Making an 8in f2.6 Pyrex mirror , and at the pitch/lap polish stage, and looking at Ronchi I had a "perfect" sphere showing after 2hours polishing. However I needed to improve polish, so have done another 2hrs TOT and MOT. Have warm pressed lap ( excellent contact ) and been doing 1/3 COC strokes as per sphere. To my horror Ronchi is telling me I,m heading for some other figure of revolution ! I have done nothing different from my first effort so am bewildered. I have done a 10in glass at f3.4 with good results, but this is weird .
My Ronchi photos are "burned out in center" ,too hi an iso, also very bright LED. In fact the bands are straight, even edged, and visually vastly better - except for turned edge!! At the moment best I can do. Outside ROC:-