- Silicone (I think mine was GE brand) sealant--uses acetic acid to keep it from curing in the tube and smells like it while it cures. It comes in plain translucent as well as in several other "designer" colors (white, almond, etc.). I chose the translucent version. The package claims that this stuff is guaranteed to remain flexible for 35 years. After two days the pads were well cured, but still slightly flexible. The material was well bonded to both the glass and metal surfaces. This was my choice for mounting the new secondary.
- Silicone II (also GE brand) sealant. This one didn't seem to smell as bad as the original Silicone. It came in several colors, one of which was black. I chose the black because I thought it would look better with all the rest of the black parts at the front of my scope. It is guaranteed to remain pliable for 50 years. In my test, the stuff seemed a bit too soft even after two days of curing--much softer than the original silicone. It was well bonded to both the metal and glass, but I was concerned that it might deform too much as I moved my scope around. The 2.6" secondary I have is fairly heavy.
- Adhesive Caulk (in a large squeeze tube, not the standard caulking tube). I used the standard white version of this. It was easily the worst choice of the three I tried. While the outer parts of the "pad" dried and got somewhat "crusty", the center portion never dried, even after several days. Maybe I'll use it in the bathroom someday, but it won't get near my telescope!
Once I had chosen an adhesive, I began removing the existing Coulter secondary from the 45 degree metal bracket. This proved to be much more difficult than I expected. Here's a cruddy ASCII diagram:
Side view:
___ /##/ /##/ /##/ /##/ /##/*/ /##/*/ /##/*/<--+-------metal bracket /##/*/ | /##/*/ v /##/*/_________ /##/ ^ /##/ +-------- bracket to "bipedal spider" mounting slot ---
# = glass mirror * = adhesive(s)
Rear view (it's really circular from this angle :-)):
________
/ \
/ \
/ \<----mirror
/ \
/ xxxxxxxx \
| +--------+ |
| x| |x |
| x| |x |
| x| bracket|x |
\ x| |x /
\ x|________|x /
\ +--------+ /
\ xxxxxxxx /
\________/
X = mystery adhesive fillets
There were fillets of some sort of spongy soft stuff around the bracket edges next to the mirror. It looked like shoe goo. After I cut through this rubbery stuff and peeled most of it off the assembly I could see foam-core double sided "tape" actually bonding the mirror to the bracket. This was very difficult to seperate and left rubber-cement-like stuff on the back of the bracket after the foam was scraped away. The back of the secondary had a 4mm mesh of some kind of fiber epoxied to the back of it so the foam tape would adhere better. It seems to me that this full-surface bond could easily have been deforming my secondary, especially when the temperature dropped. It took me a while to remove all of this (and the black paint) from the metal which was steel and not aluminum after all! As it is 1/8" thick and high quality I won't be able to use Aart Olsen's suggestion that I drill holes in the bracket to form a better mechanical bond between the metal and the silicone. I used an X-acto knife and a file to clean up the surface which would be bonded to the new mirror. Now for the risky part--attaching the mirror. As Steve Scampini noted, acetic acid and Aluminum are not combatible. Since I just laid out $90 for this new mirror with a 96% reflective coating I did not want to screw it up, so I carefully cut some plastic wrap (from a clean, new sandwich bag) to fit over the front surface and overlap about half the edge of the mirror. I laid the mirror face down on the plastic and taped the edge of the plastic to the side of the mirror to try to seal out the acetic acid fumes during curing. After lots of measuring and figuring (the new mirror is about 1/10" thinner than the original) I marked the bracket location on the back of the new mirror with a pencil. Bob Pfaff recommended using three "pads" of silicone to hold the mirror in place and Bratislav Curcic pointed out that this arrangement should minimize thermal effects as there is more circulation around the secondary and no contact between it and the metal bracket. I took this advice and made three equally sized blobs of silicone on the back of the mirror in an equilateral triangle. I used three toothpicks (plenty of these around for some reason) as spacers and carefully pressed the bracket onto the silicone. The resulting pads were about 1/2" to 5/8" in diameter and about 1/10" thick. I used some lead fishing weights on top of the bracket to hold it in place while the silicone cured. Two days later I carefully tested the bond--it was quite strong! I let the assembly sit for a few more days before removing the plastic seal. In checking the front surface I noticed a few wisps of purplish discoloration near the edge which were not present before I began the operation. I used regular scotch tape to hold the plastic onto the mirror and it did not remain comletely sealed, so I suspect that some acetic acid fumes leaked under the plastic. I doubt the effects will be noticeable, but if I were to do it again I would use thin strips of duct tape and possibly run a fan over it as it cured. There was much stronger dis- coloration on the sides of the mirror which had some aluminum coating on them and which were completely exposed to the fumes, so I would definitely recommend protecting the front surface of your mirror if you use acetic acid based silicone rubber adhesive. I used a "Sharpie" brand black permanent ink marker to carefully color all of the side and back of the mirror and the parts of the bracket which had had the pain removed. I'm too chicken to use brushed-on paint near my optics. After mounting the new mirror in my scope I re-collimated everything and am waiting for the latest weather system to move out of California so I can see the results. Thanks to all who replied to my original message!
Doug Ferrell dferrell@adoc.xerox.com Xerox Desktop Document Systems Palo Alto, CA