The Astrophotography Manual by Chris Woodhouse;

Author:Chris Woodhouse;
Language: eng
Format: epub
Publisher: Taylor & Francis (Unlimited)
Published: 2024-06-15T00:00:00+00:00

PE = wiggle ∙ 3600 ∙ ATAN(pixel size/focal length)

An example of a significant periodic error is shown in fig.5. A quantitative test uses a guide camera and an autoguider application to accurately measure the tracking error. In practice, focus the guide camera and calibrate the autoguider application at a low declination. Be sure to enter the correct focal length of the guider optics and pixel size for the guide camera. After calibration, start guiding using an exposure of 3-8 seconds but disable/disconnect the guider outputs. In this way, the guider measures the tracking error for both axes without being too severely affected by atmospheric seeing. After several minutes, the tracking graph will indicate drift on both axes and RA tracking errors. Left unguided, the peak-to-peak tracking error is an important indicator of whether unguided operation is feasible. The maximum rate of change, inferred by the steepest slope of the periodic error, is arguably more significant for those using autoguiders, as it determines the speed and aggression required to keep up with the tracking error.

fig.5 Andy Galasso’s PHD2 log viewer not only reconstructs the guider trace from a PHD2 guider log but also can analyze the data and show periodic error (left) and analyze the principal error frequencies (right), corresponding to the period of various motor and gear revolutions. In this case, for a harmonic drive, the principal periodic error is about 45 arc seconds peak-to-peak and has a period of ~430 seconds. Unfortunately, only a handful of premium mounts have low enough periodic error for unguided operation, and most will have integrated optical encoders. If your mount has periodic error correction, use it; while autoguiding can react and correct periodic errors, it is always after the event. This is another example of the 80:20 rule; fix 80% of the problem with PEC and let the autoguider have an easier time fixing any remaining tracking errors.

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