Quote:
Originally Posted by multiweb
Thanks. I understand now. All this polishing/figuring business fascinates me but I have no idea how it's done hence the silly questions. By the sound of it in your posts although there is a great deal of experience needed it seems to be an iterative process that can be tedious to achieve a high level of figuring, so there's no free lunch. What I find scary it that it's like sculpting. You can't add back if you remove too much and the potential to stuff up must be getting higher and higher as you're nearing your target figure.
|
Unfortunately figuring is a bit of a black art that cannot be learned from books. It is very difficult to get a full size polishing lap to do exactly what you want it to do. The polishing pitch is a very viscous liquid that is exposed to various amounts of compression and dragging forces that make its behaviour rather unpredictable. A sub-diameter polishing lap is easier to predict but it is not as good at controlling astigmatism. Figuring is not as bad as sculpting because if you dig a hole somewhere, then you just have to polish away the rest of the surface until the hole disappears - not too hard for spherical surfaces but a nightmare for aspheric ones.