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Ceramic v. Supremax Mirror

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8 replies to this topic

#1 hdavid

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Posted 17 January 2020 - 06:40 PM

Since ceramic glass experiences less expansion / contraction during temperature changes, does the figure and coating hold up longer with a ceramic glass mirror than it does with a Supremax glass mirror?  Seems like that might be the case in theory.  If so, roughly how much longer?



#2 Mike Spooner

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Posted 17 January 2020 - 08:24 PM

No difference. The metal coating generally has much more expansion than either substrate and other factors affect its longevity in measurable time frames. 

 

Mike Spooner


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#3 hakann

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Posted 17 January 2020 - 09:13 PM

Longer ?
A strange question.
Has you the idea of design/glass/labour/coatings costs.
Just silver high end on a a 30” is 1K” USD.
But the good news is bathroom made optics/coatings can be as good or even ’better’ ( as told )
That’s what AMT is all about.

Edited by hakann, 18 January 2020 - 05:59 AM.

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#4 hdavid

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Posted 17 January 2020 - 10:24 PM

Thanks for the responses.  On my 20" order, I'll soon have to make a choice between the two.  I'm not terribly concerned about the extra cost of ceramic since it's pretty big bucks (more than $15,000 with S&H) anyway.  I was just wondering if there were any particular advantage of ceramic over Supremax other than less expansion/contraction and hence less adjustment time.  I'll be storing the scope in my garage, which is fairly well insulated and ventilated.  It will be at or near ambient outdoor temperature most of the time, so it won't be experiencing rapid or extreme temperature changes.  I do all my viewing from my drive way.

 

Any other thoughts on the relative merits of ceramic and Supremax would be most appreciated.



#5 a__l

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Posted 17 January 2020 - 10:55 PM

http://www.reginato.it/indexen.htm

Not enough information.
Do you know who makes mirrors for them?
What ceramic are used?


Edited by a__l, 17 January 2020 - 10:56 PM.


#6 Lucullus

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Posted 20 January 2020 - 06:45 AM

Can Supremax33 be molded into cellular blanks?



#7 a__l

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Posted 20 January 2020 - 09:53 PM

I can express only my opinion.
This is an expensive procedure. What is the point of doing this for Supremax?



#8 Stephen Kennedy

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Posted 27 January 2020 - 03:40 AM

In Georgia, I imagine you have temperature changes similar to what we experience in the Sacramento area of California. they are rarely extreme and never approach what you see in places like New England or the Midwest.  My 210 mm F/7.7 Newtonian has  Pyrex primary and secondary mirrors have never given  me   any problems with temperature changes.  in fact I probably could have gotten just as good results with plate glass mirror substrates,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       .


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#9 TG

TG

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Posted 29 January 2020 - 08:55 PM

Low expansion substrate will cure one and only one problem: the change of correction with temperature. When a glass disk is cooling, the edges cool faster and the center slower. This causes the mirror to be overcorrected.

While real, the above problem is dominated by a boundary layer forming over the warm mirror. So if the mirror doesn't cool down fast enough, boundary currents will persist and ruin the image. Here's where low expansion substrates have an advantage: you can use a fan to blow off the boundary layer and be in business in a short time.

I would go for a thinner Supremax mirror with fans and a good cell than pay top dollar for exotic low expansion materials. Thinner mirrors hold lesser heat and shed it faster so the expansion properties of the material become less important.

Tanveer
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