Solar feature correlation tracker

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Abstract
We present a tracking system that stabilizes atmospheric and instrumental image motion in vacuum tower telescopes. The system is designed to lock on low contrast features, such as solar granulation or other small scale structure. A matrix diode array rapidly scans the scene of interest, usually with a field of 5 arcsec. Images are cross-correlated in real time with a previously recorded reference image of the same area. The drive signal for the image motion corrector, a small, articulated mirror, is generated by measuring the position of the cross correlation maximum. Reference pictures are updated every 30 s in order to adapt to the changing small scale solar features. Performance tests show that the residual image motion in the tracked image is 0.05 arcsec rms compared to a typical 0.5 arcsec rms for the untracked image. The system locks on any small scale structure anywhere on the sun. The bandwidth of the servo system is 40 Hz, or sufficient to stabilize image motion on a meter-class solar telescope.

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