Periodicerror correction PEC

When it is tracking the stars, an equatorially mounted telescope moves only in right ascension, so it only needs to run one motor. This makes the tracking inherently smoother.

Still, even the best drive motor has some periodic error caused by small irregularities in the gears. During one revolution of the main worm gear - which typically takes 8 minutes - the image shifts east or west, then back to its original position.

The best computerized telescopes offer periodic-error correction (PEC), which means that small irregularities in the gears can be memorized and corrected by the computer, so that tracking is extremely smooth - but only in equatorial mode. The procedure is to use a crosshairs eyepiece at high magnification and keep a star perfectly centered by making manual corrections for 8 minutes. Thereafter, the computer repeats the same corrections, somewhat smoothed out, at the correct interval. For really accurate training, use a CCD autoguider.

Larger observatory telescopes have periodic-error correction on both axes for maximum smoothness even in altazimuth mode.

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