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The reflectivity of even overcoated aluminum gradually diminishes over time. There is no question the mirrors in an SCT stay clean longer than in a standard newtonian, but there is nothing magic about the ability of the mirror coatings to stay reflective longer than in a newtonian. Perhaps they don't get damaged by cleaning because they are rarely cleaned, but the coatings deteriorate just the same.
The industry does not have quantifiable data on the deterioration of mirror reflectivity. I've seen estimates ranging from a loss of 5% per year to as low as 1% per year. OMI is undergoing testing right now to actually pin down the truth on the reflectivity loss per year, but until that data is published in several years, the best estimate I've seen was from a metal film coater who did some work for my business, and they estimated a 3% loss per year if the aluminum is well-overcoated with SiO2.
That means:
0 years--100%
3 years--91%
5 years--86%
10 years--74%
15 years--63% This is about the point when most people would see the new coating as brighter.
20 years--54%
Now, as I said, tha actual data isn't in, but the 3% loss per year was considered very conservative by most coaters when I investigated this a few years ago, so the real case might be worse.
The point?
1) If you have a reflector, consider re-caoting the mirrors somewhere between 10 and 15 years or put up with a lot of light loss.
2) Cleanliness and reflectivity are not the same thing.
3) If you paid extra for higher reflectivity coatings, consider recoating more often(say, 10 years instead of 15-20) to keep the reflectivity high (what you paid for in the first place)
4) Closed tubes are not air tight. SCT mirrors deteriorate just like Newtonians. Indeed, I have cleaned organic films off the inside of corrector plates and mirrors in SCTs, so they are attacked by smog, cooking vapors, chemical vapors--the same things that attack the newtonian mirror.
5) The human eye doesn't notice the deterioration when it occurs slowly. The way to check a newtonian mirror (look through the back) can't be done with SCTs. A difference of 30% is considered the minimum to be noticeable (it's about 0.3 magnitudes of loss), so it's not surprising we think older mirrors seem to have their coatings in excellent condition. Coating deterioration is a lot like the deterioration of vacuum tubes in the old high-fi systems--it's only noticable when compared to new. That's good news since it means people can and will be satisfied with coatings that have lost a lot of reflectivity.
So when SHOULD you re-coat? Unfortunately, I can't answer that question exactly right now, but it does seem that there are some valid reasons to do so in less than 20 years. Certainly coatings have improved a lot in the last twenty years, as well, so that if your coatings are older than that you are likely to see a big difference if you recoat. I've seen a new and old SCT side-by-side more than one time, and there was no question the newer scope had a brighter image.
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