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mirror age?
#1
I watched a demonstration and discussion of Skytools 4 on Youtube recently. I already have a Skytools 4 license, but to be honest I'm far from a "power user".

One topic that interested me was discussion of how the age of the telescope's mirror affects image quality. I hadn't really thought of that before. It makes sense to me, for example with a Dobsonian. However, one of the OTAs I use is an SCT. The mirrors are in an enclosed space that 's barely exposed to the outside world, whereas the corrector pane is exposed to the elements.

In this case, is it more likely to be the age/state of the corrector corrector pane that affects performance, rather than the mirror?
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#2
Hello,

The answer is that you are probably right, but nobody really knows, because it hasn't been thoroughly studied. I would suggest treating SCTs like refractors until we find a means of obtaining more data. The companies that put commercial coats on mirrors make claims about durability but they have offered no data to back these claims up. Professional observatories do not coat their mirrors with a protective layer, opting instead to re-aluminize regularly.

There are only two studies that I am aware of regarding optical degradation over time. As part of the development of the interferometer at MRO (MROI) (http://www.mro.nmt.edu/about-mro/interferometer-mroi/) they did an experiment with small commercially coated mirrors that were opened to the sky on clear nights. They tested the reflectivity over time and derived an equation that would predict the decline. But the mirror company they were working with went out of business before they could run more experiments. As a result, there is no way to disentangle the accumulation of dirt on the mirror from degradation of the coating.

My study is the other one. In my case I used many different telescopes at iTelescope.net and discovered that the pure refractors delivered the signal expected by my model, but that the open tube reflectors did not. For the reflectors I found a relationship between the amount of light lost and the time since they were last washed, which more or less matched the model derived at MRO.

I have a Dob with an 18-year-old commercially coated mirror that has obviously degraded. Others report similar findings. So we know it is more than just dust on the mirrors, but actual degradation. Whether or not similar degradation occurs inside a closed tube is a really good question.

What I would love to have is a handheld device that could measure the reflectivity of a mirror. I'd use it at star parties to sample mirrors and collect information about when they were manufactured and when they were last cleaned.
Clear skies,
Greg

SkyTools Developer
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#3
Thanks Greg, that's very interesting. Mirror cleanliness isn't the limiting factor in *my* imaging, but it's good to understand as many factors as possible.
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