instrument on ESO’s Very Large Telescope. The use of NACO combined with a so-called “lucky imaging” technique, allows the astronomers to obtain the sharpest ever image of Betelgeuse, even with Earth’s turbulent, image-distorting atmosphere in the way. The resolution is as fine as 37 milliarcseconds, which is roughly the size of a tennis ball on the International Space Station (ISS), as seen from the ground. The image is based on data obtained in the near-infrared, through different filters. The field of view is about half an arcsecond wide, North is up, East is left." [75]

Betelgeuse has an estimated diameter of

8.21 x 10 6 km, but "[t]he precise diameter has been hard to define for several reasons:

1. The rhythmic expansion and contraction of the photosphere [may mean] the diameter is never constant
2. There is no definable "edge" to the star as limb darkening causes the optical emissions to vary in color and decrease the farther one extends out from the center
3. Betelgeuse is surrounded by a circumstellar envelope composed of matter being ejected from the star—matter which both absorbs and emits light—making it difficult to define the edge of the photosphere [76]
4. Measurements can be taken at varying wavelengths within the electromagnetic spectrum, with each wavelength revealing something different. Studies have shown that angular diameters are considerably larger at visible wavelengths, decrease to a minimum in the near-infrared, only to increase again in the mid-infrared. [77][78] The difference in reported diameters can be as much as 30–35%, yet because each wavelength measures something different, comparing one finding with another is problematic [76] limits the resolution obtainable from ground-based telescopes since turbulence degrades angular resolution. [79]

"Assuming a distance of 197±45pc, an angular distance of 43.33±0.04 mas would equate to a radius of 4.3 AU". [77]

"Images of hotspots on the surface of Betelgeuse [are] taken at visible and infra-red wavelengths using high resolution ground-based interferometers" [78] .

Mira A is a red giant variable star in the constellation Cetus. This ultraviolet-wavelength image mosaic, taken by NASA's GALEX, shows a comet-like "tail" stretching 13 light year across space. The "tail" consists of hydrogen gas blown off of the star, with the material at the furthest end of the "tail" having been emitted about 30,000 years ago. The tail-like configuration of the emitted material appears to result from Mira's uncommonly high speed (about 130 km/s (81 mi/s)) relative to the Milky Way galaxy's ambient gas. Mira itself is seen as a small white dot inside a blue bulb.

A Lyman-alpha blob (LAB) is a huge concentration of a gas emitting the Lyman-alpha emission line. LABs are some of the largest known individual objects in the Universe. Some of these gaseous structures are more than 400,000 light years across. So far they have only been found in the high-redshift universe because of the ultraviolet nature of the Lyman-alpha emission line. Since the Earth's atmosphere is very effective at filtering out UV photons, the Lyman-alpha photons must be redshifted in order to be transmitted through the atmosphere.

The ultraviolet image at right is "of the giant spiral galaxy Messier 101 (M101) was obtained by the Ultraviolet Imaging Telescope during the Astro-2 mission of the Space Shuttle Endeavour." [80]

"M101 is an Sc-type galaxy, meaning a spiral galaxy with a relatively small central bulge and a system of spiral arms that is not tightly wound. At a distance of about 16 million light years, it is considered relatively close to the Earth." [80]

"M101 is known to contain many giant HII regions, meaning huge glowing nebulae shine as a result of ultraviolet radiation from the massive stars within them. The UIT images will be used to determine the far-ultraviolet energy outputs of these stars and nebulae. Also, the astronomers will study the ages of the nebulae, their dust contents, and the "initial mass functions" of their stars, meaning the relative numbers of stars of different masses when they first formed in the nebulae. This is equivalent to finding the relative numbers of newborn babies of different weights. The investigators will also determine the total mass of all the young massive stars in each HII region or nebula." [80]

"UIT is a 15-inch (0.38-m) telescope which was designed and built at the Goddard Space Flight Center, Greenbelt, MD. . The exposure time was 1310 seconds and the photograph was made at an effective wavelength of 1520 angstroms (152 nanometers), with a bandwidth of 354 angstroms (35.4 nanometers). The photograph was obtained during nighttime portion of Endeavour's orbit on March 11, 1995. The region shown here is about two-thirds the apparent diameter of the full moon. The original UIT image was recorded on black and white film the image is displayed here with color coding indicating intensity of the ultraviolet light." [80]

The Large Binocular Telescope is located on Mount Graham (10,700-foot (3,300 m)) in the Pinaleno Mountains of southeastern Arizona, and is a part of the Mount Graham International Observatory.

The first image taken shown at right combined ultraviolet and green light, and emphasizes the clumpy regions of newly formed hot stars in the spiral arms.

The Very Large Telescope (VLT) is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The UTs are equipped with a large set of instruments permitting observations to be performed in the near-ultraviolet. It includes large-field imagers, adaptive optics corrected cameras and spectrographs, as well as high-resolution and multi-object spectrographs and covers a broad spectral region, from the deep ultraviolet (300 nm).

The first launch of a V-2 by the Naval Research Laboratory was an effort to place an ultraviolet spectrograph over 160 km above the desert at White Sands Proving Ground in New Mexico. The spectrograph would record how much high-energy solar radiation reaches the Earth's upper atmosphere.

The first image at right shows the launch complex. On either side of the rocket are extension ladders to allow technicians access to the nose of the rocket for a final instrument check, color image. On June 27th, liquid oxygen is pumped into one tank and kerosene into a separate fuel tank. This was followed by filling the smaller hydrogen peroxide and permanganate tanks for the turbopumps.

On June 28, 1946, the missle is launched on a flight that lasts 354 s, reaches an altitude of some 100 km, and crashes back to Earth.

The third image show the impact crater created by the V-2 when it returned from its suborbital flight. An approximate size of the crater is indicated by the crane brought in to help find the photographic film taken during the measurements. Excavation of the crater continued throughout the summer, but the cassette carrying the film was never found.

Opacity depends on the frequency of the light being considered. For instance, some kinds of glass, while transparent in the visual range, are largely opaque to ultraviolet light.

For a given medium at a given frequency, the opacity has a numerical value that may range between 0 and infinity, with units of length 2 /mass.

"The absorption of a photon of wavelength λ(nm) in a superconductor is followed by a series of fast processes in which the photon energy is converted into a population of free charge carriers known as quasiparticles in excess of any thermal population. For typical transition metal superconductors this conversion process is of order of a few nanoseconds. At sufficiently low temperatures (typically about an order of magnitude lower than the superconductor's critical temperature Tc) the number density of thermal carriers is very small and the number of excess carriers N0 created as a result of the absorption of a photon of wavelength λ is inversely proportional to the photon wavelength." [81]

"In general N0 can be written [approximately as]:" [81] 7 x 10 5 λ Δ ( T ) . over lambda Delta (T).>

"Here the wavelength is in nm and the energy gap is in meV." [81]

The Markarian galaxies are a class of galaxies that have nuclei with excessive amounts of ultraviolet emissions compared with other galaxies.

The nuclei of the galaxies had a blue colour that in a star would be classed from A to F. This blue core did not match the rest of the galaxy. The spectrum in detail tends to show a continuum that Markarian concluded was produced non-thermally.

The First Byurakan Survey commenced in 1965 using the Schmidt telescope at the Byurakan Astrophysical Observatory. . The purpose of the survey was to find galaxies with an ultraviolet excess. [82]

Seventy galaxies with UV-continuum appeared on lists, and the term "Markarian galaxies came into use. [83] [84] [85] Two more lists brought the number of galaxies up to 302 in 1969. [86] [87]

XUV is strongly absorbed by most known materials, but it is possible to synthesize multilayer optics that reflect up to about 50% of XUV radiation at normal incidence. This technology, which was pioneered by the NIXT and MSSTA sounding rockets in the 1990s, has been used to make telescopes for solar imaging (current examples are SOHO/EIT and TRACE).

Mars Science Laboratory (MSL) is a robotic space probe mission to Mars launched by NASA on November 26, 2011, [88] which successfully landed Curiosity, a Mars rover, in Gale Crater on August 6, 2012. [89] [90] [91] [92]

Rover Environmental Monitoring Station (REMS): Meteorological package and an ultraviolet sensor provided by Spain and Finland. [93] It measures humidity, pressure, temperatures, wind speeds, and ultraviolet radiation. [93]

Mars Hand Lens Imager (MAHLI): This system consists of a camera mounted to a robotic arm on the rover, used to acquire microscopic images of rock and soil. It has white and ultraviolet LEDs for illumination.

"Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths, whereas silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm. [94] [95] [96]

Although optical telescopes can image the near ultraviolet, the ozone layer in the stratosphere absorbs ultraviolet radiation shorter than 300 nm so most ultra-violet astronomy is conducted with satellites. Ultraviolet telescopes [10 nm - 400 nm] resemble optical telescopes, but conventional aluminium-coated mirrors cannot be used and alternative coatings such as magnesium fluoride or lithium fluoride are used instead. The OSO 1 satellite carried out observations in the ultra-violet as early as 1962. The International Ultraviolet Explorer (1978) systematically surveyed the sky for eighteen years, using a 45 cm (18 in) aperture telescope with two spectroscopes. Extreme-ultraviolet astronomy (10–100 nm) is a discipline in its own right and involves many of the techniques of X-ray astronomy the Extreme Ultraviolet Explorer (1992) was a satellite operating at these wavelengths.

Astron was based on the Venera spacecraft design and was operational for six years as the largest ultraviolet space telescope during its lifetime.

"The Soviet Astron orbital station [. ] designed primarily for UV and X-ray astrophysical observations [. ] was injected into orbit on 23 March 1983. The satellite was put into a highly elliptical orbit, with apogee

2,000 km. The orbit kept the craft far away from the Earth for 3.5 out of every 4 days. It was outside of the Earth's shadow and radiation belts for 90% of the time. The spacecraft was over 6m long, and its main instrument was Soviet-French 5m long UV telescope." [97]

The Far Ultraviolet Spectroscopic Explorer (FUSE) detected light in the far ultraviolet portion of the electromagnetic spectrum, between 90.5-119.5 nanometres, which is mostly unobservable by other telescopes. Its primary mission was to characterize universal deuterium in an effort to learn about the stellar processing times of deuterium left over from the Big Bang. The telescope comprises four individual mirrors. Each of the four mirrors is a 39-by-35 cm (15.4-by-13.8 in) off-axis parabola. Two mirror segments are coated with silicon carbide for reflectivity at the shortest ultraviolet wavelengths, and two mirror segments are coated with lithium fluoride over aluminum that reflects better at longer wavelengths. Each mirror has a corresponding astigmatism-corrected, holographically-ruled diffraction grating, each one on a curved substrate so as to produce four 1.65 m (5.4 ft) Rowland circle spectrographs. The dispersed ultraviolet light is detected by two microchannel plate intensified double delay-line detectors, whose surfaces are curved to match the curvature of the focal plane. [98]

The Galileo spacecraft has on board an ultraviolet spectrometer (UVS) and an extreme ultraviolet spectrometer (EUV).

The Cassegrain telescope of the UVS had a 250 mm aperture and collected light from the observation target. Both the UVS and EUV instruments used a ruled grating to disperse this light for spectral analysis. This light then passed through an exit slit into photomultiplier tubes that produced pulses or "sprays" of electrons. These electron pulses were counted, and these count numbers constituted the data that were sent to Earth. The UVS was mounted on Galileo's scan platform and could be pointed to an object in inertial space. The EUV was mounted on the spun section. As Galileo rotated, EUV observed a narrow ribbon of space perpendicular to the spin axis. The two instruments combined weighed about 9.7 kilograms and used 5.9 watts of power. [99] [100]

Kosmos 215 . was used to study radiation and conduct optical observations of the Earth's atmosphere. It was equipped with eight telescopes for optical observation, [101] and one for ultraviolet astronomy. [102] It was primarily used to study the Sun, although several other X-ray emissions were detected. Kosmos 215 performed ultraviolet photometry of 36 A and B stars from parallel telescopes and two UV photometers with maximum responses at 274.0 and 227.5 nanometres. [103]

## Contents

On 15 May 2015, a brief, single radio signal at 11 GHz (2.7 cm wavelength) [11] was observed in the direction of HD 164595 by a team led by N. N. Bursov [12] involving Claudio Maccone at the RATAN-600 radio observatory. The signal may have been caused by terrestrial radio-frequency interference or gravitational lensing from a more distant source. [13] [14] It was observed only once (for two seconds), by a single team, at a single telescope, giving it a Rio Scale [15] score of 1 (insignificant) or 2 (low). Discussions in the media from 29 August 2016 onwards featured speculation that the signal could be caused by an isotropic beacon from a Type II civilization. [16]

The senior astronomer of the SETI Institute, Seth Shostak, stated that confirmation by another telescope is required. [17] Astronomer Nicholas Suntzeff of Texas A&M University stated that the signal is in a military frequency band, and that it could have been a satellite downlink, implying that some such systems may be kept secret and therefore would be unknown to SETI scientists. [11]

SETI and METI studies followed with the Allen Telescope Array and the Boquete Optical SETI Observatory. [18] [17] Also, scientists at Berkeley SETI Research Center at the University of California, Berkeley observed HD 164595 using the Green Bank Telescope as part of the Breakthrough Listen program. No signal was detected at the position and frequency of the transient reported by the RATAN group. [19] [20]

The Special Astrophysical Observatory of the Russian Academy of Sciences has since released an official statement that the signal is of a "most probable terrestrial origin". [21]