plaster of paris

[multiweb (Marc)]

I have a question for the engineers out there. My C11 primary mirror is currently held by a stainless steel bush that goes the length of the bore in the glass with an annular gap of ~2mm. The gap is filled with RTV. I'm thinking of getting rid of the RTV and replace it with plaster of paris. The only thing I'm a little worried about is the coefficient of expansion of the glass vs. the plaster which is very low. The idea is to have a bonding agent between the bush and the glass that is stiffer than the RTV, won't stress the mirror and will expand/contract similarly or very close to the glass. any thoughts?

[LewisM]

Plaster will chip, crack and break.

Get a firmer silicone. Mold making RTV silastic can be bought stiffer. I used to cast resin model parts from RTV molds, and you can buy varying degrees of firmness silicones (2 part) from Dow Corning.

[Wavytone]

Definitely use RTV. Anything rigid - dental plaster comes to mind - will create localised strain in the mirror potentially affecting the image.

[multiweb (Marc)]

Thanks for the replies. My beef with RTV is that it bonds to the glass and has an incredible pull when it cures. I wonder if I put a thin layer of paper or plastic between the glass face and the RTV so it doesn't come directly in contact with the glass would do the trick.

[mental4astro (Alexander)]

A smear of valesine on the glass is an excellent separating agent, better than paper. If you are concerned about the oil, you can use something like Gladwrap, again better than paper. You can tack it into place with a little water on the glass and then place the Gladwrap on. Single sheet of the stuff, not crumpled up. Gladwrap is much thinner than paper, non porous and won't degrade. Paper can have residual chemicals in it and will be affected by residual chemicals in the RTV materials. The chemicals in the paper can also affect the longevity & stability of the RTV. Paper will also absorb moisture, which again makes for another weak link in using paper as a separating agent long term for sensitive equipment.

Just be careful with which RTV you use. Many are not stable long term. Some will crack/dry, others realease condensate from the chemical reaction that sets it off and these spread like oil, others release volatiles that are not great for optics.

[multiweb (Marc)]

Glad wrap might work if it doesn't react with the RTV. Celestron used to sandwich cork sheets along with lollypop wood to that effect.

[LewisM]

Or just sell the Celestron

Some RTV's vulcanise with acetic acid (vinegar), so be careful!

Just why would a Frenchman talk about Plaster of Paris...just too patriotic Marc

I'd avoid "paddlepop stick" wood too - maybe some thin laminate would be better?

[multiweb (Marc)]

Originally Posted by LewisM

Or just sell the Celestron

No way. It's a good one.

Originally Posted by LewisM

Some RTV's vulcanise with acetic acid (vinegar), so be careful!

Good point. Will keep some for the salad dressing.

Originally Posted by LewisM

Just why would a Frenchman talk about Plaster of Paris...just too patriotic Marc

Just the stuff I used to play with as a kid and that's the first thing that came to mind (after matches).

Originally Posted by LewisM

I'd avoid "paddlepop stick" wood too - maybe some thin laminate would be better?

Well it makes sense as spacers. It compresses well and it's light.