March 11 General Meeting: Mirror Making and Testing

Saturday @ 8 p.m.

A telescope mirror must conform to a particular shape very accurately, down to a fraction of a wavelength of light over its whole surface. So it may seem surprising to hear that you can grind and polish your own mirror to these tolerances yourself, by hand, using very simple tools and techniques. Richard Ozer and Paul Zurakowski, of the Chabot Telescope Maker's Workshop (associate director and optical testing guru, respectively), will lead us through the process in our March meeting. They will cover a number of related issues: Why would you want to do it yourself? How is it done? What are you getting yourself into? Why does it work?

The secret of getting a good mirror is the testing and figuring stage, wherein you determine how your mirror surface differs from its desired shape, and correct it. Paul will show us how the Ronchi and slitless testers work, and how to interpret what you see with them. The next step is to use that information to guide your choice of corrective strokes. They will tell us how to do that, but Richard will also show us how to use a computer program to interpret what you see.

The Ronchi and slitless tests are generally considered to be qualitative, not quantitative measurements. But Paul has correlated his results with quantitative measurements from interferometry and the caustic test, and found them to be good to about one-tenth of a wavelength of light, which makes for an excellent mirror.

After the presentation, Paul and Richard will have a mirror and tester set up so that you can get first-hand (first-eyeball?) experience with what you see through the tester, and what that tells you.