Have been working on a pair
of 25" by 1 1/2" thick plate glass blanks, (ex Sea World Aquarium [Napier NZ]). The dolphins had their turn looking through this glass. Now in its recycling mode will be turned to focusing in on the cosmos.
About 85 hours of work (all by hand) have gone into these two mirrors so far, from hog to preliminary figuring. Had intended the pair to be close to parfocal at around f5.15. However my mechanical method for measuring sagittas was perhaps a little
loose. Ended up at f5.3 for #1 and f5.05 for #2. Wasnt planing on a binoscope so no worries.
Had previously ground a 25"f4.68 from 3/4" plate, but have in practice found it difficult to use due to its lack of stiffness. As stiffness goes up with the cube of thickness, the current thicker mirrors have an edge stiffness about 8x , and a central stiffness of over 20x
that of the old 3/4" glass.
Have got both mirrors to about the same stage figure wise. Picture[Ronchi] is of #1 taken outside ROC. Some work to be done but its a start.