[ATM] Re: Second Thoughts on Mirror Support

[jonah at eecs.berkeley.edu (Jeff Anderson-Lee)]

Don D'Egidio wrote:

>The proof is in the seeing, pun intended. :-) I didn't want to start a 'you can't do that, or you can't do this' kind of discussion. The mirror in my Discovery TD shows no problems with being RTV'd in place. Many in our club have looked through the scope and have the same opinion. The original poster asked what would happen in his situation with using RTV and a 9 point flotation system. I gave him my honest experiences with using my scope for 3 years now, without a flotation system. I will keep the mirror RTV'd for now.
>  
>
I'm glad you did start this discussion. It could be that for some cases, 
multiple (>3) RTV pads on a solid substrate may be easier and cheaper 
and quicker to do, yet give perfectly reasonable results.  The trick is 
in properly quantifying the "some cases".  We have a proof of existence 
case with your mirror.  Now we just need to spin the numbers a bit.

>"Peter" <peter@kitgear.com> wrote:
>
>>... RTVing an optic is a terrible thing to do.
>>    
>>
Not always, as many RTV'd optics owners know.

>>"IMHO gluing a mirror to a solid substrate having
>>a different CTE is asking for trouble. Mounting a
>>commercial scope this way is just a way to save $."
>>
RTV isn't glue.  (for the umpteenth time.)  Also, plate glass and steel 
have about the same CTE (coefficient of thermal expansion), so RTVing a 
plate glass mirror to a steel plate might not be unreasonable under some 
circumstances.

Jeff Anderson-Lee