> So anyways, I have this piece of plate glass about 190mm in diameter
> and I think about 7mm thick. It used to be a solar filter, but is
> now stripped. It is very flat and has no wedge, but it has a fair
> amount of small scale surface roughness which must be cleaned up.
How abut birefringence ? How homogenous is the glass (try it with polarizer)? In any case, 7mm is not that bad for a window 190mm dia. I'd try to polish with glass on top (pitch will support the window well), use very slow strokes, and avoid contacting the glass with hands (use just fingertips around the edge). In any case, use soft pitch, and do very frequent cold pressing (NO weights - just glass!). I'd use rouge as well.
> I know it is pretty thin, but I will be
> using a 12mm minor axis diagonal, so it will be nearly weightless.
> Maybe it will work, mayby it won't.
Don't be afraid of sagging - it doesn't matter for the window. You can deform the window quite a bit without affecting the wavefront at all - it is thickness difference which matters, NOT the flattness.
Bratislav