Re: restart

Steve Strickland (lensnut@tpoint.net)
Tue, 1 Aug 1995 02:56:59 -0500

>I'm going to restart a project i started in 1968 but need some help and
>suggestions. I started a 15 inch 2.5 thick mirror and got to the polishing
>stage
>when i discovered i couldn't get the figure to behave. On closer inspection i
>noticed bubbles in the glass that weren't there before. The glass was
>apparently
>two pyrex discs of 1.25 inch thickness cemented together. When the mirror
>became
>concave on one side it looks like an uneven stress occurred that had a
>tendency
>to pull the discs apart. In 1985 I separated the discs. I would like to finish
>the original mirror which is about f5 and grind the second blank for a long FL
>or about f7 or 8. I saw some people making use of tile tools for grinding.
>What
>kind of matrix is needed to hold the tiles and where can they be obtained?
>Should I try to make a full size tool for the f5 mirror and what can I use
>for
>the base of the tool because I used sub diameter tools originally , tools on
>top. I have enough large ceramic tile squares, 1/4 inch thick and 2-5/8 inches
>on a side. Maybe these are too large. Is it best to put them on plaster or
>concrete or some other base material? Any suggestions welcome
> Buck

Buck, I'd like to have a more detailed description of the figuring defects you're seeing. Are they macro-ripple? Dog biscuit? Astigmatism? Zonal errors? What? Have you checked the blank with cross-polarization? Details may give some clues as to how to proceed.

Making a plaster/ceramic tool is easy. I use dental plaster, Royal Rock brand (it's locally available). I cast it right on top of the optic. I glue 1" unglazed ceramic tiles using Bondo auto body filler. I trim the tiles with a glass cutter. It takes me about 4 hours to make the tool with 3 hours of that being curing time for the plaster.

Seems to me that bonding two pieces of Pyrex together would create a bad situation for optics work. You have undoubtedly done the correct thing by separating the pieces. The drawback is that you are now working with a thin optic, one that is less than 6 to 1 thickness ratio. This means that you will need to take special care in fabrication and mounting of the mirrors.

Please post a detailed description of what's happening to your figure. Perhaps you could attach a small sketch of the changes over time. Please include details of the polishing technique used. 

--
Steve Strickland
Lensnut@tpoint.net