If you are only refiguring, and not grinding, you probably don't need to bother casting a tool from plaster. I'm currently refiguring an 8" F6 mirror than is > 1/2 wave undercorrected. I took an 8" diameter of formica counter top (leftover from a dob construction project) and bonded it to another piece of 3/4" plywood with standard wood glue. I then poured the pitch lap on the formica surface and channeled it with a soldering iron. The lap will conform to the curve of the mirror and there should be no problems unless you have a very fast mirror. (sounds like yours is about F7, so, no worries) Don't try to get away with one layer of 3/4" wood - it's probably not stiff enough. At least two layers (1.5" total thinkness) is necessary. Oh yea, it's a very good idea to seal the exposed wood with something (varnish or water seal works OK). ________________________________________________
Greg Granville - N3SPE - (day)814-865-3310 ARL-PSU gag1@psu.edu (eve)814-339-7244