Is there an oxide layer on aluminum reflectors for telescopes?

Is there an oxide layer on aluminum reflectors for telescopes?

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I was watching a Nottingham Science video linked here (periodic videos) and they mention modern reflectors are made of aluminum.

I can understand why mirrors are preferred over lenses, but what inhibits the aluminum oxide coating that would naturally form on the surface of the mirror? Why do they work so well?

The aluminum oxide layer forms as soon as oxygen comes in contact with the mirror surface.

Aluminum oxide is tough, firmly attached and transparent; it serves to protect the aluminum surface. Rubies and sapphires are slightly impure aluminum oxide

Optical coating

An optical coating is one or more thin layers of material deposited on an optical component such as a lens or mirror, which alters the way in which the optic reflects and transmits light. One type of optical coating is an anti-reflective coating, which reduces unwanted reflections from surfaces, and is commonly used on spectacle and camera lenses. Another type is the high-reflector coating, which can be used to produce mirrors that reflect greater than 99.99% of the light that falls on them. More complex optical coatings exhibit high reflection over some range of wavelengths, and anti-reflection over another range, allowing the production of dichroic thin-film filters.

UC Observatories Advanced Coatings Lab

The Advanced Coatings Lab (ACL) is a part of the University of California Observatories (UCO). It was developed in response to a need for improved optical coatings for ground-based astronomy, in particular, silver-based reflective coatings for telescope mirrors and broad-band anti-reflection (AR) coatings for lenses, windows and prisms.

What are optical coatings?

Optical coatings are thin films applied to mirror and lens surfaces to enhance reflection (for mirrors) or suppress reflection (“anti-reflection” for lenses and other transmissive optics). They can consist of one or multiple layers of various materials, and are generally quite thin, perhaps 200-400 nm in thickness for visible-light coatings – this is roughly one half a wavelength of visible light, or1/300 the diameter of a human hair.

Note that the terms “thin films” and “coatings” are used interchangeably, although a coating can include multiple thin films.

Why are they important?

Astronomy is an observational – rather than experimental – science. Except for within the solar system (the realm of planetary science), we cannot visit and manipulate the objects we study – we can only observe and analyze the light we receive from these objects. Most of this light is very faint (it’s dark at night!) and so our observations are limited by how quickly we can gather enough light for our analysis. Bigger telescopes mean more light collected more quickly, and has driven the increase in telescope size over the last few centuries. However, telescopes and the instruments that gather the light are not 100% efficient and light is lost at every surface it strikes.

For example, vacuum-deposited aluminum has been the standard telescope coating since the 1930s. It is both highly reflective and when exposed to air it forms a transparent oxide layer that protects it against further chemical change. However, these aluminum coatings absorb about 10% of the light that falls on them. In most modern large telescopes, there are 3 mirrors, so roughly 30% of the light is lost. Similarly, the instruments used to analyze the light typically will contain 1 or more mirrors, and 8-10 lenses. An uncoated glass surface reflects approximately 4% of the light that hits it anti-reflections coatings typically drop this to 1% per surface. Nevertheless, with

18 lens surfaces, this is still an 18% loss. In fact, most telescope-plus-instrument combinations lose about 50% of the total light that reaches them. Improved coatings with even modest gains can reduce these losses significantly when applied to the large number of optical surfaces involved.

Silver is more reflective than aluminum over most wavelengths (except for the deep UV). However, silver does not form a protective oxide, and it is prone to tarnishing and corrosion. Therefore, to use silver, we must over-coat it with protective layers of transparent materials that prevent reactants such as sulfur, chlorine and water from reaching it. Developing silver-based coatings has been a major goal of the Advanced Coating Lab since its inception.

Silica sol-gel is a porous material with a refractive index ideal for AR coatings on most glass, preventing almost all loss at a particular wavelength. It was developed as an anti-reflection coating at Lawrence Livermore National Labs (LLNL) where they are using powerful lasers – so sol-gel coatings tuned to the proper wavelength would prevent laser light from being absorbed and (over-)heating the lenses and windows through which the laser light passed. At ACL, our work has been to broaden the “pass-band” (the range of wavelengths) over which sol-gel based coatings would be effective.

How are they deposited?

The simple explanation: the coating material is heated to evaporation on either a bulk scale (resistive heating), locally (e-beam) or atomic scale (sputtering). The evaporated material that falls on the mirror or lens builds up the coating.

For optics, almost all coatings are deposited using Physical Vapor Deposition, or PVD. In these processes, a material is evaporated or sputtered in a vacuum, crosses some distance, and lands on the substrate. Materials may be heated by being in contact with a hot resistive metal through which a large current is passed examples are hot filaments (traditionally used for aluminum deposition), or “boats” or crucibles wrapped with heater wires. Generally, only materials with relatively low evaporation temperatures can be deposited this way. More refractory materials need to be heated by a beam of energetic electrons in an electron gun (“e-gun”). Such depositions may be modified by striking the growing film with energetic ions from a ion source this is call Ion Assisted Deposition, or IAD. Alternatively, materials can be sputtered by very energetic ions generated by a magnetron (“magnetron sputtering”) or a directed ion source (“ion beam sputtering,” or IBS).

The vacuums appropriate for PVD are so good that most evaporated atoms travel from the source to the substrate without collision with any residual gas. (Another way to say this is that the source-to-substrate distance is less than 1 “mean free path.”) This means that the deposition is “line of sight,” and due to a process called “self-shadowing” very thick films will become rough and porous, and “pinholes” can be created in films of any thickness. IAD helps by adding energy to the growing film, allowing the atoms/molecules to move around on the surface and pack more densely. Residual gas pressure is usually low enough to prevent a lot of gas from becoming entrapped in the film. Film thickness varies with the distance from source to substrate, and with the angle of the substrate relative to the incoming stream, so maintaining thickness uniformity with PVD is a challenge that must be faced.

The desired film material can be directly evaporated or sputtered: for example, pure metal films of Aluminum or Silver are always deposited this way, as are some compounds like magnesium fluoride. Metal oxides (like aluminum oxide, Al2O3) can also be deposited starting from the oxide material, but a preferable method is to deposit them “reactively” – we evaporate the metal in the presence of a small amount of oxygen, and the metal and oxygen chemically combine at the surface. Usually, the oxygen is supplied as a plasma generated by an ion source or magnetron, although some metals with a strong affinity for oxygen (such as aluminum or yttrium) will oxidize well in a neutral gas. Similarly, nitrides are almost always deposited using reactive deposition in a nitrogen plasma.

Chemical Vapor Deposition (CVD) is another way to deposit coatings, although they are rarely used for optics. In this case (always “reactive”), chemical reagents are introduced as vapor into a vacuum, usually at high temperature, and the desired material condenses on the substrate. CVD can deposit thick coatings quickly, and conformally – that is, the thickness is the same everywhere, independent of substrate shape. CVD is used for coating many items for wear resistance or decoration. However, the films tend to contain more trapped contaminants than to PVD films. A newer variant of CVD called Atomic Layer Deposition (ALD) introduces carefully-chosen reagents sequentially to form mono-layers on the substrate this process seems to have the best properties of PVD and CVD, and shows promise for optical coatings. It is inherently slow, however, since the films can only be grown at a few molecular layers per minute, so thick films are impractical with this process.

What are the challenges?

Coming soon: performance, uniformity, durability.

The Advanced Coatings Lab has received National Science Foundation support via Award AST-1005506. NSF is not responsible for the content of this website.

All the Satellites in Space Could Crack Open the Ozone Layer

Mega-constellations might end up being a massive problem.

  • A huge number of low-Earth orbit satellites could cause environmental problems.
  • Byproduct aluminum oxide reflects sunlight and helps to deplete ozone.
  • There are unintended consequences in legality and even the study of astronomy.

The hole in the ozone layer, Earth&rsquos protective chemical shield that absorbs most of the sun&rsquos ultraviolet rays, has slowly healed over the last few decades since the global ban of chlorofluorocarbons (CFCs). But scientists are now raising the alarm about puncturing a new hole in the ozone layer&mdashthis time without any noticeable CGCs in sight.

Dive deeper. ➡ Get unlimited access to Pop Mech's best-in-class space content , starting now.

Instead, the surprising cause is deterioration of the aluminum in megaconstellation satellites like SpaceX&rsquos Starlink network.

For our purposes, a satellite is a human-made object put into low-Earth orbit (LEO) for a planned lifespan. There are about 5,000 active and defunct satellite sin LEO, with over 40,000 Starlink sats planned in the future, plus satellite projects from national space agencies and private companies around the world, researchers from the University of British Columbia say in their new Scientific Reports study.

The human-made distinction may seem obvious, but it hasn&rsquot always been. That&rsquos because, as reports, scientists spent decades favorably comparing satellite &ldquojunk&rdquo to the amount of material deposited and burned up in our atmosphere by meteorites. As long as meteorites were so much more of the material by volume while doing almost no harm to the planet, how bad could human-made satellites be?

Well, as it turns out, it&rsquos a matter of quality rather than quantity. That&rsquos because meteorites are made of a different constellation of minerals and elements than our custom-manufactured sky robots.

&ldquoWe have 54 tonnes (60 tons) of meteoroid material coming in every day,&rdquo lead study author Aaron Boley told &ldquoWith the first generation of Starlink, we can expect about 2 tonnes (2.2 tons) of dead satellites reentering Earth&rsquos atmosphere daily. But meteoroids are mostly rock, which is made of oxygen, magnesium and silicon. These satellites are mostly aluminum, which the meteoroids contain only in a very small amount, about 1 [percent].&rdquo

Aluminum is key to everything at stake here. First, it burns into reflective aluminum oxide, or alumina, which could turn into an unwitting geoengineering experiment that could alter Earth&rsquos climate. And second, aluminum oxide could damage and even rip a new hole in the ozone layer. Let&rsquos look at each threat separately and try to figure it out.

Misadventures in Geoengineering

Geoengineering is the umbrella term for technologies that seek to alter the climate or other physical realities about the planet. The major meaning that most people associate with the word is solar geoengineering, an experimental idea to fight climate change. Yes, this includes launching reflective aerosols that will &ldquoblock the sun&rdquo back into space and ostensibly cool the planet, which is what Bill Gates eventually wants to try.

But we just don&rsquot know how large-scale geoengineering could affect the planet&rsquos climate. (In the sci-fi flick Snowpiercer, geoengineering has turned Earth into a lifeless iceball whose only survivors must crowd aboard an unceasing train. That&rsquos probably our worst-case scenario.)

Aluminum oxide scatters more light than glass, with a refractive index of about 1.76 compared with just 1.52 for glass and about 1.37 for plain aluminum. The researchers write:

Another Hole in the Ozone?

What, then, of the ozone layer? Once again, aluminum oxide comes to the forefront. As aluminum burns, it can chemically react with ozone in the air to form aluminum oxide, thereby depleting the naturally protective supply of ozone in the atmosphere. The atmosphere can absorb a small amount of these chemicals without ill effect, but with tens of thousands of satellites in play, the quantities will naturally go up.

That&rsquos in addition to the ozone damage done by each rocket launch to put satellites into LEO. &ldquoRockets threaten the ozone layer by depositing radicals directly into the stratosphere, with solid-fueled rockets causing the most damage because of the hydrogen chloride and alumina they contain,&rdquo the researchers write.

While satellites typically dissolve above the stratosphere where most ozone is contained, the particulate can drift down into the stratosphere in order to react there with ozone, scientist Gerhard Drolshagen, an expert on meteoroid material, told Aluminum oxide will sink to that level and subsequently cause losses.

An Uncertain Future

So, where does all of this leave us? Well, a huge number of satellites are already approved to be launched in coming years&mdashif anything, this study will help us better understand some of the eventual consequences of that. It&rsquos filled with descriptions of existing space-governing laws (or lack thereof) with commentary about what could change.

First, the authors say, is an inadequate policy governing end-of-life rules for satellites. Something stricter would take into account all the factors associated with, like with Elon Musk&rsquos Starlink megaconstellation, a huge array of satellites that are made to frequently be replaced.

Second, the Inter-Agency Space Debris Coordination Committee recommends, but cannot enforce, that satellites include collision avoidance and deorbiting technologies that add cost. It&rsquos easy to see how that cost stacks when tens of thousands of satellites are planned at once.

The lack of a unified body of rules is a huge problem, the researchers write:

The researchers also say the high number of planned satellites is a threat to simple astronomy, because of the light pollution and effective sky clutter created by these satellites. &ldquo[T]here is little recognition that Earth&rsquos orbit is a finite resource, the space and Earth environments are connected, and the actions of one actor can affect everyone,&rdquo they conclude. &ldquoUntil that changes, we risk multiple tragedies of the commons in space.&rdquo

Deposition Method Developed for Telescope Mirrors

Researchers based out of the University of California Santa Cruz have begun developing a protective coating for telescopes by reworking a method commonly used in microelectronics.

"It turns out that improving the performance of mirrors is all about thin-film materials, and that's what I do. So then I got hooked," said Nobuhiko Kobayashi, a professor of electrical engineering in the Baskin School of Engineering at UC Santa Cruz.

Working alongside astronomers Joseph Miller, Andrew Phillips and Michael Bolte, Kobayashi received funding for the research project from the National Science Foundation, along with support from UC Observatories director Claire Max. All of this is geared toward the development of protective coatings for large, silver-based telescope mirrors.

Telescope Coatings

Most telescopes use aluminum instead of silver for the reflective layer of the astronomical mirrors, despite silver&rsquos superior reflective properties.

Researchers based out of the University of California Santa Cruz have begun developing a protective coating for telescopes by reworking a method commonly used in microelectronics.

"Silver is the most reflective material, but it is finicky to work with, and it tarnishes and corrodes easily," Phillips said. "You need barrier layers on top that can keep anything from getting through to the silver without messing up the optical characteristics of the mirror."

Telescopes already in operation could become more efficient and cost-effective if their mirrors were recoated in silver, noted Bolte. He went on to add that the new coating would effectively make the mirror bigger.

"The reason we want bigger telescopes is to collect more light, so if your mirrors reflect more light it's like making them bigger,&rdquo he explained.

Coating Development

The new coating being developed at UC Santa Cruz could make this possible, according to the university, using the technique called atomic layer deposition. The process builds a thin film of material over time with consistent uniformity, thickness control and substrate surface conformity. After a pilot study, the use of ALD resulted in better protective coatings for samples of silver mirrors, rather than more traditional physical deposition methods.

"Atomic layer deposition performs significantly better," Phillips said. "The problem is that the systems used in the electronics industry are designed for silicon wafers, so they're too small for a telescope mirror."

Sizing Up Technology

During the pilot study, the team used an ALD system at Kobayashi&rsquos lab that was designed for microelectronics, which later inspired them to design a larger setup that could accommodate telescope mirrors. After filing for a patent and finding Structured Materials Industries, a Piscataway, New Jersey-based thin-film deposition systems manufacturer, the team had its new system in the lab by July of this year.

Working alongside astronomers Joseph Miller, Andrew Phillips and Michael Bolte, Kobayashi received funding for the research project from the National Science Foundation, along with support from UCO director Claire Max. All of this is geared towards the development of protecting coatings for large silver-based telescope mirrors.

Currently, the system can accommodate a mirror that is 0.9 meters in diameter, but, Phillips noted, there is no reason that the system cannot be scaled up to cater to larger mirrors. The mirror segments for the Thirty Meter Telescope will be 1.4 meters across, for example. The twin Keck Telescopes in Hawaii, where UC Obersvatories is a managing partner, are comprised of hexagonal segments that are 1.8 meters across, totaling in 10-meter primary mirrors.

Research Motivation

Using silver on TMT mirror segments has been a major motivation in the team&rsquos research on new coating technologies, Bolte said. The researcher also anticipates that the coating will be used on existing telescopes, given that traditional aluminum-coated mirrors last about three to five years before recoating becomes necessary.

"We hate to lose telescope time, and we lose a lot of nights recoating segments at Keck," Phillips said. "We'd like to have a silver coating that could last five to 10 years."

Currently, researchers are using physical deposition to coat mirror blanks in silver, along with an initial barrier layer to protect the silver while it is transferred to the ALD system. From there, ALD is used to form final barrier layers.

"Right now, it's a hybrid process, but we're following the development of atomic layer deposition for the silver coating as well," Phillips said.

This research could have an impact on astronomy, according to Bolte.

"This is the last trick we have to make existing telescopes more efficient," he said. "It could really make a big difference."

Previous Research

A team of researchers at NASA&rsquos Goddard Space Flight Center has been working to develop the perfect coating to protect highly reflective aluminum telescope mirrors that the agency said are sensitive to three bands of wavelength: infrared, optical and far-ultraviolet.

Aluminum has been the key to developing the mirrors themselves, but the problem the team encountered was that the aluminum substrate easily oxidizes, leading to a loss of reflectivity.

NASA said that one approach involved the physical vapor deposition of a thin layer of xenon difuoride gas onto the aluminum surface. The fluorine ions that are formed then prevent the aluminum from oxidizing.

Maksutov-Cassegrain Telescopes

A Questar 3.5″ Maksutov-Cassegrain telescope.

Small Maksutov-Cassegrain telescopes are a godsend for urban observers who need a compact scope with good optics. And “Maks” are back in style, so there’s a good selection on the market. But Maks aren’t for everyone. Here’s how to tell if a Mak is right for you .

Maksutov-Cassegrains are another type of compound telescope, similar to Schmidt-Cassegrains. They have a spherical mirror to collect light and a curved lens up front to correct for aberrations. But the corrector lens on a Mak has a simple spherical curvature which is easy to manufacture. And the secondary mirror is simply a thin layer of aluminum deposited on the back of the lens. So unlike a Newtonian or SCT, a Mak requires no alignment.

The downside of the Mak’s optics? To keep aberrations small, Maks are made with a long focal ratio… typically f/12 to f/15. That means you get a higher magnification with a given eyepiece and a narrower field of view than with an f/10 Schmidt-Cass or f/6 or f/8 Newtonian. So Maks aren’t great if you want wide, sweeping views of the Milky Way. They’re much better for objects that require high magnification such as planets, the Moon, double stars, globular clusters, and planetary nebulae.

A Skywatcher 150mm (6″) Maksutov-Cassegrain telescope on an equatorial mount.

Maks are great for urban observers for two reasons. They are compact and easy to transport. And the higher magnification will darken the washed-out city sky and bring out more contrast in deep-sky objects.

They are rugged and robust, so Maksutov-Cassegrains are used in harsh environments in industrial and military applications. More than a few field photographs in National Geographic have been made with Maks. But because the corrector lens on a Mak is quite thick, these scopes get heavy at higher apertures. That’s why you won’t find commercially-made Maks with apertures larger than 7 inches (175 mm).

The most famous (and expensive) Maksutov telescope is the Questar. First made in 1954, a Questar is like a fine Swiss watch. These scopes have superb mechanics and razor-sharp optics almost without aberration. Questars are widely used for terrestrial observing and nature photography. And NASA used Questar telescopes on its early space missions. They are, however, outrageously expensive for their small apertures. A basic 3.5″ Questar costs $4,000 and up.

Less elaborate Mak-Cass scopes, such as the 6″ Skywatcher version shown above, go for just over $1,000 (without the mount). That’s still more expensive than a Schmidt-Cass of the same size.

Is there an oxide layer on aluminum reflectors for telescopes? - Astronomy

Astronomical telescopes continue to demand high-endurance high-reflectivity silver (Ag) mirrors that can withstand years of exposure in Earth-based observatory environments. We present promising results of improved Ag mirror robustness using plasma-enhanced atomic layer deposition (PEALD) of aluminum oxide (AlO x ) as a top barrier layer. Transparent AlO x is suitable for many optical applications therefore, it has been the initial material of choice for this study. Two coating recipes developed with electron beam ion-assisted deposition (e-beam IAD) of materials including yttrium fluoride, titanium nitride, oxides of yttrium, tantalum, and silicon are used to provide variations in basic Ag mirror structures to compare the endurance of reactive e-beam IAD barriers with PEALD barriers. Samples undergo high temperature/high humidity environmental testing in a controlled environment of 80% humidity at 80°C for 10 days. Environmental testing shows visible results suggesting that the PEALD AlO x barrier offers robust protection against chemical corrosion and moisture permeation. Ag mirror structures were further characterized by reflectivity/absorption before and after deposition of AlO x barriers.

RFO Telescopes and Equipment

Reflector Telescope

The West Wing houses a 40″ reflecting telescope. This telescope’s focal ratio is f/3.6, with a focal length of 3,600 mm. The entire telescope weighs about one third of a ton. It is supported below the building by three concrete piers.

Mounted on the side of the giant 40″ scope is an 8″ reflecting spotting scope which also functions for observing. There is also a Telrad finder. The telescope is an alt-azimuth mounting with a computer drive integrated with planetarium software so that the operator can choose a target and command the telescope to slew to that point in the sky. The software of the drive keeps the altitude and azimuth of the telescope constantly adjusted so that it tracks an object in the sky.

The telescope mirror was purchased from Jeff Baldwin. The Project 40 team finished rough grinding, fine grinding, polishing and figuring the mirror on a hand-built polishing machine. Optical testing used a large zone mask for initial figuring and two years of interferometric measurements and figuring to reach the final curve. The telescope mirror was coated with NASA-grade aluminum and aluminum oxide as a donation by Viavi (formerly JDSU and OCLI) in Santa Rosa. The mount was hand-crafted of tubular steel welded together to form a rocker box. The altitude and azimuth drive design uses a SiTech controller and interface to a Windows 7 computer running TheSky X for finding and slewing to objects. Each axis has a computer-controlled server motor plus gear train with the required torque to move the massive mounting.

How it works: The light from an object enters at one end of the telescope and travels the length of the tube where it is reflected off a 40″ diameter parabolic mirror. The light then travels back in the opposite direction where it hits a tilted secondary mirror near the end where the light first entered. The secondary is set at an angle of 15º. The light is reflected from the secondary mirror to a diagonal tertiary mirror just outside the telescope tube, and is again reflected to the final direction through a coma-correcting lens stack and an eyepiece for use by the observer.

Refractor Telescope

The white dome houses a two-meter-long refractor. Attached to the side of the telescope are a spotting scope and a Telrad finder. A tracking motor moves the telescope to counteract the Earth’s rotation, so that an object in the eyepiece will stay there over a period of time as the Earth moves. This telescope made its way to RFO from Dominican University in San Rafael.

How it works: Light enters at one end of the telescope through an 8″ piece of refracting glass which slightly bends the light. The light rays converge together at the opposite end of the telescope where a small 45º mirror directs the light out to an eyepiece.

Robotic (CCD) Telescope

The East Wing houses a beautiful Ritchey-Chretien 20” telescope, donated to RFO by the University of San Francisco in 2016. The RC design is the most popular choice for professional observatories: even the Hubble Space telescope is of the same design. This telescope is not set up for visual observing: camera equipment is attached to the back of the scope, and RFO docents use the system to do astrophotography and research. The entire system is controlled by computer software.

This sensitive camera system (called a “CCD” for charge-coupled device, referring to the camera’s chip) is capable of imaging objects hundreds of millions of light-years away. It can also image asteroids and comets to help ascertain their positions and orbits, and can provide data on a star’s light, giving astronomers insight into the star’s physical properties. Contributions to professional research projects are made with data generated from image analysis. Take a look at our Astrophotography and Research projects.

Mount: AstroPhysics GTO3600
Camera: deep-cooling Apogee U42
Spectroscope: DSS7 mounted on an SBIG STl-8XME
Autoguider: Lodestar Autoguider mounted on a 400mm refractor
Filters (9): Optec LRGB, Ha, and photometric BVRI

How it works: In the diagram at right, light enters from the left and reflects off the primary mirror at the rear of the scope, bouncing back to the secondary mirror. The light is reflected back through an opening in the primary mirror, coming to focus inside the camera.

RFO thanks the University of San Francisco for their generous donation of this equipment.

Solar Telescopes

RFO has two methods for observing the Sun—safe direct observation through heavily filtered telescopes, and radio antennae which record fluctuations in activity.

The telescope at right rear is a powerful 80mm Lunt refractor with a red hydrogen-alpha filter. It images the solar chromosphere (atmosphere) and shows prominences, filaments and granules.

The nearer scope is an 80mm Orion refractor with a neutral density filter. It images the solar photosphere (surface) and shows sunspots in high detail. Our “Sun Spotter” apparatus is in the center.

How they work: The light path diagram for our 8″ Refractor applies to both these refractors.

Funds for the Lunt telescope were provided by the California State Parks Foundation and private donations. The telescope was dedicated in memory of beloved observatory docent and solar astronomer Merlin Combs.


The Radio Telescope consists of antennae attached to the Observatory’s walls which feed radio and audio signals to the indoor classroom computers.

We can hear the Sun’s activity and see radio graphs projected onto an indoor screen for public observing. Radio astronomer Dean Knight examines the fluctuations appearing on today’s graph.

About Telescopes

There are three basic types of telescope design (refracting, reflecting, and catadioptric), and the Robert Ferguson Observatory operates all three types.

Size refers to the primary mirror or refracting glass used in the telescope–not the length of the telescope or the magnification. The larger the primary mirror or refracting glass, the more light is gathered. The “object”—a galaxy, for example—appears brighter to the observer.

Magnification is dependent on the focal length of the telescope and the specific eyepiece used.

Filters of various types can be used to help emphasize certain features of the object being observed. Oddly enough, a green filter can bring out surface features of the red planet Mars.

Other devices attached to a telescope may include one or more “spotting scopes”—smaller telescopes that provide a wider field of view to assist the operator in finding objects. Telrads are devices which generate a red laser ring “target.” When the operator moves the telescope so that the desired object is centered in the target, it will also be visible in the telescope’s eyepiece.

Title: Plasmonic Three-Dimensional Transparent Conductor Based on Al-Doped Zinc Oxide-Coated Nanostructured Glass Using Atomic Layer Deposition

Transparent nanostructured glass coatings, fabricated on glass substrates, with a unique three-dimensional (3D) architecture were utilized as the foundation for the design of plasmonic 3D transparent conductors. Transformation of the non-conducting 3D structure to a conducting 3D network was accomplished through atomic layer deposition of aluminum-doped zinc oxide (AZO). After AZO growth, gold nanoparticles (AuNPs) were deposited by electronbeam evaporation to enhance light trapping and decrease the overall sheet resistance. Field emission scanning electron microscopy and atomic force microcopy images revealed the highly porous, nanostructured morphology of the AZO coated glass surface along with the in-plane dimensions of the deposited AuNPs. Sheet resistance measurements conducted on the coated samples verified that the electrical properties of the 3D network are comparable to that of the untextured two-dimensional AZO coated glass substrates. In addition, transmittance measurements of the glass samples coated with various AZO thicknesses showed preservation of the highly transparent nature of each sample, while the AuNPs demonstrated enhanced light scattering as well as light-trapping capability.

New mirror-coating technology promises dramatic improvements in telescopes

Materials scientist Nobuhiko Kobayashi wasn't quite sure why the astronomer he met at a wine-tasting several years ago was so interested in his research, but as he learned more about telescope mirrors it began to make sense.

"It turns out that improving the performance of mirrors is all about thin-film materials, and that's what I do. So then I got hooked," said Kobayashi, a professor of electrical engineering in the Baskin School of Engineering at UC Santa Cruz.

The astronomer was Joseph Miller, former director of UC Observatories (UCO), whose interest led to a thriving collaboration between Kobayashi and UC Santa Cruz astronomers Andrew Phillips and Michael Bolte. With funding from the National Science Foundation and support from current UCO director Claire Max, the researchers are developing new protective coatings for large silver-based telescope mirrors by adapting a technique widely used in the microelectronics industry.

According to Phillips, most astronomical telescope mirrors use aluminum for the reflective layer, despite the superior reflective properties of silver. "Silver is the most reflective material, but it is finicky to work with, and it tarnishes and corrodes easily," he said. "You need barrier layers on top that can keep anything from getting through to the silver without messing up the optical characteristics of the mirror."

Existing telescopes could substantially increase their efficiency by recoating their mirrors with silver instead of aluminum. "It is by far the cheapest way to make our telescopes effectively bigger," said Bolte. "The reason we want bigger telescopes is to collect more light, so if your mirrors reflect more light it's like making them bigger."

The new coating technology being developed at UC Santa Cruz could make that feasible. The researchers are using a technique called atomic layer deposition (ALD), which gradually builds a thin film of material, one molecular layer at a time, with excellent uniformity, thickness control, and conformity to the surface of the substrate. In a pilot study, ALD provided much better protective coatings for silver mirror samples than traditional physical deposition techniques.

"Atomic layer deposition performs significantly better," Phillips said. "The problem is that the systems used in the electronics industry are designed for silicon wafers, so they're too small for a telescope mirror."

The results of the pilot study, which used an ALD system in Kobayashi's lab designed for microelectronics, convinced the team to design a larger system that could accommodate telescope mirrors. They filed for a patent on their concept and found an equipment vendor willing to work with them to build the system. The vendor, Structured Materials Industries (SMI) in Piscataway, New Jersey, makes thin-film deposition systems for the microelectronics industry.

"We gave them the concept and our requirements, and they did the engineering design work and fabrication," Kobayashi said.

The new system was delivered to his laboratory in July and has performed well in initial testing. The researchers will use the system to demonstrate that it works for telescope mirrors and other large substrates and to continue perfecting the coatings. The system can accommodate a mirror up to 0.9 meter in diameter, and there is no reason the design could not be scaled up to accommodate even larger mirrors or mirror segments, Phillips said. The 10-meter primary mirrors of the twin Keck Telescopes in Hawaii are composed of hexagonal segments 1.8 meters across, and the mirror segments for the Thirty Meter Telescope (TMT) will be 1.4 meters across.

According to Bolte, the desire to use silver on the TMT mirror segments is a major driver of their research on new coating technologies. But he expects the technology will also be used to recoat the mirrors of existing telescopes. An aluminum-coated mirror lasts about three to five years before it needs recoating, a process that puts the telescope temporarily out of action.

"We hate to lose telescope time, and we lose a lot of nights recoating segments at Keck," Phillips said. "We'd like to have a silver coating that could last five to ten years."

At this point, the researchers are using a physical deposition process to put the silver coating on the mirror blanks along with an initial barrier layer to protect the silver while the mirror is transferred to the ALD system. Atomic layer deposition is then used for the final barrier layers.

"Right now, it's a hybrid process, but we're following the development of atomic layer deposition for the silver coating as well," Phillips said.

Bolte said the new technology could have a big impact in astronomy, in the same way that the advent of digital detectors to replace photographic plates gave new life to small telescopes throughout the world several decades ago. "This is the last trick we have to make existing telescopes more efficient," he said. "It could really make a big difference."

Watch the video: DIY Vacuum chamber for telescope coating mirror03 (February 2023).