[ATM] Drum head mirrors.

[Jim Burrows]

Our favorite method of describing astigmatic mirrors surfaces, 
Zernike's orthogonal polynomials, are also used to describe the 
vibration modes of drum heads.  During a 2-minute spell figuring my 
4.25" RC cass secondary with a starlap on a pottery wheel, I heard 
the usual squeal (shriek) of good contact plus an unusual lower pitch hum.

The starlap petals were fairly narrow, so they must have been acting 
like violin bows - when I did a 2D webcam Hartmann test on the 
mirror, there was an obvious tetrafoil astigmatic component on the 
mirror surface (Zernike polynomial R44) of magnitude 80 nm.  This 
hurts the RMS a fair amount, 80/3 nm, so it has been added to the 
deviations to get rid of.

         -- Jim Burrows
         -- http://home.earthlink.net/~burrjaw
         -- mailto:burrjaw at earthlink.net
         -- Seattle N47.4723 W122.3662 (WGS84)