What is the cause of all of these sharp, concentric rings around bright stars in this HST image?

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ESA's HST page heic1819 - Photo Release; Hubble reveals cosmic Bat Shadow in the Serpent's Tail is of course beautiful and stunning, but my eyes are drawn to the diffraction artifacts of the bright stars.

I'm assuming the crosses are due to four vanes supporting the secondary mirror, but are the tight concentric rings due to Hubble's large aperture, or it's smaller secondary mirror blocking that aperture, or something else, perhaps image processing?

Without a scale for reference, it's hard to get an angular frequency and compare to an Airy-like diffraction pattern to get a diameter, and that's where it gets more puzzling, because you need a narrow wavelength range to get a coherent oscillation for so many cycles (I think I can see perhaps 15 or more sharp, distinct rings), and to zeroth order stars are mostly black-body.

Is this a bit of a puzzle, or am I missing something obvious (e.g. filters)? Or both?

Here's a cropped, monochromed, ROI:

Further stretched in contrast and size:

The diffraction pattern at the focal plane created by a circular aperture is called an Airy Disk or Airy Pattern. Both the outer opening and the inner hole plus secondary contribute to the exact function. This is usually not easily observed with ground based telescopes because the seeing fluctuations due to air turbulence smears it out. These images use filters that are a few 100 Angstroms wide, so although not monochromatic, they are narrow enough to clearly see the pattern. The spectral shape of the star's light within the filter makes the effective width of frequencies more narrow.

Diffraction rings not visable?

Question on my new Meade LX85 6" f/10 ACF UHTC Catadioptric Telescope (OTA.

I'll keep my question short and sweet.

No matter how long outside, the star or the power all I see going thru focus is a nice

round doughnut with absolutely no diffraction rings. Is hard to imagine nothing I have

tried produced only a single doughnut. I see tons of pics with multiple rings and some

like mine with none. I have read plenty and don't know what to do. Don't want to send

it back if there is something basic and I just don't get it, but it seems to me I should

have seen a few nice round concentric rings.

Thanks for any thoughts on this.

#2 dustyc

Use the highest power eyepiece you have. When I tried to do the same with my old C8 I had to use an 11mm plossl to get a good pattern.

Edited by dustyc, 27 February 2021 - 09:39 PM.

#3 decep

In 2 years, I have never seen diffraction rings either. Even with a 12" SCT.

I think it comes down to "seeing". I do not think I have ever had anything other than average seeing. The air is never stable enough to see the diffraction rings where I live (GA).

#4 Jon Isaacs

Seeing diffraction rings requires stable seeing and a scope that is thermally stable. And the larger the scope, the more difficult it is because everything is concentrated into a smaller area. Seeing diffraction rings in a 60mm is not difficult.

#5 Stellar1

Same here, have never seen them in my CPC1100, I gave up trying after many attempts and just figured my model didn’t come with those lol, views were great anyway.

#6 maroubra_boy

The only way to see diffraction rings is with sharp focus of a bright star at high magnification.

Think about it, as you defocus, the energy of the star is being spread out, and the larger the doughnut, the more and more this energy is being spread out. With the diffraction rings, as they are much fainter than the star at the centre, the energy is diluted even faster as you defocus. You will not see diffraction rings around a defocused star if the doughnut is large and especially if the optical quality of the scope isn't great.

With my C8 I was only able to ever see one diffraction ring at most. With my 9" Deluxe Russian Mak, I can see as many as 8 rings around Sirius. The difference being entirely optical quality - the better the photons are controlled to go where they are supposed to, the faint energy of the outer diffraction rings will also fall into place where they should. With my 7" Deluxe Mak, I could see up to 7 rings. A friend's standard Russian 6" Mak only shows 3 diffraction rings. When defocused, what use are the defuse diffraction rings anyway? It is the central doughnut we want. A large doughnut for course/gross collimation, and then a small, tight doughnut to do the collimation fine tuning as with a large doughnut the energy is spread out a large amount, and this will also cancel any remaining small mis-collimation that may still be present. A small doughnut will reduce this error and show up whatever misalignment remains.

If the scope is not at equilibrium or insulated, the rings will still be there, but disrupted. Before I wrap my 9" Mak, I am able to see the rings without problem, but all broken up and shimmering.

Edited by maroubra_boy, 27 February 2021 - 11:33 PM.

#7 Dave Ponder

I think 25-30X per inch is a good start for magnification. Like others have stated, “seeing” is very important as well as thermal equalization. Then there is collimation. If you want to see the rings, make an artificial star. Take a flashlight, cover the end with aluminum foil, make the tiniest hole you can with a fine needle, put the flashlight 40 ft or more away if possible inside and see what you get. You should see a nice diffraction pattern. You may see that your collimation needs work. (I am not starting a discussion as to the best separation distance between the scope and the artificial star). I remember thinking something was wrong with my first SCT because I could not see the rings. My scope ended up being just fine after I collimated the scope, allowed the scope time to “cool down”, used high power on a really good night of seeing.

#8 maroubra_boy

Yes, I forgot to mention how important good, stable seeing conditions are as well. Poor seeing will mean the diffraction rings will be disrupted and broken up.

Collimation needs to be done at high magnification too in order to be sure that errors are eliminated.

If you collimate using a devise of some kind, laser or whatever, you should still verify by star testing, always. THIS is the final proof of the pudding with all scopes. Professional instruments will have initial collimation done with a laser, but always verified by star testing. And star testing is done with a small doughnut, not large. Remember, a large doughnut will even out whatever small error may still be present. A small doughnut means a much tighter image and shows up these wee errors need tweeking.

Edited by maroubra_boy, 28 February 2021 - 12:11 AM.

#9 maroubra_boy

There was one C8 I looked through that showed up to 5 diffraction rings! Absolutely stunning unit! There are some mighty fine mass production SCT's out there, but it means looking through a lot of them to find them. This particular C8 I was pitting it against a 7" Intes Mak to see which I was going to keep. The Intes won by a bee's willy. If I hadn't come across that Russian Mak, I would have been totally happy with that C8.

In that shoot out, both scopes were collimated so tightly you could just about hear them squeal. I still know where that C8 calls home - good to keep tabs on where the good scopes are!

Edited by maroubra_boy, 28 February 2021 - 12:19 AM.

#10 qns

Thank you all for the comments, CN is a great resource. I'm feeling encouraged to keep

up the good fight. I have two weeks to work on this so I'll try one of those DIY artificial stars at about 100', jack up the magnification far as I can while paying attention to those other parameters and take a pic of what I'm seeing.

The search for the mysterious and elusive diffraction rings continues. I did read someplace where a poor main mirror finish could result in a "busy" round doughnut with no diffraction rings. So much has been written about them it is natural to believe one has a problem when they are not showing up.

#11 Jon Isaacs

The brightness of the diffraction rings also depends on the size of the central obstruction. SCTs are generally between 0.3D and 0.4D. 0.0D would be a refractor, A newtonian could be anywhere from 0.14D on up but is generally around 0.2D-0.25D.

#12 maroubra_boy

The search for the mysterious and elusive diffraction rings continues. I did read someplace where a poor main mirror finish could result in a "busy" round doughnut with no diffraction rings. So much has been written about them it is natural to believe one has a problem when they are not showing up.
Thanks again,
Joe

The only way to see whatever diffraction rings your scope can show is with the star in sharp focus.

You have been proving to yourself all this time that the faint diffraction rings cannot be seen with the star as a doughnut all along. Make sure your scope is well collimated, focus the bright star sharp, have your scope acclimated or insulated and under good seeing conditions whatever rings are to be seen will be shown to you.

These rings are of low energy. Defocus the bright star & all you are doing is disperse into nothing the little energy that these feeble rings have.

Edited by maroubra_boy, 28 February 2021 - 03:45 AM.

#13 luxo II

OP you need to focus sharply - no donuts - and use a 6-7mm eyepiece. It is also possible that you won’t see diffraction rings- just a “ circle of least confusion” I’ve owned two SCTs like that before.

Edited by luxo II, 28 February 2021 - 04:11 AM.

#14 PETER DREW

There is a common confusion between extrafocal rings and diffraction rings. Best to define which before seeking advice.

#16 btschumy

There is a common confusion between extrafocal rings and diffraction rings. Best to define which before seeking advice.

Yes, I'm not sure what maroubra_boy is referring to when he says he sees 8 diffraction rings around Sirius. I've never heard of seeing so many diffraction rings. The number (and brightness) of rings is related to the size of the central obstruction, as Jon referenced in his post above. I would think you'd have to have a massive obstruction for 8 rings to be visible. I wouldn't consider this a good thing.

#17 maroubra_boy

What I have noticed with scopes that are of a known quality, such as these Intes Maks, is the better the quality, the more diffraction rings are seen around the very brightest of stars. This is because the photons are all going where they are supposed to, so the energy of these outer rings would will all be going exactly to the position that they should. And this too depends on the prevailing seeing conditions.

The central obstruction of Maks tends to also be smaller than that of mass production SCTs, so it not the size of the central obstruction that's at play. Most of these SCT's struggle to show one diffraction ring. But with these Russian made optics, the standard 1/6th wave scopes can show up to four rings, and my 1/8th wave scopes have shown 7 rings (a 7" Mak I had) or 8 rings (my current 9" and a friend's 10") with the brightest of stars, and the central obstruction of these scopes is between 26% and 29%. With the few really good mass production SCT's I have encountered, these too showed multiple diffraction rings, but these scopes are very few and far between. Mass production Maks can also show one or a few diffraction rings, again depending on the optical quality of each individual instrument.

And yes, this can be a problem with particularly bright stars, like Sirius, no denying that. And thankfully this is not a problem often encountered. Nor is it a problem with the Moon or planets as these are extended objects and not point sources of light, so there is no diffraction pattern associated with these.

#18 AJK 547

The only way to see diffraction rings is with sharp focus of a bright star at high magnification.

Think about it, as you defocus, the energy of the star is being spread out, and the larger the doughnut, the more and more this energy is being spread out. With the diffraction rings, as they are much fainter than the star at the centre, the energy is diluted even faster as you defocus. You will not see diffraction rings around a defocused star if the doughnut is large and especially if the optical quality of the scope isn't great.

With my C8 I was only able to ever see one diffraction ring at most. With my 9" Deluxe Russian Mak, I can see as many as 8 rings around Sirius. The difference being entirely optical quality - the better the photons are controlled to go where they are supposed to, the faint energy of the outer diffraction rings will also fall into place where they should. With my 7" Deluxe Mak, I could see up to 7 rings. A friend's standard Russian 6" Mak only shows 3 diffraction rings. When defocused, what use are the defuse diffraction rings anyway? It is the central doughnut we want. A large doughnut for course/gross collimation, and then a small, tight doughnut to do the collimation fine tuning as with a large doughnut the energy is spread out a large amount, and this will also cancel any remaining small mis-collimation that may still be present. A small doughnut will reduce this error and show up whatever misalignment remains.

If the scope is not at equilibrium or insulated, the rings will still be there, but disrupted. Before I wrap my 9" Mak, I am able to see the rings without problem, but all broken up and shimmering.

Alex.

Alex, thanks for the explanation. I’ve learned a lot in this thread!

FWIW, I have a 2019 replacement C8-A that the Celestron QA team selected from their QA’d inventory stash, then Celestron bench tested. collimated. and then sent it me for testing and collimation refinement. All I can say is this particular C8-A can REALLY take mag., has presented me with thrilling visual views of targets like the Vallis Alpes rille, Ina IMP, the small ‘Craterlets’ next to Aristarchus peak, etc. Totally happy with this particular unit.

My question is ‘What star should I select (Sirius?) to get an idea on the optical quality of this C8-A with respect to the diffraction ring development”. I’ve side-by-side tested this C8-A against a friends hand built Planetary Newt 8” (Null tested and is

1/8 wave), and my C8-A easily develops more solid. progressively fainter diff rings around the Airy disk during an evening of excellent stable conditions. After numerous nights of side-by-side testing, I honestly couldn’t detect where his dedicated planetary Newt out performed the C8-A in resolving capabilities. The only area I could see where the 8” newt performed a bit better was in slightly better contrast due to less CO size.

Any further insight is much appreciated.

*** Honestly, I’ve always thought the less # of diffraction rings . the better the optics! Dumb me!

Use an artificial star to star test

Alternatively, you can do a star test in any kind of conditions using an ‘artificial star’. This is a piece of kit that uses a white LED and a small length of fibre-optic cable to make a star-like source of light.

Place it 25 to 50m (80 to 160ft) from the end of the telescope and you can star test even when it’s cloudy.

Persistent rings in and around Jupiter’s anticyclones – Observations and theory

We present observations and theoretical calculations to derive the vertical structure of and secondary circulation in jovian vortices, a necessary piece of information to ultimately explain the red color in the annular ring inside Jupiter’s Oval BA. The observations were taken with the near-infrared detector NIRC2 coupled to the adaptive optics system on the 10-m W.M. Keck telescope (UT 21 July 2006 UT 11 May 2008) and with the Hubble Space Telescope at visible wavelengths (UT 24 and 25 April 2006 using ACS UT 9 and 10 May 2008 using WFPC2). The spatial resolution in the near-IR (∼0.1–0.15″ at 1–5 μm) is comparable to that obtained at UV–visible wavelengths (∼0.05–0.1″ at 250–890 nm). At 5 μm we are sensitive to Jupiter’s thermal emission, whereas at shorter wavelengths we view the planet in reflected sunlight. These datasets are complementary, as images at 0.25–1.8 μm provide information on the clouds/hazes in the troposphere–stratosphere, while the 5-μm emission maps yield information on deeper layers in the atmosphere, in regions without clouds. At the latter wavelength numerous tiny ovals can be discerned at latitudes between ∼45°S and 60°S, which show up as rings with diameters ≲1000 km surrounding small ovals visible in HST data. Several white ovals at 41°S, as well as a new red oval that was discovered to the west of the GRS, also reveal 5-μm bright rings around their peripheries, which coincide with dark/blue rings at visible wavelengths. Typical brightness temperatures in these 5-μm bright rings are 225–250 K, indicative of regions that are cloud-free down to at least the ∼4 bar level, and perhaps down to 5–7 bar, i.e., well within the water cloud.

Radiative transfer modeling of the 1–2 μm observations indicates that all ovals, i.e., including the Great Red Spot (GRS), Red Oval BA, and the white ovals at 41°S, are overall very similar in vertical structure. The main distinction between the ovals is caused by variations in the particle densities in the tropospheric–stratospheric hazes (2–650 mbar). These are 5–8 times higher above the red ovals than above the white ones at 41°S. The combination of the 5-μm rings and the vertical structure derived from near-IR data suggests anticyclones to extend vertically from (at least) the water cloud (∼5 bar) up to the tropopause (∼100–200 mbar), and in some cases into the stratosphere.

Based upon our observations, we propose that air is rising along the center of a vortex, and descending around the outer periphery, producing the 5-μm bright rings. Observationally, we constrain the maximum radius of these rings to be less than twice the local Rossby deformation radius, LR. If the radius of the visible oval (i.e., the clouds that make the oval visible) is >3000 km, our observations suggest that the descending part of the secondary circulation must be within these ovals. For the Red Oval BA, we postulate that the return flow is at the location of its red annulus, which has a radius of ∼3000 km.

We develop a theory for the secondary circulation, where air is (baroclinically) rising along the center of a vortex in a subadiabatic atmosphere, and descending at a distance not exceeding ∼2× the local Rossby deformation radius. Using this model, we find a timescale for mixing throughout the vortex of order several months, which suggests that the chromophores that are responsible for the red color of Oval BA’s red annulus must be produced locally, at the location of the annulus. This production most likely results from the adiabatic heating in the descending part of the secondary circulation. Such higher-than-ambient temperature causes NH3–ice to sublime, which will expose the condensation nuclei, such as the red chromophores.

Contents

Tethys was discovered by Giovanni Domenico Cassini in 1684 together with Dione, another moon of Saturn. He had also discovered two moons, Rhea and Iapetus earlier, in 1671–72. [14] Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory. [15]

Cassini named the four new moons as Sidera Lodoicea ("the stars of Louis") to honour king Louis XIV of France. [16] By the end of the seventeenth century, astronomers fell into the habit of referring to them and Titan as Saturn I through Saturn V (Tethys, Dione, Rhea, Titan, Iapetus). [14] Once Mimas and Enceladus were discovered in 1789 by William Herschel, the numbering scheme was extended to Saturn VII by bumping the older five moons up two slots. The discovery of Hyperion in 1848 changed the numbers one last time, bumping Iapetus up to Saturn VIII. Henceforth, the numbering scheme would remain fixed.

The modern names of all seven satellites of Saturn come from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus). [14] In his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope, [17] he suggested the names of the Titans, sisters and brothers of Kronos (the Greek analogue of Saturn), be used. Tethys is named after the titaness Tethys. [14] It is also designated Saturn III or S III Tethys.

The name Tethys has two customary pronunciations, with either a 'long' or a 'short' e: / ˈ t iː θ ɪ s / [18] or / ˈ t ɛ θ ɪ s / . [19] (This might be a US/UK difference.) [ citation needed ] The conventional adjectival form of the name is Tethyan, [20] again with either a long or a short e.

Tethys orbits Saturn at a distance of about 295,000 km (about 4.4 Saturn's radii) from the center of the planet. Its orbital eccentricity is negligible, and its orbital inclination is about 1°. Tethys is locked in an inclination resonance with Mimas, however due to the low gravity of the respective bodies this interaction does not cause any noticeable orbital eccentricity or tidal heating. [21]

The Tethyan orbit lies deep inside the magnetosphere of Saturn, so the plasma co-rotating with the planet strikes the trailing hemisphere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere. [22]

Tethys has two co-orbital moons, Telesto and Calypso orbiting near Tethys's trojan points L4 (60° ahead) and L5 (60° behind) respectively.

Tethys is the 16th-largest moon in the Solar System, with a radius of 531 km. [6] Its mass is 6.17 × 10 20 kg (0.000103 Earth mass), [7] which is less than 1% of the Moon. The density of Tethys is 0.98 g/cm³, indicating that it is composed almost entirely of water-ice. [23]

It is not known whether Tethys is differentiated into a rocky core and ice mantle. However, if it is differentiated, the radius of the core does not exceed 145 km, and its mass is below 6% of the total mass. Due to the action of tidal and rotational forces, Tethys has the shape of triaxial ellipsoid. The dimensions of this ellipsoid are consistent with it having a homogeneous interior. [23] The existence of a subsurface ocean—a layer of liquid salt water in the interior of Tethys—is considered unlikely. [24]

The surface of Tethys is one of the most reflective (at visual wavelengths) in the Solar System, with a visual albedo of 1.229. This very high albedo is the result of the sandblasting of particles from Saturn's E-ring, a faint ring composed of small, water-ice particles generated by Enceladus's south polar geysers. [9] The radar albedo of the Tethyan surface is also very high. [25] The leading hemisphere of Tethys is brighter by 10–15% than the trailing one. [26]

The high albedo indicates that the surface of Tethys is composed of almost pure water ice with only a small amount of darker materials. The visible spectrum of Tethys is flat and featureless, whereas in the near-infrared strong water ice absorption bands at 1.25, 1.5, 2.0 and 3.0 μm wavelengths are visible. [26] No compound other than crystalline water ice has been unambiguously identified on Tethys. [27] (Possible constituents include organics, ammonia and carbon dioxide.) The dark material in the ice has the same spectral properties as seen on the surfaces of the dark Saturnian moons—Iapetus and Hyperion. The most probable candidate is nanophase iron or hematite. [28] Measurements of the thermal emission as well as radar observations by the Cassini spacecraft show that the icy regolith on the surface of Tethys is structurally complex [25] and has a large porosity exceeding 95%. [29]

Color patterns Edit

The surface of Tethys has a number of large-scale features distinguished by their color and sometimes brightness. The trailing hemisphere gets increasingly red and dark as the anti-apex of motion is approached. This darkening is responsible for the hemispheric albedo asymmetry mentioned above. [30] The leading hemisphere also reddens slightly as the apex of the motion is approached, although without any noticeable darkening. [30] Such a bifurcated color pattern results in the existence of a bluish band between hemispheres following a great circle that runs through the poles. This coloration and darkening of the Tethyan surface is typical for Saturnian middle-sized satellites. Its origin may be related to a deposition of bright ice particles from the E-ring onto the leading hemispheres and dark particles coming from outer satellites on the trailing hemispheres. The darkening of the trailing hemispheres can also be caused by the impact of plasma from the magnetosphere of Saturn, which co-rotates with the planet. [31]

On the leading hemisphere of Tethys spacecraft observations have found a dark bluish band spanning 20° to the south and north from the equator. The band has an elliptical shape getting narrower as it approaches the trailing hemisphere. A comparable band exists only on Mimas. [32] The band is almost certainly caused by the influence of energetic electrons from the Saturnian magnetosphere with energies greater than about 1 MeV. These particles drift in the direction opposite to the rotation of the planet and preferentially impact areas on the leading hemisphere close to the equator. [33] Temperature maps of Tethys obtained by Cassini, have shown this bluish region is cooler at midday than surrounding areas, giving the satellite a "Pac-man"-like appearance at mid-infrared wavelengths. [34]

Geology Edit

The surface of Tethys mostly consists of hilly cratered terrain dominated by craters more than 40 km in diameter. A smaller portion of the surface is represented by the smooth plains on the trailing hemisphere. There are also a number of tectonic features such as chasmata and troughs. [35]

The western part of the leading hemisphere of Tethys is dominated by a large impact crater called Odysseus, whose 450 km diameter is nearly 2/5 of that of Tethys itself. The crater is now quite flat – more precisely, its floor conforms to Tethys's spherical shape. This is most likely due to the viscous relaxation of the Tethyan icy crust over geologic time. Nevertheless, the rim crest of Odysseus is elevated by approximately 5 km above the mean satellite radius. The central complex of Odysseus features a central pit 2–4 km deep surrounded by massifs elevated by 6–9 km above the crater floor, which itself is about 3 km below the average radius. [35]

The second major feature seen on Tethys is a huge valley called Ithaca Chasma, about 100 km wide and 3 km deep. It is more than 2000 km in length, approximately 3/4 of the way around Tethys' circumference. [35] Ithaca Chasma occupies about 10% of the surface of Tethys. It is approximately concentric with Odysseus—a pole of Ithaca Chasma lies only approximately 20° from the crater. [36]

It is thought that Ithaca Chasma formed as Tethys's internal liquid water solidified, causing the moon to expand and cracking the surface to accommodate the extra volume within. The subsurface ocean may have resulted from a 2:3 orbital resonance between Dione and Tethys early in the Solar System's history that led to orbital eccentricity and tidal heating of Tethys's interior. The ocean would have frozen after the moons escaped from the resonance. [37] There is another theory about the formation of Ithaca Chasma: when the impact that caused the great crater Odysseus occurred, the shock wave traveled through Tethys and fractured the icy, brittle surface. In this case Ithaca Chasma would be the outermost ring graben of Odysseus. [35] However, age determination based on crater counts in high-resolution Cassini images showed that Ithaca Chasma is older than Odysseus making the impact hypothesis unlikely. [36]

The smooth plains on the trailing hemisphere are approximately antipodal to Odysseus, although they extend about 60° to the northeast from the exact antipode. The plains have a relatively sharp boundary with the surrounding cratered terrain. The location of this unit near Odysseus' antipode argues for a connection between the crater and plains. The latter can be a result of focusing the seismic waves produced by the impact in the center of the opposite hemisphere. However the smooth appearance of the plains together with their sharp boundaries (impact shaking would have produced a wide transitional zone) indicates that they formed by endogenic intrusion, possibly along the lines of weakness in the Tethyan lithosphere created by Odysseus impact. [35] [38]

Impact craters and chronology Edit

The majority of Tethyan impact craters are of a simple central peak type. Those more than 150 km in diameter show more complex peak ring morphology. Only Odysseus crater has a central depression resembling a central pit. Older impact craters are somewhat shallower than young ones implying a degree of relaxation. [39]

The density of impact craters varies across the surface of Tethys. The higher the crater density, the older the surface. This allows scientists to establish a relative chronology for Tethys. The cratered terrain is the oldest unit likely dating back to the Solar System formation 4.56 billion years ago. [40] The youngest unit lies within Odysseus crater with an estimated age from 3.76 to 1.06 billion years, depending on the absolute chronology used. [40] Ithaca Chasma is older than Odysseus. [41]

Astigmatism, cause for concern?, or just newbie

• topic starter

Weather has been fairly poor, so I haven't had much opportunity to test the optics in my new scope much, but I've tried collimation using my eye, and I'm fairly certain I must be relatively close (defocused star reveals a spider/secondary that is more or less in line), but I have yet to be impressed yet (My prior telescope on my signature I feel did a better job), perhaps it's just the skies, or thought perhaps it might be the temperature of the scope, though it has now been outside for a while.

My major concern is the very obvious image rotation inside and outside of focus, even after being outside for around 2 hours (it is a BK-7 mirror). I can see it on Jupiter, and can see it even more obviously on stars. I don't think I've pinched the optics, at least the Mirror locking screws are just hand tightened.

Do you think this is cause for concern, or am I not doing something right?, any suggestions most welcome, this is after all my first Reflector, and I might of done something very wrong.

#3 Guest_**DONOTDELETE**_*

• topic starter

Hmmm, well perhaps I'm mistaking Astigmatism for something else, the image itself doesn't rotate, just the defocused image, still if this is astigmatism, then I appear to have lots of it.

One of the more obvious sights I've been disappointed with is Jupiter. I guess I was expecting to see more then 2 bands, but that doesn't appear to be the case, would you on a poor night still make out a decent amount of detail on jupiter surface?, apart from the greater contrast, my konus 2.25" did just as well.

In addition I've marked the mirror with vivid, that shouldn't be a factor in the warranty right?.

#4 Guest_**DONOTDELETE**_*

• topic starter

I find it interesting that the weather has been so poor since I got this scope, now my brand new scope is probably faulty!. Sometimes it seems that someone is out to get to me. or perhaps they're watching, maybe I should be careful to look out for not-so-carefully placed advertising of products by friends and family..

Tonight it has been fairly transparent when the clouds did clear, at least I've eliminated cool down, and eyepieces from the astigmatism question (1 x 25mm Kellner, 2 x 9mm Plossl's, 1 x 25mm plossl), the air was fairly still, and I did get a nice look at the Jewelry box on crux (Southern Cross), very nice, the contrast of the gold star is amazing(I am very new to navigation of the skies, perhaps I should try to quote some made up bizzare catalogue type, I might get away with it ), much better then that 60mm konusstart. shame it wasn't sharp, but it looked good on the 25mm plossl revealing a multitude of stars(don't try the 9mm you can't focus very well at all).

Do you think that there is anything else I can do to check/fix, or is it definitely a matter of getting a straight replacement?

#5 Neil Carroll

Whoa. Before you decide your scope is faulty, let's look at few things. First that scope needs to be collimated very well before you try to star test it again. Simply being out of collimation can show astigmatism. Just using your eye to collimate and thinking that it is "more or less in line" and "realitivly close" is not good enough. A scope this far out of alignment would certainly show astigmatism even on the very best mirrors.
Has the focuser been squared to the tube? Has the secondary been properly aligned? These two things can be off even if a laser is used and shows a perfect return path. Squaring the focuser and centering the secondary with a sight tube is the first thing a person should do with a new scope (or a new used scope). Proper collimation may eliminate your astigmatism.

Second. your mirror locking screws. Even just hand tighting these screws can cause the optics to pinch. Remember, it takes very little pressure to change the surface of the mirror one millionith of an inch. To see if, indeed, the mirror clips are pinching the mirror, you can do a star test. Use the highest power eyepiece you have and focus/defocus on a very bright star. The image should break into concentric circles. The spider viens and central obstruction should be surrounded by concentric circles. If these circles appear pinched into a smooth cornered triangle, then the clips are pinching your mirror.

Astigamtism may also be induced by the secondary. If it is not flat, astigmatism will show. This can be caused by a poor secondary OR a secondary that is pinched. How is the secondary mounted?

I would check all of these things out before I went back to the vendor. I have seen astigmatism while using the very best mirrors available. Every time it was attributed to one of the reasons above and not the primary itself.

#6 matt

Jamie: Neil is right (Hi Neil!). The "sticky" threads on the reflector forum will give you helpful links on collimating your newtonian and understanding whatever problems your optics might have.

As for Jupiter, it's pretty low in the sky right now, so don't expect excellent views these days. Maybe looking at some close double stars.

An acid test I like for large telescopes is looking at big globulars, and in the South you have more than a few decent ones: if the stars are sharp and you can separate them (meaning the outside of the globular is made of sharp stars, not some mushy thing that appear to be stars on second thought), you can be happy with yout optics.

In addition to the factors Neil mentioned, check to see if you have any astigmatism in your eye. Defocus the image a bit, then tilt your head back and forth. If the elongation rotates with your head, you have astigmatism in your eyes. It will be especially noticeable at low powers (large exit pupils).

#8 Guest_**DONOTDELETE**_*

• topic starter

Hey thanks for all the good advice, unfortunately the smallest eyepiece I have is only 9mm (139x magnification), so I can get the focuser to the point that I see rings, but I don't think they're perfectly concentric, which would probably mean that I haven't got collimation right, I just figured I would be close enough to make some fine adjustments, but the astigmatism effect got me concerned enough that I posted here.

How I collimated by eye is to hold my eye back from the focuser, until only the outline of the secondary mirror filled the view (therefore forcing me to be on the focal axis providing the secondary was centred with the focuser), then made adjustments on the screws on top of the scope until the primary was centred in the view. From here I adjusted the mirror tilt screws until the black dot on the primary was centred with the reflection of my eye (which should be centred if my eye is held centre on the focal plane). For centering the secondary (don't worry I did this before everything else!), I couldn't be as accurate, but I tried to use the sides of the focuser tube to keep it centred. I guess how I reasoned that the scope was in line, was that if the reflection of my eye is centred perfectly with the dot, then light is getting reflected back to it's source through the secondary.

Now everything here I could of done wrong, but I guess thats why I'm here, to be corrected would save me far more time then to return the scope .

Neil, I was looking at picking up a laser collimation tool today (with a dart board pattern and preview hole for checking out the return path), I was hoping that this would be the be all and end all of tools for myself. What other tools would you recommend?

#9 kiwisailor

Be sure to check the focuser alignment first, I've posted a method you can use to do this in the collimation thread.

I use a cheshire and laser.

#10 Guest_**DONOTDELETE**_*

• topic starter

The first step in collimating a Newtonian reflector is to check your focuser alignment, you can do this by getting a lenght of threaded rod of the same diameter as the bolt that holds your secondary mirror in place. Remove the mirror and it's bolt and fit the threaded rod (the rod needs to be about the same lenght as your secondary and its bolt). Clamp the rod in your secondary holder with some nuts.

Ok, now you have a line to adjust your focuser (or prove that its correctly aligned) to the left or right of your tube. A laser is easy for this step, but you can do it with a sight tube. If the laser beam falls on the rod, no adjustment is required on this axis. If it dosen't, then you need to adjust the focuser, by placing shims under it, or if it has collimating screws in its base, losening the mounting bolts and adjusting the longitudinal pairs to center the laser beam on the rod(or sight tube cross hair)

You now have the focuser correctly adjusted for one axis, now you need to adjust the focuser so it's at a right angle to the lenghtwise axis of your scopes tube. Remove the rod, turn your laser on and using a plastic ruler measure the distance between the top of your scopes tube (OTA) and the laser beams exit from the focuser. Now measure where the beam falls on the other side of your OTA, if the measurements are not the same, you need to adjust the pairs of screws (or shim) to make the laser beam fall equal it's eixt from the focuser.

The good news, is that you only need to do this part of scope collimation once a year (or after you've pulled it apart).

--------------------
If at first you don't succeed, skydiving is not for you.

Sky-Watcher 8" F5 Newt
Mmmmm Moonlite CR2

#11 Neil Carroll

Jamie, The laser is a great tool. But should be used AFTER the focuser and secondary are all set. This is the most difficult part of collimation, BUT, It is usually only needed when you first get a scope and then only rarely after. It can take me an hour sometimes to get the focuser squared, The secondary is not as difficult.

My best recommendation would be to get a copy of "New Perspectives on Newtonian Collimation: Principles and Procedures" by Vic Menard and Tippy D'Auria. It is about 8 bucks.

If you want to spend the money, this book also comes with the Tectron Collimation Tools. there are three. As sight tube, cheshire and the auto collimator. With proper use these instruments preclude the necessity of a laser. Personally I use the laser for quick touch ups in the field.
The laser is no good to align your focuser and secondary with. The laser can show a perfect return path without the secondary and focuser being square.

After "finding" astigmatism in what was supposed to be a near perfect mirror I got the book and tools out and spend about a 5 hour session with the book and a couple of my scopes and learned the "right way" to do it. The most productive time I have spent on my scopes.

The set of tools is about $115.00 (?) or so. and can sometimes be picked up on Astromart or CN Classifieds. For a little less. If you only get one . get the sight tube. The other two tools can get you into better collimation than any laser. #12 Neil Carroll #13 Guest_**DONOTDELETE**_* • topic starter Well, I've purchased the laser collimation tool, and a barlow ($29NZ for a fully multicoated glass 2x Barlow, I thought that was very impressive). I collimated using the laser tool (after collimation of the laser tool, it has about a 6cm diameter rotation over 7 or so metres). It was raining last night, however it cleared at about 1.00am, so after allowing a cool down I gave it another go.

Now the more I read, the more pin point accuracy seems to be important (I thought good accuracy would be enough), and I have noted several things, for example while racking the focuser completely out, there is a *small* deviation of about 2 mm with the laser pointer, however at least the secondary is pointing at the centre (or close to, I did mark it) now, it was off by about 10cm which can't of been good for things (ahh and I thought I could collimate with my eye ).

Well the quality of the image is better, considering a poorer night for viewing, especially with low magnifications (25mm) such with stars looking a little closer to a point (still not there yet), at least I can cleanly split Alpha & Beta Centauri (just to give you an idea how decollimated it was before hand), just higher magnifications suffer, the 9mm with barlow (about 280x) looks reasonably good on the moon, but you can tell that it isn't perfectly sharp, and don't try looking at stars at this magnification, as they are far from pin point.

The astgmatism effect is still there though (one side of the focus (I can't remember which) starts to form those concentric circles, whereas on the otherside of focus it is far more static filled, and you can't make out any real detail on it), but I will check the focuser is in the correct place, try to accurately line up the secondary with the focuser, and try tweaking the collimation on the laser collimator. but I'm a little relieved now that I have improved the quality, I hope though for razor sharp views with at least 280x, is this a little unreasonable in this instrument?

#14 kiwisailor

Reflectors F5 and below have to be "bang on", the "by eye guys" all own F6 or longer reflectors.

You might find that your mirror is glued in place, as well as retained by clips. Some like glue, some don't, mine had three dobs of Chinese gunge as well as clips, I think it is better without.

Once you've got the focuser sussed, you could try removing the mirror, setting up two threads at right angles (at the mirror end) and then adjusting the tilt of the secondary so that the laser beam hits the center of the threads (the mechanical center of your OTA).
Then put your mirror back in, and see if the beam hits your center mark on the primary, if it doesn't then your primary isn't centered in the tube, take it out and move it in its holder (if you can) and then put it back in.

Once you have the beam falling on the ceter of your primary, then adjust the angle of the primary to reflect the laser beam back into its exit hole.

Its a good idea to check that the laser is collimated before using it, with the secondary removed, turning it on and rotating it in the focuser is one way, the beam shouldn't move as you rotate.

You can probably get some of the slop out of the focuser by adjusting it. There's a thumb screw on top which locks the tube) on either side are a couple of small grub screws, tweak them and it should get rid of some on the "dreaded draw tube flop"

#15 Jon Isaacs

>>>My major concern is the very obvious image rotation inside and outside of focus, even after being outside for around 2 hours (it is a BK-7 mirror).
-------------

Your scope is a 10 inch F5 GS Newtonian. My understanding is that astigmatism is quite common in these but that fortunately the cause is a pinched secondary mirror and that once fixed, the optics are quite good.

I bought one used from astromart that suffered from astigmatism and I tried all the tricks, collimation, rotating the primary with no luck. When I tried to remove the secondary from its mount, it was quite difficult and after cleaning things up a bit and reinstalling it, the astigmatism was completely gone and the optics have performed very nicely. It does not take much pressure to warp a mirror sufficiently to cause astigmatism.

Since then I have read that this is quite common in these scopes, I believe there are user groups for these scopes that have some detailed information about dealing with this problem.

If and when you do remove the mirror, take care because these are first surface mirrors and easy to damage.

#16 Guest_**DONOTDELETE**_*

• topic starter

that's very interesting information. Do you by any chance have any pictures of how the secondary mirror is mounted on the GS scope? I'm wondering because I have an 8" f/5 synta scope, and I'm wondering if astigmatism in my scope coul dbe caused by the same thing.

#17 Jon Isaacs

I don't have any pictures but I will try to describe the secondary mount as I remember it. Its been a year so my memory is pretty vague. The mount is made of plastic and it surrounds the mirror in most directions. In back there is a hole about an inch in diameter that has some lightweight flexible foam which pushes the mirror forward. There is a clip on one end that holds the mirror in place.

Mirrors should be held gently. My undertanding is that the Synta DOBs use Silicone to mount the secondary, at least some of them do.

But the main thing is to isolate the cause of the astimatism by carefully noting the orientation in when the focuser is inside and outside of focus and then rotating the primary mirror 90 degrees and determining if the astigmatism reverses or not. Another method is to just rotate the secondary 120 degrees and see if the axis of the astigmatism changes. If it does not change then it is probably the secondary and hopefully it is just pinched.

#18 Guest_**DONOTDELETE**_*

• topic starter

your description was enough to visualize how the secondary mirror is mounted. Yes, synta secondaries are glued on to a piece of cork with silicone, which rules out a tight clip (However, I'm pretty much convinced that the mirror clips on my primary mirror are too tight, so I'll loosen those up and do a star test).

rotating the secondary mirror is a great idea though, and I'll try it to see if that changes the orientation of in and out focus (even if it's not a pinched secondary, but the secondary isn't flat, that would show when rotating the secondary). If that doesn't work, I'll try loosening the primary mirror clips. and if that doesn't make a difference, I'll rotate the primary and see if that changes things. either way, I should be able to pinpoint the problem.

#19 Jon Isaacs

It is not really possible to rotate the secondary, as it fixed by geometry, one must do this by rotating the primary. Even with a glued secondary it is possible that somehow the glue is causing it to deform. Main thing to do is to see if you can isolate the source of the astigmatism and then go from there.

#20 Guest_**DONOTDELETE**_*

• topic starter

I was thinking of unglueing the secondary. rotating it 180 degrees and putting it back. if that changed the orientation of the astigmatism, I'd know it's the secondary that's the problem. wouldn't that work? a 180º rotation would maintain the geometric requirements (horizontally oriented elipse, as viewed from focuser). and the orientation of the in and out focus astigmatism would be shifted 180º too (if the secondary is the problem)?

I don't have the scope near me at the moment (it's down in the car), so can't really check the mirror. but off the top of my head is seemed it would be possible to rotate the secondary in that way.

#21 Kevin201

Not to seem rude but maybe its your eyes? I know because I have astigmatism so I view with eyeglasses. If I dont the image looks elongated. Not perfectly round.

#22 Guest_**DONOTDELETE**_*

• topic starter

I also have mild astigmatism, and don't view with eyeglasses. This makes a difference when viewing a focused image (the slightly elongated image changes direction when I turn my head). however, when the image is out of focus at high mag (so the airy disks can be seen), it's elongated and doesn't change shape or orientation when I turn my head. which, I believe, indicates that it's an optical problem in the telescope, and not my eyes.

If my logic in distinguishing mechanical defects from biological defects in the optical train is wrong, I'd be happy to know

#23 Guest_**DONOTDELETE**_*

• topic starter

I've had 2 fine nights for viewing, so I've had a chance to test the scope, but I did try a few things first.

Set up a piece of paper opposite the focuser with concentric circles, then centred this piece of paper with the laser collimator. From here I measured the point from the centre of the paper's circles to the rim of the telescope, then I measured the width of the focuser, worked out the centre, and measured from this to the rim of the telescope. Both were right on, so I'm fairly sure the focuser is at least vertically squared, however horizontal alignment remains unchecked. I then used the circles to try and get the secondary closer to the centre of the focuser.

The secondary was a disaster when I removed it to try and reseat it in the cell. I've heared about the secondary being too tightly held in the cell, so I removed the secondary, took out the mirror from the cell, and then attempted to lightly sand down the mirror housing. however I could not resist cleaning the secondary, mirrors are obviously a lot less scratch resistant then eyepieces (despite using Japanese brand lens cleaning tissues), very glad these secondary mirrors only cost 34NZ, I am however using it anyway, gathering there is much more surface area unscratched then scratched to affect the image. Generally I'm probably not as careful as I could be (I am trying hard to reindoctrinate myself about patience), but I have worked out that it is best to clean my eyepiece lenses at night, and not under direct light, as the dust is excited and as soon as you clean the lens, dust lands on it, and it becomes far too tiresome to clean them, as well as increasing the risk of scratching them, too bad this did not help me with the secondary Anyway to the collimation under light polluted skies, I adjusted the brightness using Hadar (double star isn't it, can't see the second?), magnified at 280x (2x Barlow + 9mm plossl), used a out of focus star pattern to get it more or less concentric, though if I was to take a guess, when it get's closer to focus, the image looks a little squashed or oval, whether this is because of the atmosphere (it was a little distorted), I am not certain, but thought it might be more to do with the secondary alignment. Well the images of stars aren't perfect points of light, at least at (2x barlow, 9mm plossl) 280x, though drop down to 140x (9mm plossl) things become much crisper, drop down to 50x (25mm plossl) stars are numerous, and sharp, but not much shaper then 140x, Alpha & Beta Centauri still split cleanly, I'm guessing that if they are currently separated by about 22 arc seconds, the resolution would be around 4-6 arc seconds before they were no longer cleanly split, and 1-2 arc seconds before it would be difficult to identify them as separate stars. As with eye astigmatism, one of the easy tests is a piece of paper with a small (1-2mm) hole in it, as the eye lens is roughly 7mm in diameter, you concentrate light through just a small section of the least curved surface, therefore if you notice that things seem sharper, most likely astigmatism is the problem, I tried this, but didn't notice a real improvement in quality, I think it's more the eyepiece lenses, or something else within the optical train, as they appear to get fairly badly astigmated towards the edge, but a lot more closer to perfect near the centre. Would I be guessing right that this is more atmosphere related, and that my scope is close to being well collimated? Thanks for the great help you guys have offered, I think I might be getting closer to sorting this collimation/astigmatism issue out Reflection Artifacts in the ASI6200 Here's the data. There are two folders for both orientations of the filters. I have tilt in both images that I did not try to correct out, the wizard nebula was my first light to just see how the camera performs. Reflective side was towards the stars on the Wizard nebula. The bubble nebula was after I found out I had the filters oriented the wrong way, and I flipped them around for this image. The filters reflective side is now towards the camera. You'll find integrations and single images for HA and OIII on both objects. One other thing I learned through lots of testing was that there's a problem with the ZWO 2" filter wheel if anyone is using it. When in bi-directional mode (the default), the sensor stops moving the wheel as it reaches the filter. In clockwise motion it stops with one offset, and counterclockwise it stops with a different offset. Meaning if you take your lights in sequence and the filter wheel does HA, SII, OIII, then goes to start the sequence again It moves backwards to get back to HA in bi-directional mode. Then you shoot your flats, and you find out dust motes can't be corrected out because half of your frames were shot with the wrong filter wheel offset due to how the filter wheel approaches and stops at each filter when in bi-direcitonal mode. Switching to unidirectional (one way) mode eliminates this problem. See thread here where I tested and discovered this issue. Because of this issue, you'll see a big fat uncorrectable donut on my OIII integration. Raw Images.zip I can see a very big change in the orientation swap in your data. Odd that when I flipped the filter I did not see the same effect. What you are talking about is related to issue #2 I presented in this thread the strange artifacts around stars. I noted both a ((o)) effect and a halo effect on relatively bright (mag 4) stars. Neither the ((o)) or the halo effect went away when I flipped the filter. This is very good data to have. I sent an email off to Chroma to get them engaged and get their thoughts. It has not had any problems on an 8300, 16200, or KAI-11002. Those are much less sensitive sensors than the IMX455 though. Diffraction Diffraction : Diffraction is the spreading of waves around obstacles. Diffraction takes place with sound with electromagnetic radiation, such as light, X-rays, and gamma rays and with very small moving particles such as atoms, neutrons, and electrons, which show wavelike properties. Observing at the Diffraction Limit Get Articles like this sent to your inbox Constant Contact Use. DIFFRACTION AND INTERFERENCE Light exhibits two key properties that are characteristic of all forms of wave motion: diffraction and interference. Diffraction is the deflection, or "bending," of a wave as it passes a corner or moves through a narrow gap. As depicted in Figure 3. grating A mirror with very fine grooves that separates light into its different colours on reflection. Doppler Effect The apparent change in the wavelength of waves due to the relative movement of the source of the waves relative to the observer. , which causes shadow edges to not be perfectly sharp, but fuzzy. -limited 800 nm imaging with the 2.56 m Nordic Optical Telescope p. L1 J. E. Baldwin, R. N. Tubbs, G. C. Cox, C. D. Mackay, R. W. Wilson and M. I. Andersen DOI: . limits this to a value proportional to the wavelength of the light observed divided by the diameter of the telescope. Einstein Observatory- . spikes are caused by a non-circular obstruction in front of the primary mirror, such as the vanes holding a secondary mirror or an irregular-shaped imaging camera. inherent in the system. Sometimes means less than 'Wave manufacturing error.or less. ( Hg. Green light ) . limited A measure of optical quality in which the performance is limited only by the size of the theoretical diffracted image of a star for a telescope of that aperture. Direct motion Another term for prograde motion. GRATING A device used to produce the spectrum in astronomical spectroscopes consisting of many narrow parallel apertures or mirrors. DIFFUSE NEBULA . Fringe: Blurred fringe surrounding an image caused by the wave properties of light. No detail smaller than the fringe can be seen. (see fringe for more) Diffuse ionized gas: Also referred to as the "Reynolds Layer", Diffused Ionized Gas is nearly fully ionized gas in the Milky Way. Limited The point at which optical quality is good enough that the limits of viewing detail are determined by the physical properties of light, and not any optical defects in the telescope. See also RESOLUTION. - Spreading out of light as it passes the edge of an obstacle. Dobsonian telescope - A telescope with a stable altazimuth mount that rotates easily. (a) A property which distinguishes wave-like motions. When a wave is incident upon a barrier which is broken by a narrow slit (of comparable size to the wavelength), then the slit will act as a new isotopic source of secondary waves. [CD99] . grating A flat optical surface, transparent or reflecting, ruled with many parallel grooves at precisely spaced distances. The active parts are not the grooves but the flat sections left between them, which act like a large number of precisely spaced slits. rings. Concentric rings surrounding the image of a star as seen in a telescope. : The bending of light as it passes over the edge of an object. diode: A diode is a device that allows electrical current to flow in one direction only. dirrect current (DC): Direct current is the flow of electricity in one direction. of light established its wave nature. . Limited Electromagnetic waves diffract around the edges of opaque objects or on passing through or reflecting off a finite aperture, like a dish, lens or mirror. Even if such a wave is perfectly collimated, so that the beam emitted is parallel, it will eventually spread out. of seismic waves provided the first clear-cut evidence for a lunar crust, mantle, and core analogous to those of the earth. The lunar crust is about 45 mi (70 km) thick, making the moon a rigid solid to a greater depth than the earth. of light establishes its nature as a wave. Doppler Effect Apparent change in wavelength of the radiation from a source due to its relative motion away from or towards the observer. is the slight bending of light as it passes near the edge of an object. Every cross in this image is due to a single set of struts within Hubble itself. The color image was made from separate exposures taken in the visible and near-infrared regions of the spectrum with Hubble's ACS camera. glasses to study colour and spectra. Ecliptic Exoplanets Voyager Cosmics Rays Constellations Cosmology Dwarf Planets Dwarf Galaxies Meteors Faster Than Light Shape Oscillation Infinity Wormholes Iron Weather Speed of Light Escape Velocity Active Galactic Nuclei Curious Minds Online We have 1769 guests and no members online . of light, provide a spectrum of radiation that falls on it. disk - (n.) . grating Optical device used to disperse light into a spectrum by means of a series of closely spaced, equidistant, parallel grooves ruled on to its surface. is most noticeable when an electromagnetic wave passing around an obstacle or through an opening in an obstacle (such as the slit between your fingers) is all of the same frequency, or monochromatic. grating by itself is really no better than a prism for creating an astronomical spectrum. The grating must be built into a device called a spectroscope or spectrograph for this to be done. pattern constantly interrupted by brief gaps or flickers, or is it continuously unified? pattern is stationary. On this scale 1 to 3 is considered very bad, 4 to 5 poor, 6 to 7 good, and 8 to 10 excellent. . A laser beam must be focused by a suitable lens or shaped mirror, in order to concentrate the beam onto a small enough spot to cause damage. In order to focus the beam at the right spot, the range to the target must be known. Laser weapons incorporate an automatic range finder for this purpose. limit for an optical telescope like Hipparcos, which had a mirror of diameter d = 29 cm and measured light of wavelength &lambda = 500 nm? The spreading of light as it passes a sharp edge of an opaque object. Diffuse (bright) nebula A cloud of ionized gas, mostly hydrogen, with an emission-line spectrum. Disk The visible surface of the Sun (or any heavenly body) projected against the sky. gratings, red light is dispersed more than blue light, the opposite of prisms. The computer that assigned object types sometimes encountered an object that it did not believe was a real celestial object, such as a scratch on the plate or a spike caused by a very bright star. ablation 𠈪shen, fashion, passion, ration �straction, action, attraction, benefaction, compaction, contraction, counteraction, , enaction, exaction, extraction, faction, fraction, interaction, liquefaction, malefaction, petrifaction, proaction, protraction, putrefaction, redaction, . pattern' being observed with only one slit, which I interpreted as another word for interference suggesting an interference pattern. When light is passed through a prism or a grating to produce a spectrum, the type of spectrum you will see depends on what kind of object is producing the light: is it a thick or thin gas, is it hot or cool, is it a gas or a solid? As the years progressed, Newton completed his work on universal gravitation, of light, centrifugal force, centripetal force, inverse-square law, bodies in motion and the variations in tides due to gravity. His impressive body of work made him a leader in scientific research. - MetaGuide requires only a video camera and Windows 7, Vista, XP or 2000 computer (not Win98, ME, NT, Mac, or Linux) with recent DirectX installed to perform the core analysis of a star. MetaGuide and its install package are compatible with Win7/64. ring, and 2.8% in the second ring. The diameter of the Airy disk only depends on the f-ratio of the telescope, being smaller for lower f-ratios (faster). The Airy disk of an f/5 scope is half the size of the disk of an f/10 scope, this should give you a hint about the title of this article. CheMin: CheMin is short for 'Chemistry and Mineralogy', and it is an X-ray and X-ray fluorescence analyzer.[46][47][48] It will identify and quantify the minerals present in rocks and soil and thereby assess the involvement of water in their formation, deposition, or alteration. It is surrounded by alternating rings of light and dark (these are due to - any light passing through an aperture is diffracted, and the effect is inversely proportional to the size of the aperture.) An optical system of good quality increases the relative brightness of the central Airy disk compared to . When we pass starlight through a prism (or similar device suitable for telescopes, such as gratings), we see a forest of absorption lines from hydrogen, helium, sodium, and so on. pattern. left: These two stars are on the limit of resolution according to the Rayleigh criterion. With an all-sky view of the Milky Way, the SUV-sized instrument uses a collection of mirrors, gratings, and CCD sensors to measure the distance to more than 1 billion Milky Way stars, about 1 percent of our galaxy's estimated total. As the moon occults a source, a Fresnel pattern is produced. Lunar occultations can be performed in the radio, infrared, optical, and X-ray (in fact, the EXOSAT satellite was launched specifically to do X-ray lunar occultations). and fluorescence instrument a radiation detector a descent imager a gas chromatograph mass spectrometer a tunable laser spectrometer a pulsed neutron source and detector a meteorological package with ultraviolet sensor Electrical power. The stars are easily discernable from galaxies because of their spikes are an image artifact caused by bright starlight traveling through the telescope&rsquos optical system. grating film mounted in a 2x2-inch slide. You can buy these from Edmund Scientific for about10 for package of 15: scientificsonline.com (item #3054510)
A thick 3x5 card
A sheet of thick cardboard to use as a cutting pad.

Infrared image of Alpha Centauri AB, with

effects from partially closing the
mirror covers of the 1.5-m telescope. (A
Digitized Sky Survey image of Alpha
Centauri may become available at the
Nearby Stars Database, or see one
at Astronomy Picture of the Day.) .

A glory is produced by a process similar to rainbows, but with a part of the light that takes a different set of angles of

, reflection, and refraction as it passes through water droplets. The angle of a glory from the light source is much smaller than a rainbow.

The planetarium team provided thousands of

gratings and special lenses to teachers and students, to encourage people to learn more about the nature of light and its spectra.

In most situations, light behaves like a wave with properties like wavelength and frequency, and is subject to

and interference.
In some situations, light can be considered to behave like a particle called a photon. This particle has no mass, but carries a fixed amount of energy.
SpaceBook home .

While later physicists favored a purely wavelike explanation of light to account for the interference patterns and the general phenomenon of

, their findings owed a great deal to Newton' theories.

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Limited
MIRA software from Axiom Research
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Starry Night AstroPhoto Suite (MaxIM DL plugin) .

, a property of light. According to Vince Huegele, an optical physicist at the NASA Marshall Space Flight Center, the light rays do not shoot straight by the rim of the gaps, or a pinhole, but bend around the edge. This wave effect creates a pattern of rings that resembles a bull's eye.

It is the first Mars mission to use X-ray

, which can detect how X-rays are scattered from a sample and thus identify minerals. When minerals are formed they capture information about the environmental conditions, which could reveal if the Gale Crater was once a good place for life.

You can use the Spectrum Constructor to explore the three types of spectra and how they appear through a

grating when the brightness of each wavelength is measured with a spectrometer.
Making Connections
The Rainbow .

The lunar sky is always black because

of light requires an atmosphere. The astronauts also experienced gravitational differences. The moon's gravity is one-sixth that of the Earth's a man who weighs 180 lbf (pound-force) on Earth weighs only 30 lbf on the Moon.

Characteristics of light, such as frequency and

, that mimic properties of waves.
weak nuclear force
The force of nature that converts neutrons into protons and is responsible for radioactive decay.

grating, grism(*), echelle(*), Fabry-Perot etalon, .

Odds are the seeing will never get good enough for them to demonstrate that a half-meter shaving mirror will blow eighteen centimeters of optical perfection clean out of the water, and if they start talking about faint galaxies you can always change the subject to

rings and modulation transfer functions, .

The question that usually pops into your mind is that how sharp can be the magnification of your telescope? The circular telescope aperture diffracts waves of light and forms bright and dark rings. When you focus the image of the star becomes a small dot that has faint

In the same year a model for F-actin was proposed by Holmes and colleagues. The model was derived by fitting a helix of G-actin structures according to low-resolution fiber

data from the filament. Several models of the filament have been proposed since.

As well as the galaxies there are also five bright stars, which are much closer to us than the galaxies. The telltale

spikes - four sharp lines of light emanating at 90 degree angles, caused by light diffracting in the telescope - are unmistakable signs of the stars in the picture.

SPECTRUM: The range of color produced when light is split up by a prism of

grating.
SPECTRUM, ELECTROMAGNETIC: The distribution of light separated in order of some varying characteristic such as wavelength or frequency.

DETONATION PHENOMENA IN CHARGES WITH AN AXIAL CAVITY

Precursors

Obviously the observed precursors that induce initiation of neighboring charges with a larger detonation velocity than for each individually, do not correspond to those discussed in Chapters XIV and XVI. The difference is that conservation of momentum requires that the velocities of the ejecta increase as their density decreases. This is true, but in Chapter XIV an additional virtual acoustic mass of the displaced volume was present, which limited the velocity, even for zero-density particles, to at most u′p = 3 up,∞. This prerequisite is absent for axially cavitated charges, therefore the precursor velocities are not limited by such acoustical mass. The dependence on the area mass of the foils indicates that momentum transfer is essential.

No explanation of the effects shown in Figure XXII-2 , and Table XXIII-2 exist at presence. One approach may be to consider cascaded collisions of particles as described in Chapter XIV . Within this view such super velocities of tiny particles and the luminous events can be approached.

'Captain, There Be Planets Here!'

About 450 light-years from Earth, in the constellation Taurus, a dense, dark, interstellar cloud has slowly started to reveal its secrets. It happens to be a very active nursery for young stars resembling our own sun about 4.6 billion years ago. Embedded in this cloud, which has been carefully studied by the Hubble Space Telescope, are very young stars called HL Tau and XZ Tau, each no more than 1 million years old, give or take. You can easily see the nebulae formed by the complex blobs of gas ejected by the young stars.

HL Tauri and surroundings (credit: NASA/HST)

The star HL Tau (more properly called HL Tauri) is 10,000 times too faint for you to see with your naked eye. Even a large telescope has a hard time seeing it clearly through all the dust and gas blocking the view. But other kinds of telescopes can easily pierce the light-years of dust clouds. Since 1975, astronomers had known that HL Tau had some kind of disk of gas orbiting it. The disk is about 40 times the diameter of our solar system. Later on, astronomers studied HL Tau using radio telescopes and detected a dense knot of carbon monoxide molecules centered on the star. Caltech astronomers Anneila Sargent and Steven Beckwith were able to study this clump in more detail and discovered it really was a disk-like region rotating in the same way that planets orbit our sun: faster toward the center and slower toward the edge. Based on observations made in 1986 from the Millimeter Wave Interferometer of the Owens Valley Radio Observatory, it was determined that the disk had about 10-percent as much mass as our sun. Though not enough to build a second companion star, it would be plenty to make a lot of planets, with dust and gas to spare and throw away into the surrounding Taurus dust cloud.

In addition to dust and carbon monoxide molecules, astronomers had also detected water ice in 1975, and micron-sized silicate ("beach sand") dust grains in 1985. Searches for methane ice in the very cold outer limits of the disk haven't turned up anything yet. HL Tau's disk seems to be pretty bland in terms of interesting pre-life molecules! But its boringly simple, or absent, chemistry is countered by the disk being a very complex and active region of space. Along the axis of the spinning disk, small nebulae called Herbig-Haro objects can be seen several light-years away, such as HH-150 and six cloud clumps aligned along a jet called HH-151. These gas clouds are like puffs of smoke being ejected at very high speeds by events happening as the HL Tau interacts with its surrounding disk. We still don't know exactly what is going on after all these years. Could magnetic fields be involved?

Recent studies of the magnetic field of this disk by astronomers Ian Stephens and Leslie Looney led to a surprising result: Instead of the field oriented with a distinct axis pointing along the HH151 jet, the field was much more complex. It was always thought that the blobs of gas would be ejected along some well-defined axis provided by the magnetic field, like the barrel of a cannon. In the absence of an ordered "poloidal" magnetic field to define a unique direction, the cause of the cloud alignment in the jet remains a mystery. To make matters more interesting, in 2007, astronomer Michihiro Takami, using the Subaru Telescope, detected the faint emission from a counter-jet also aligned with the HH-151 jet and HL Tau. So whatever the events that are occurring in this disk of gas and dust, it tends to favor ejecting two streams of matter in a symmetric way along the polar axis of the disk.

Once astronomers caught the scent of the nearby HL Tau dust disk, there was only one direction to go: higher resolution to see more details and how the disk is actually shaped. By 2011, astronomers Woojin Kwon, Leslie Looney and Lee Mundy had used the Combined Array for Research in Millimeter-wave Astronomy and detected a flattened shape. It is not face-on but tilted so it looks like an ellipse inclined slightly downwards to the line of sight. The measurements also suggested that the larger dust grains in the disk, possibly as big as sand grains, had settled toward the plane of the disk and a halo of finer micron-sized dust grains probably engulfs the whole disk. The disk is also gravitationally unstable, but they were not able to see any details of the kinds of shapes that result.

Then, in 2014, astronomers used the even higher-resolution capabilities of the new Atacama Large Millimeter Array (ALMA) to create the now-famous image you have seen on the nightly news. What it confirms is all the previous observations about the size, shape and tilt of the dust disk, but it also could begin to see some of the details of its internal shape. What astronomers found was simply amazing. As many as eight dark bands concentric with the star can easily be seen! The gaps are about 450 million to 1 billion miles wide. What are they?

The HL Tauri protoplanetary disk (credit: ESA/ALMA)

The system is less than 1 million years old, and this seems too short a time to form planets within these dark rings -- but who knows? Could you really form Jupiter-sized, dust vacuum cleaners in less than a million years? Some calculations using gravitational instabilities in the disk have predicted Jupiter formation times as short as 100,000 years! This is not the first time such rings have been seen. In 2005, the Hubble Space Telescope found a dark ring in the disk surrounding the star TW Hydrae located 176 light-years from Earth. The ring is 2 billion miles wide and 8 billion miles from its star and, like HL Tau's rings, may eventually reveal a forming Jupiter-sized planet.

Meanwhile, Hubble has just completed a survey of debris disks orbiting a number of stars that are no longer enshrouded by dust clouds. The collection of 23 stars reveals some interesting clues to how these disks evolve in time between 10 million and 1 billion years after planet formation has probably stopped. The large planet seen by Hubble orbiting inside the debris disk of the bright star Fomalhaut is probably the last stages of such a planet-forming disk system. Meanwhile, the disk irregularities observed around the star HD 181327 resemble a huge spray of debris possibly caused by the recent collision of two bodies. When our infant Earth was struck by a Mars-sized planet to form our Moon, a similar spray of debris probably formed!

Fomalhaut debris disk and planet (credit: NASA/HST)

Theoretical Work: A Story in Progress

For decades, astronomers have worked with supercomputer simulations of the basic laws of gravity, fluid and gas dynamics and radiation transport to create physically consistent models of what these protoplanetary disks should look like as they evolve over time. The mechanism of planet formation has also been explored through a variety of calculations and physics-based models. For example, astronomers Phil Armitage at the University of Colorado and Wilhelm Kley at Tubingen University have arrived at similar models for how a massive planet like Jupiter forms from such a disk and excavates a swept-out ring that resembles the HL Tau rings. The models show how the planet attracts mass from the edges of the ring, and how the process develops spiral "gravity waves" in the disk that then cause the orbit of the forming planet to change over time. Other simulations like the ones by Ken Rice at the University of California, Riverside, show that for much more massive disks, gravitational instabilities can lead to very rapid large-planet formation and the creation of a strong spiral wave resembling what you see in the shape of a spiral galaxy.

A forming planet and ring (credit: Wilkelm Kley/Tubingen)

So the addition of new high-resolution data like that for HL Tau is at long last allowing astronomers to see the hidden details of planet and disk formation and, from this, create the next-generation physics-based models for how planets form. The rings are still something of a mystery and may not actually involve planet sweeping. Until we can detect an actual planet inside one of these rings, this connection is still an unproven theoretical possibility.

Stay tuned for new discoveries and more clues to how our own solar system formed!