Ok so after much deliberation I decided to go with a long slow newtonian, 10 inch, about f8 or so, as a planetary scope for the school that I work at. This is a step up from my first project in that it is both slightly larger, thinner (19mm) and plate glass. However, I don't anticipate too much trouble with this.
However, as a concurrent project, I would like to polish a flat to use for autocollimation and other tests. It is a 19mm thick piece of plate glass, 10 inch diameter, perforated with a 25mm diameter hole, all water jet cut. Being plate glass, of this thickness/diameter, I know this isn't the best substrate and thickness for a flat of this size. Is this project a waste of time? My plan is to start from about 600 grit with a penny tool, alternating equal amounts MOT and TOT, and just work up to 1200 grit and finally pitch. Does this sound like a workable plan? I would perhaps use the acetone test to check for flatness during grinding-or perhaps even an engineers straight edge with a light.I plan to use the Raleigh water test for final figuring. I have tried making elliptical flats and it was a disaster (I started grinding them from an ellipse instead of a circle), so this project might be overly ambitious..