> (For those interested in the mathematics, how much force is needed to
> deflect a 3/8 thick * 6" square disk, supported along two opposite edges, by
> 1/4 wavelength in the middle?
> Taking E = 7 * 10^10 N/m2 and dc = 5pL^4/384EI for a uniformly loaded beam
> (load p per unit length, dc = central deflection) I get 2.4 Newtons (8.6
> ounces). The lap is likely to weigh at least this much so, unless you have
> an optically flat surface to support your mirror while polishing, it will
> flex all over the place).
Fortunately, things are not that bad as they look. Think about two perfect parabolas that differ in focal length fraction of mm. Which one is "off" by 1/4 wave ? In othe words, your mirror is allowed to flex, but as long as it is flexing fairly uniformly, it won't matter that much. In my experience, a 10mm (3/8") for a 6" mirror is not that bad. Just use good support (piece of old carpet will do), and rotate the work often. And NO pressure in final stages of polishing, of course.
Bratislav
PS start with few long f/ until you gain experience/confidence.