>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).
Check "Amateur Telescope Making- Book 3" page 349 for a non-vacuum method. Excell can be used to graph out the petals and they can be scaled on a photocopier. I just played with this and didn't make a corrector but it looks do-able.
As for the vacuum method you may be surprised how cheep it is. I picked up a piece of steel scrap 3/4 in thick, 12in dia for free at a steel yard. Al will cost about $1.50/lb if you have to buy it ( again from a scrap dealer). My lens 8" lens cell, more complex than the pan, was $20 (as I recall) to have turned. If you don't know someone with a lathe, go to a small "back yard" machine shop, and you'll probably have to pay cash. For an 8" pan you should be able to have the whole thing done for under $30.