[ATM] testing flats with a green laser

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

This evening at the NCA-CCCC telescope making workshop, as usual, Jim McPherson brought in an interesting project. He had purchased a roughly 10 inch quartz opitcal window that was supposed to be 1/10 wave (or something like that) and was wondering if we had a way of testing that claim. Well, I had received a gift from some folks at NASA-Goddard of a heliostat complete with a 12 inch flat mirror. But I didn't know how flat that was, either. In any case the 12 inch flat had a deteriorated aluminum coating, and we couldn't see any lines when we put one on the other. Plus, I had neglected to bring in the monochromatic light box that I had made - it's in my garage. The aluminum coating on the 12 inch flat swamps any interference fringes.
So, I decided to kill a couple of birds with one stone, as it were. I used some paper towels and our trusty mixture of CuSo4 and HCl to strip off the aluminum coating, then we washed the known flat, and Jim happened to have a GREEN laser pointer. We found a couple of small Korean-War era lens blanks that one side frosted, shone the laser through the frosted blanks, putting all of this into a darkened room, and the interference fringes showed up well. Not as good as in my little monocrhomatic light box, but well enough for everybody to see them. And they were straight! 
Jim also had another piece of glass that was supposed to be flat by about a wave or two every inch, which is not all that flat, frankly, by optical standards. It was quite fun to watch it when we made a stack of all three of these pieces of glass. I began to see Moire' patterns!

Bottom line - if you don't have a mnochromatic light box, and you want to test flats, you can use some double-frosted glass and a green laser pointer.

Guy



Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
or else 
http://tinyurl.com/r6fh2

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