Non-uniform illumination (was: Re: ATM Foucault testing an hyperbolic mirror?)

["James W. Burrows" <burrjaw@earthlink.net>]

At 11:48 2002-12-12 -0500, James Lerch wrote:

>I'm currently thinking I'm going to need a point source light, like a
>LED/Laser pointer reflected off a ball bearing (or something) to test such a
>fast mirror surface.
>
>Of course this brings up an interesting question in my mind.  The cause of
>the  inability to fully illuminate the mirror at F/2, does this translate
>over to any possible error for slower mirrors?

In good ol' shadow-matching Foucault, non-uniform illumination would 
certainly introduce errors - the difference in illumination would result in 
a longitudinal reading shift to balance the zone brightnesses.  I've used a 
gas laser pointed at a 1/8" ball bearing as a point source which works very 
nicely, but not for shadow-matching tests because you get pretty Fresnel 
zone-type patterns from surface imperfections on the ball bearing.  I 
suppose a modern laser pointer, which shows a "speckle" pattern like a gas 
laser, would do the same.

         -- Jim Burrows
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