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hbaidu / open-phd-guiding

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Calibration should recognize and correct for dec drift #425

Open GoogleCodeExporter opened 8 years ago

GoogleCodeExporter commented 8 years ago 
When there is a significant polar alignment error, dec drift can have a 
substantial effect on calibration.

During RA calibration, the camera angle is off since the motion of the star has 
a component in the N/S direction in addition to the W motion due to the guide 
pulses.

During Dec calibration, the Dec guide rate is off since the motion of the star 
has an additional motion in the N/S direction beyond the motion caused by the 
guide pulses.

We can correct both of these errors by measuring the N/S displacement at the 
end of the W-E calibration and determining the Dec drift rate (px/sec).

For RA, we can use the elapsed time for the W calibration to find the Dec drift 
during the W calibration. The camera angle correction, a, we get from sin(a) = 
dec_drift/RA_cal_distance.

For Sec, we would use the elapsed time for the N calibration and the measured 
drift rate to determine the drift distance.  The corrected dec rate is 
(dec_distance_pixels + drift_distance) / sum_dec_pulses.

Original issue reported on code.google.com by andy.gal...@gmail.com on 22 Apr 2015 at 3:56