Have you ever tried to make a convex optical surface?
If so, you know that it’s much more challenging than a concave one, since the rays of light do not come to a focus at all.
Some of us* at the Amateur Telescope Making workshop here in Washington DC have made several attempts at doing this, pretty much without success. I would like to show you some weird images that we got when we tried to ‘figure’ the convex surface by performing a Ronchi test from the back side, looking through what was supposed to be a flat.
What we find is that even though the glass itself is very clear and free of visible strain when seen by the naked eye or when using crossed polarized filters, it looks like we are looking through an extremely murky and totally un-annealed piece of ancient Venetian glass, causing all sorts of weird striations in what should otherwise be nice, smooth Ronchi lines.
These pictures go in order from outside the radius of curvature to inside the ROC.
You might well think that the glass itself has lots of strain left in it, causing the very weird patterns that you see here. I can prove that this is not the case by showing you a short video that we made with crossed polarizing filters of the 5-inch diameter blank itself and two pieces of plastic (the protective covers for one of the filters). Judge for yourself.
This is not the first time that this strange phenomenon has occurred.
Any suggestions from those with actual experience would be extremely welcome.
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* Me, Nagesh K, and Oscar O.
Guy's Math & Astro Blog 