Mirror Life expectancy in a Schmidt-Cassegrain

[Stardumb (Nigel)]

Hello stargazers,

As a newbie, I am going through that phase that obviously you all once went through regarding purchasing your first scope.

As much as I would love to buy a new scope, being unemployed, I need to be a bit frugle when it comes to spending money on an instrument. Hence, I probably will end up buying a second hand `scope`.

I have read all the relevant guides regarding choosing a scope, but would like some clarification regarding the mirror life expectancy, in particular of Schmidt-Cassegrain design telescopes.

Most of the sellers of second scopes, quote that the scope has had limited useage . If a Schmidt-Cassegrain scope was say, 9 years old, with extremely limited use, could I expect that the mirror would still be in good condition? How could I check/test for imperfections in the mirror coating of a Schmidt-Cassegrain?

regards,

Nigel

[erick (Eric)]

I get the feeling that it depends on the quality of the original mirror. I bought a many-hand C8 that is probably early 80s vintage. It would seem to be the original mirror, unchanged. I think it looks great and performs well.

I understand that SCTs, being enclosed, should keep their primary and secondary mirrors in good condition for longer than reflectors - presuming the inside is kept dry and moisture isn't allowed to build up?

Edit:- put the call out for an expert from the site to come with you to check out a prospective purchase?

[astroron (Ron)]

I bought My SCT 8" in the USA in September 1990 and the mirror is as good as the day it was purchased.
I have observed with it over a couple of Thousand hours over the last nearly 19 years and it is still in good nick
So to answer your question, there should be no problem with an old SCT if it has been looked after
Cheers

[Blue Skies (Jacquie)]

I've seen a lot of old SCTs with perfect coatings on their primaries. The ones I haven't have either had the dust cap left off the back for a long time, or someone removed the corrector plate and had a fiddle with things inside (Why, I'm not sure). Usually its just the corrector that gets dirty. I don't think I've ever heard of an SCT primary having to get recoated, probably because they are quite well protected inside the tube.

[Don Pensack]

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.

[erick (Eric)]

Thanks Don, that is useful information.

Like an old CRT TV set - looked fine for years until you put it against a new one and realised the deterioration that had occurred, un-noticed.

[Barrykgerdes]

Originally Posted by erick

Thanks Don, that is useful information.

Like an old CRT TV set - looked fine for years until you put it against a new one and realised the deterioration that had occurred, un-noticed.

I bet the program material now seen on the new set has deteriorated more than the old CRT TV. Now that is something I do notice!

Baz

[renormalised (Carl)]

Originally Posted by Barrykgerdes

I bet the program material now seen on the new set has deteriorated more than the old CRT TV. Now that is something I do notice!

Baz

I wouldn't even think of sullying a good plasma or LCD screen TV with the commercial channels. SBS or the ABC are fine...pay TV is fine, but 7,9 and 10 aren't worth the effort. Especially 10....trash TV. I was going to say lowest common denominator, but that'd be an insult to the denominator!!!!.