Hi grinders/polishers/ and figurers,
Just like to report that I have finished figuring, (first iteration),the 25" f4.72 by 3/4" thick float glass mirror, after 33hrs of fighting all the contortions
that astigmatism could throw at me.
The present spell of cool (7-8deg C) damp weather, has given me the ideal conditions to successfully use my #40G pitch laps, to gently move the mirrors figure in the right direction.
(Neither of the 22"ers presented such a challenge, though "stig" did present in both but not so aggressively).
When using the 85lpi Ronchi test I found the 25"er very sensitive to the position of edge
supports while on the testing rig, (if not exactly +-45deg from mirror bottom then the mirror would show physical warp as
astigmatism). The further the edge supports from the ideal, the worse the stig appeared to be, but when placement of supports was
correct, stig disappeared. Neither 22" was as sensitive to edge support placement.
Testing is done at night with temps around 5deg C so as to approximate those temps which one would experience, when mirror would be working when night viewing in my part of the world, Autumn/Winter/Spring seasons particularly.(Dosent get really dark here in Summer anyway). Now just waiting for the weather to clear so star
testing can be undertaken.