To: atm{at}shore.net,"Vladimir Galogaza" 
From: Jim Burrows 
Reply-To: Jim Burrows 


At 09:39 2003-07-03 -0700, Mel Bartels wrote:

> >>>
>  Briefly: Cover the mirror with a mask containing a grid of small holes.
>  Take two images at a known spacing.  Analyze the spots and viola - a
>  complete surface profile.
><<<
>
>I've done some thinking along these lines and discussed it with Jim Burrows.
>He pointed out that the diffraction effects between the holes are significant
>and suggested ignoring the focusing light from the holes and instead
>concentrating on the diffraction pattern.  Perhaps Jim can discuss this
>further.

"Analyze the spots" is the tough part.  I've been fiddling with a
webcam version of the Hartmann test for 2 years, trying to get astigmatism
estimates, that being the bugbear of today's thin mirrors, and a parallel
universe to ATM's trusted bench tests.  Without making this post too long,
here are the past trials and tribulations.

1)  Get a defocused webcam (lens removed) whole-mirror image of a point
source and try the Misell iteration to find the mirror surface.  This was
used to find the cruddy Hubble primary's surface up in space.  Result - the
iteration wouldn't converge.

2)  Take a defocused image of the mirror masked by "a grid of small
holes".  The Misell iteration converged - whee!  Result: it said there
was huge roughness (~ 1/4 wave) in every mirror patch.

3)  Modify the Misell iteration so it just estimates the mirror patch
slopes with low-gain feedback to the surface profile.  Everything works
except the current mystery of the primary astigmatism estimates being
unreasonably large.  It may be coming from the webcam pixels not being
quite square, or insufficiently accurate measurements of source and image
positions, or - who knows?

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

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