>RE : Silicone rubber to glue secondary
>...
>To get it off, use knife blade to cut away the silicon, and then use
>solvent to remove the residual silicon bits. If you do this for the purpose
>of realuminizing the secondary, make sure that ALL silicon is removed.
>Otherwise it will outgas in the chamber, and your mirror won't be well
>coated (not to mention nasty calls from aluminizing mob!).
>
Silicone glue holds glass extremely tenaciously, and you really can't dissolve it away. I had a long talk with General Electric lab technicians about this. The glass-to-silicone adhesion is so intimate that you more or less can't get it all off, especially if the glass is rough like the ground back of a secondary. The way to get *almost* all of it off is to repeatedly "raise" the silicone with kerosene, naphtha, turpentine, etc., which it absorbs causing it to swell, and then shave it off with a razor blade. The solvent lubricates the process and it is very easy, not like cutting dry silicone. At every iteration you take more off, but never all of it. Finally you get real close and you can give up. I also had a good talk with Clausing about this very slight remainder outgassing and causing problems with aluminizing and he said close enough is good enough--a couple of microns of glue can't hold very many volatiles. I haven't had trouble with re-aluminizing silicone glued mirrors.
I've never had as much confidence in the silicone-metal bond. I usually drill three small holes in the metal plate and have the glue spots passing through each hole to a wider bead on the other side, so that you get a mechanical hold in addition to the glue bond. On another telescope I glued a bit of fishing line to the back of the secondary and tied it to the spider. I've never had a diagonal fall but you never know!
My favorite silicone glue is the kind that smells of vinegar when it cures. It may be my imagination but the "improved", less smelly kind doesn't seem as strong.
Aart M. Olsen aart@uiuc.edu 217-333-7467 College of Veterinary Medicine Univ of Illinois at Urbana-Champaign