Glueing secondary mirror

[dark star]

I have glued the secondary mirror for the 14 inch dobsonian I have been making to it's wooden holder. I used silicone glue with matchstick spacers so the secondary can be removed again, as recomended in lots of places. The glue is Unibond frame silicone. After leaving it to set for 24 hours as stated on the bottle I can pull the secondary off the holder! Am I using the wrong kind of silicone glue and can anyone recomend a specific type/ brand?

I used three large blobs.

Glad I tried pulling the secondary off before putting my telescope together!

David

[Astrobits]

Did you coat the wood with epoxy first? Silicone doesn't stick to plain wood, or any wood product ( e.g MDF ), very well.

Nigel

[gooseholla]

I did mine two weeks ago. I used three blobs of normal clear silicone, put the spacers in then placed it on the blobs, left it for 24 hours with the mirror pushing down onto the holder, which was held perfectly flat. My mirror cannot be pulled off or moved. Did you do it so that the 45 degree angle was perfectly flat and the mirror pressing down onto the holder, or the holder pressing down onto the mirror?

John

[Tinker1947]

Its properly best to stick to tried and tested ways, but there is a new type of adhesive that's works on just about anything, even under water.....

http://www.orbitalfasteners.co.uk/en/categories/ct1-unique-all-in-one-sealant-construction-adhesive?gclid=Cj0KEQjw_9OoBRChj9vMo5CHrdUBEiQAJ6YRPbAX9Na4XCSTaHC8LZxUNJilOnxzJWKnzHlQdk-zefUaAkUR8P8HAQ

[Peter Drew]

Dow Corning silicone is particularly good. More expensive but you get what you pay for. 

[dark star]

I coated the wood with varnish, could this cause a problem? The blobs of silicone were rather large, maybe they were too large? Also the silicone was white, not clear. RTV silicone is also recomended, is this correct?

David

[Astrobits]

I have always used clear silicone which is specified as useable on GLASS ( as in greenhouses ). I suspect that the frame silicone you used was not intended for glass but PVC frames and was therefore not suitable for this application. I haven't tried varnish as a base coat. I use Quick-setting Epoxy as it cures in a few minutes. A quick roughening of the Epoxy surface with abrasive paper or a file before the silicone is applied. I apply only one blob of silicone at the centre of the flat as they are usually small enough to be held firmly with just the one, about 3/4-1" final diameter, without putting any stress on the mirror.

Nigel

[Ajohn]

I haven't used it for some time but dow corning did 2 grades of clear silicone, sealant and adhesive. The latter is for sticking glass together. It's also probably possible to buy small tubes from aquatic shops - aquarium repair and also for windscreen seal repair from motor factors. The correct stuff has a much stronger smell as it cures than the sealant types.

I've also used double sided tape for this - the type that is sold to hold down carpet  tiles, it's thicker than the usual type. Even that might not stick all that well to bare wood. It worked for at least 10 years.  the scope was sold so  pass after that.

John

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[Horwig]

Silly question, was the silicone fresh?

When I built mine, I used some old silicone left over from building work, that didn't set at all, and the secondary slipped off. Re did it with fresh silicone, (Travis Perkins glazing) and have had no issues.

Huw