How to Train and Apply Periodic Error Correction
1. Create a tracking log that records the magnitude of the mount’s periodic error over three or more
revolutions of the worm. See “Error! Reference source not found.” on page Error! Bookmark not defined.
for details how to create a tracking log.
2. Open the tracking log in the Compute PEC Curve tab.
3. Fit an optimal periodic error curve to the tracking log data.
4. Save and permanently store the optimized periodic error curve to the mount’s firmware.
5. Turn on the Apply PEC checkbox to apply periodic error corrections.
Click the Train PEC button (page 131) to get started training your mount’s periodic error.
The importance of applying periodic error correction to tracking depends on several factors:
• The magnitude of the mount’s periodic error. All mounts have slightly different periodic error due to
machining variations. The greater the periodic error, the more noticeable the back and forth “drift” in
hour angle and more elongated stars in photos.
• The scale of the imaging system. Tracking demands for an imaging system with an image scale of four
arcseconds per pixel or larger are less demanding than on a system with an image scale of one arcsecond
per pixel or less.
• The local seeing and weather conditions. A turbulent jet stream caused by nearby mountain ranges (here
in Golden, Colorado, for example) can produce average seeing errors from three to five arcseconds per
pixel and rarely better. A laminar, more stable atmosphere in regions like Florida or Southern New Mexico
in the USA often produce periods of sub arcsecond seeing.
As an example of periodic error versus seeing versus image scale, if the mount’s peak to peak periodic error is two
arcseconds, and the local seeing conditions average three arcseconds, then the tracking error caused periodic
error is less than the tracking errors caused by seeing conditions, so correcting the mount’s periodic error is not
critical.
However, if the local seeing averages one arcsecond and the optical system’s image scale is one arcsecond per
pixel or smaller, you want to make sure that the periodic error is as small as possible for optimal results.
TheSky Professional is used to compute, retrieve, and save a periodic error correction curve to the Bisque TCS
firmware.
When the Apply PEC checkbox is turned on, the resulting corrected periodic error of the mount should be less
than one arcsecond (usually 1 arcsecond peak to peak error or less), which is normally less than the errors
introduced by local seeing conditions (typically 1-5 arcseconds, depending on the location, weather conditions,
etc.).
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