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Progress in Wavefront Sensing for Solar Applications (1Jun92)
(from NSO, NOAO Newsletter No. 30, 1 June 1992)

A critical area of Adaptive Optics (AO) technology is the wavefront
sensor system for measuring the rapidly varying incoming wavefront. The
Sun presents unusual problems for wavefront sensing. Unlike objects in
the nighttime sky, the Sun does not provide natural, high-contrast
point sources, and creation of artificial beacons bright enough to be
visible against the solar disk remains problematic with current
technology. Conventional wavefront sensors such as shearing
interferometers and Shack-Hartmann systems appear to have some
limitations when used with targets such as solar granulation.
Therefore, a program has been carried out at NSO/SP to investigate the
performance and limitations of various wavefront sensor concepts for
solar imaging.

The Lockheed AO system is based on the Hartmann test. This system
appears to work well, but is best suited for targets such as sunspots
and pores. A more universal system is under development at SP that uses
a programmable liquid- crystal mask. Two other wavefront sensing
techniques have recently been tested on-line and in parallel at the
Sacramento Peak Vacuum Tower Telescope in an attempt to critically
compare their performances. These two tests were the classical
Shack-Hartmann test using a lenslet array and a CCD camera, and the
"focal volume" technique. Both tests gave similar magnitudes and forms
for the residual instrumental aberration, but quantitative differences
were present in the measurements which may be significant.  Further
analysis of the results should resolve this point.

Sergio Restaino, Richard Radick

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