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Mark II Correlation Tracker (1Dec94)
(from NSO, NOAO Newsletter No. 40, 1 December 1994)

Two years ago, the NSO copy of the correlation tracker that was developed
jointly during the 1980s with the Kiepenheuer Institut f�r Sonnenphysik
(KIS) in Freiburg, Germany, quit working because of a hardware failure.
This correlation tracker was based on custom-built hardware components. A
second copy was built for use at the German Vacuum Tower Telescope on
the Canary Islands. Since KIS is currently completing a Mark II correlation
tracker in conjunction with the Instituto de Astrofisica de Canarias, KIS has
loaned its copy of the original tracker to NSO. This unit is frequently used
at the Sac Peak VTT, effectively without spares or backup. This high-risk
situation has led to efforts at NSO/SP to develop its own Mark II tracker.

A study has been performed to compare different tracking algorithms. In
particular, the performance of the "absolute difference algorithm" and the
"FFT cross-correlation algorithm" were compared via computer simulation
using data taken at the NSO VTT under varying seeing conditions. The
simulations clearly show a performance advantage for the FFT algorithm in
conditions where the image contrast is low. The ability to track low-contrast
images is important to a user-friendly system and to the possible use of such
a tracker in a Shack-Hartmann type wavefront sensor. Based on the results
of the simulation studies, it was decided to design and build a Mark II
correlation tracker system that uses FFT cross-correlation. The initial design
has been completed and is based purely on commercially available hardware
components. The system should therefore be easy to maintain and clone for
use at other NSO sites. First engineering runs are projected to occur early
next year.

Thomas Rimmele, Richard Radick,
Fritz Stauffer, Phil Wiborg

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