To: atm{at}shore.net
From: "James W. Burrows" 
Reply-To: "James W. Burrows" 


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
         --              mailto:burrjaw{at}earthlink.net
         --              http://home.earthlink.net/~burrjaw
         -- Seattle      N47.47233, W122.36620 (WGS84)

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