Re: [ATM] holographic null test on round robin mirror C

[Andy Saulietis <iss@pvtnetworks.net>]

Matt writes,

I was referring to the classical Hartmann test in which a real
star is used, the imaged spots are both inside and outside
focus at a precisely known separation distance, and simple
geometry is used to calculate the best focus spot diameter.

>exactly local wavefront slope deternimation seems to be accurate enough for
>the largest professional telescopes , since they're using it for wavefront
>sensing in adaptive optics.

This will not scale well for small apertures, see below.

>a Hartman mask or a small lenslet array in front of a webcam style CCD is
>all it takes. The size of the defocused pattern can be made arbitrarily
>small, by getting the system closer to focus . Even on a 640x480 size webcam
>ccd, with 6um square pixels, the pattern would fit. Defocus each
>spot/subaperture to 100um diameter, and you could still fit 10x10 to 20x20
>subapertures . Calculating spot centroids even for large blur size is a
>photometric problem similar to doing star photometry, where centroids are
>calculated to subarcsec accuracy although seeing blurs star images to
>several arcsecs . If exposure is not saturating the image and still in the
>linear area, centroid calculation works .

The problem is that even with  centroiding to 1% of the spot diameter,
the resulting slope errors are too large to be useful for mirror testing
and figuring. I used to design star trackers for NASA, and know
that centroiding  at these accuracies is not a trivial problem,
requiring extensive calibration models for each pixel in the
detector array.


>today's largest telescopes all use derivatives of the Hartman, for example
>Shack Hartman, where the mask is replaced by a lenslet array . This is the
>most widespread adaptive optics wavefront sensor . There are a few other
>ideas for wavefront sensing, like curvature sensing using a vibrating
>mirror, or the pyramid wavefront sensor, shearing interferometry , etc. The
>Shack Hartman is dominating now for active and adaptive optics systems.

Jim Burrows has a functional Hartmann derivative test but it's not
the classical type..he uses the overlapping spot diagram interference
pattern to determine slopes. Perhaps he can weigh in on the subject?
He and I participated in the development of the modified Harmann
test, and the analysis that just using a CCD to do a classical Hartmann
test was not practical. Actually, I was hoping that it *would* work
because of it's inherent simplicity and the potential ability to
test *any* conic surface. Finally, the Shack-Harmann test may
be the one of choice..but is it practical for an ATM to use?








_______________________________________________
ATM mailing list http://www.atmlist.net/