> How difficult is it to just grind in the appropriate curve without
> deforming the glass at all? As you may be able to tell, I have a
> real aversion to warping a large piece of glass (not to mention the
> expense of having the vacuum pan made).
It really depends on a power of the plate. If you are making f/3 16" Schmidt camera, you have to grind it in. In many cases you can polish in the desired correction. As a rough rule of thumb, calculate the desired Schmidt shape, and its maximal departure from sphere (or flat, depending what you start with). If it is in order of say up to 5 microns (10 waves), it can be easily done by polishing only. (pitch and Cerium will polish away roughly 2 microns per hour with appropriate pressure, fast polishing pads even more). I'd attempt anything up to 10 micrometers to polish (with ring tool and polishing pads), knowing that you can put the correction on both sides. Above that you have to grind the curve in. Whether by vacuum method, zonal grinding, ring tools, flexible tools, it's up to you. Just for the record, maximal departure of my 6" f/4 Wright corrector plate (more than 2.5 times the power of equivalent Schmidt) was only 6 microns.
Bratislav