"Lloyd E. Sponenburgh" lloydspinsidemindspring.com fired this volley in
. 3.70:

Some ordinary flint glass about 1/2" thick should
do.


Oh... I might add that, you don't need fine precision here. I figured my
12" mirror to a precision parabolic curve accurate to within 1/8th
wavelength of sodium light, but you don't need that level of accuracy.

It would even be possible to laminate thinner glass using two-part
acrylic as the cement. You'd get an aberration ring wherever you
penetrate a layer, but that wouldn't matter too much for this
application.

Lloyd