thin plate mirrors

I'm almost finished figuring a friends 16 inch pyrex mirror and will be making a new scope.

I'm thinking 18 inch f-4.8 is within my capabilities.

The cost and risk in getting a big piece of pyrex here to New Zealand is a worry.

I can get 28mm thick plate at very reasonable cost. (around 1/4 of pyrex cost.)

I'm sure with a decent 18-27 point flotation cell it would work well.

Has anyone else tried a big plate mirror and had success???

I have seen a 20 inch made with 19mm plate which was ridden not surprisingly with astigmatism.
Phil B

Re the edge support I assumed at least 3 points would do am I far off the mark here?

Phil B

Hello Phil,
I have a 19" plate glass mirror, which is 1" thick. It is polished and slightly overcorrected. If I ever get the truss ota together, I'll finish figuring it, No problems have been encountered, the blank was slumped and has a 100% support cell. The final figuring will be done in the cell

One thing to be very careful of with a thin mirror is to not grind astigmatism into it during polishing. I think you will need to do short sessions to avoid distortions from friction heating.

One the cell, I would consider gluing it to the cell and skipping edge supports. The thinner the mirror is, the better this works, since there is more issue with potato-chipping with edge supports of any kind, and less with lateral sagging (front-to-back) with glued supports. Just make sure you make the cell out of material with similar expansion characteristics to the mirror, with enough of a silicone blob to have some give to accomodate what little differential expansion there is, and not out of something that will warp over time (i.e. not wood).

Jarad