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Performance of astronomical mirrors that are exposed to the environment for long periods of time can experience a slow degradation of performance. They usually need to receive some kind of regular maintenance, which may involve cleaning and/or re-coating.

While I'm told that the aluminum degrades and must be replaced from time to time, this is not consistent with my understanding of the behavior of aluminum in air. I'm pretty sure that a freshly evaporated layer of pure aluminum will quickly develop a native oxide layer of perhaps 20Å of aluminum oxide as amorphous $\text{Al}_2 \text{O}_3$ (alumina) which is then pretty much impermeable to diffusion of anything else the atmosphere might expose it to, including various pollutants.

I can understand that it might in some cases ultimately be simpler to strip and replace the aluminum after a number of cleanings due to accumulated damage from the cleaning, but is there any evidence that the aluminum must be replaced due to chemical degradation of the aluminum itself?

I found this article, Reflectivity Degradation Rates of Aluminum Coatings at the CFHT, Magrath, B. 1997, Pub. Ast. Soc. Pac. 109, 303 that seems to show that - at least at one observatory location - a representative test mirror exposed to the observing environment (in the dome) that was washed once a month showed no significant degradation in reflectivity over 32 months, similarly to a mirror that remained protected but not cleaned.

However, the test mirror that was exposed and not washed showed a continuous loss in reflectivity, dropping by about 10% over the same period. A fourth test mirror undergoing regular $\text{CO}_2$ cleaning lost about 5% reflectivity.

This is a small, limited test of course, less than three years, only one location, one wavelength, and only one parameter (reflectivity). But it gives a bit of support to the idea that while mirrors get dirty and require regular cleaning, the aluminum itself may not actually deteriorate.

So I'm asking: Do primary mirrors in large observatories undergo regular removal and re-coating of the aluminum? Why?, where "why" addresses the specific need for the removal and replacement. If these primaries actually do receive regular re-coatings of aluminum, is it because the aluminum actually degrades?

I'm looking for an answer that carries some backup references. If just for example, your answer is of the form You silly goose, everybody knows aluminum tarnishes like the Dickens!, please back it up with a reference to measurements of aluminum tarnishing. Thanks!

From Magrath 1997:

GIF ("blink comparator")

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From http://global.kyocera.com/fcworld/charact/chemistry/chemiresist.html - alumina can potentially be remarkably resistant to chemical attack compared to other materials - note logarithmic scale.

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The individual figures:

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  • $\begingroup$ note: I've already flagged the overlap of this question with the linked question and asked for moderator assistance in resolving the overlap. Here I'm zeroing in on the need to remove and replace the aluminum, and if it's because of degradation of the aluminum itself, the nature of that degradation. $\endgroup$ – uhoh Jul 17 '16 at 3:11
  • $\begingroup$ Don't worry about the overlap. We'll just let both questions stand. They're different and both answers may be useful to someone. $\endgroup$ – called2voyage Jul 18 '16 at 12:21
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    $\begingroup$ This link mentions that on the Hale there are old motors involved in optical accessories that are above the primary, and these deposit oil on the surface. So the environment for that scope is a lot worse than might be expected... I don't really know enough to contribute a good answer though; things might be very different for other observatories. $\endgroup$ – Andy Jul 18 '16 at 12:43
  • $\begingroup$ @Andy thanks for that - very interesting reading and a great data point. Wow every two years! The air in San Diego county can be pretty dirty at times, the altitude is comparably low and the wind probably picks up dirt from exposed soil everywhere, city & traffic nearby, ocean nearby, sometime giant fires nearby. With a lot of scratchy particulates, it's possible that it is too hard to clean the mirror thoroughly (including oil and water spots) without scratching it in the process. I'm really after the cause and need for the metal replacement, and my hunch is that it's damage from cleaning. $\endgroup$ – uhoh Jul 18 '16 at 13:08
  • $\begingroup$ @Andy I am surprised however by the statement that the coating is 8 Angstroms - that's virtually transparent. I'd expect something like 1000 Angstroms as a ballpark figure. 500 might be OK, but it's hard - especially here - to get reliable uniformity so you usually go long. See this question and this plot in one of the answers. Oh!, here's a screenshot from Google maps with some of the fine San Diego soil, good wine country I hear - oranges too! $\endgroup$ – uhoh Jul 18 '16 at 13:25
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This might not be the type of telescopes your are thinking about, but as IACTs (Imaging Atmospheric Cherencov Telescopes) in the end also measure light in (or near) the optical range, their mirrors are of similar build.

The important arrays (H.E.S.S., VERITAS and MAGIC) have, at least partially, mirrors made of glass or with a glass surfaces, coated on the front surface with aluminium (Al) and an additional single protective layer (e.g. SiO$_2$ , Al$_2$O$_3$ ). But as they are exposed to the environment the whole time without any luxury such as protecting domes or similar, the coating degenerates pretty fast.

In the case of H.E.S.S., which is located in the Khomas Highland of Namibia, the mirrors have to endure all kinds of environmental factors, beginning with the sand from the dessert, extreme temperature changes between day and night to curious birds picking at their reflected images. So after five years the optical efficiency of the mirrors was so bad, that one could actually see ones hand through from the mirror. To prolong the lifespan of the instrument, the mirrors were refurbished with a slightly different aluminium coating. In the following image the time of the mirror recoating is marked with the blue shaded area.

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Image source: https://www.mpi-hd.mpg.de/hd2012/pages/presentations/Stegmann.pdf

The degeneration for the mirrors of the other observatories mentioned above where of similar timescales and they also did some recoating campaigns.

For further information see for example the following papers, which contain some overview about the different possible techniques:

If you ever happen to be in Namibia, go visit the World's largest optical telescope:

enter image description here

Image source: https://www.lsw.uni-heidelberg.de/projects/hess/HESS/hessmultnu.phtml

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  • $\begingroup$ This is really interesting! Before I asked this question, I was used to putting mirrors away in nice clean containers in clean, environmentally controlled rooms. You ordered the coating that gave the desired response and assumed it wouldn't change. I thought the Hale telescope was dirty but yikes - birds! These are good examples, thank you! I will try to look into them, and the links are excellent. $\endgroup$ – uhoh Jul 20 '16 at 18:00

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