[ATM] Re: Second Thoughts on Mirror Support

[salguod at minbaritm.com (Doug Williams)]

Hi guys,

Here is my rather uneducated opinion.  I am only chiming in because I am
considering RTV for my mirror cell, and I think that Don hit on an important
point.

It seems to me that there are three keys to making a successful RTV cell.
First, you want to glue the mirror to something stiff that has the same
coefficient of thermal expansion as the mirror.  If you can't get an exact
match, you at least need to be close.  Secondly, (as Don points out) when it
comes time to observe, you want to cool your mirror to the ambient
temperature.  So putting the scope out to cool early and/or installing
cooling fans would probably make a big difference with an RTV mirror cell.
Thirdly, you probably want to glue your mirror at the same temperature that
you will do the bulk of your observing.  That would minimize thermal stress
on the system.

Of course, all the already stated considerations such as proper glue
thickness and placement of the RTV also apply.  And a thicker mirror should
also work better with RTV.  Does all this make sense, or am I barking up the
wrong tree?


Doug
P.S.  I'll have to search the archives, but didn't we talk about another
silicone glue a while back that is similar to RTV but that is more flexible?
That would help as well, don't you think?

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf Of
Don D'Egidio
Sent: Wednesday, February 04, 2004 17:04
To: The Amateur Telescope Makers List
Subject: Re: [ATM] Re: Second Thoughts on Mirror Support

Hey guys,

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.

Maybe I don't experience many problems because the telescope is stored in my
shed and is basically at ambient temperature at all times. There is very
little cool down needed, unless the temp is dropping fast during the night.
Even then I don't see any astigmatism caused by the RTV.

Don

----- Original Message -----
From: "Peter" <peter@kitgear.com>
To: "The Amateur Telescope Makers List" <atm@atmlist.net>
Sent: Wednesday, February 04, 2004 15:02
Subject: RE: [ATM] Re: Second Thoughts on Mirror Support


> Andy,
>
> You got it!! RTVing an optic is a terrible thing to do.
>
> "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 $."
>
>
> Andy Saulietis
> ISS Enterprises
> 10 Milky Way
> PO Box 19
> Mayhill NM 88339
>
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>

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