However, in the real world, this is not the case.  Slight imperfections in the shape of the 

worm, manufacturing tolerances in the gears and bearings, dirt, and misalignment can all 

cause slight variations in the speed of the worm gear while tracking.  These variations 

generally occur in a pattern that repeats itself every time the worm makes a complete 

revolution, hence the term   periodic error.   

Periodic Error Control (or PEC) is meant to handle these imperfections of the worm and 

worm gear combination of your mount.  This involves   training   the PEC by making 

appropriate corrections to keep a star centered during one revolution of the worm.  The 

Gemini software records these corrections, and then automatically plays back the 

corrections for every rotation of the worm, offsetting the periodic error.  It also provides 

sophisticated algorithms for manipulating this correction data to cancel the periodic error 

more effectively than would just a simple record/playback. 

PEC can be especially useful when manually guiding a long exposure image by reducing 

the amount of guiding correction needed.  It is possible to train the PEC during the first 

few minutes of the exposure, so that after the training is complete, PEC will reduce the 

number of corrections required for the remainder of the exposure. 

Using PEC together with an autoguider can also reduce the amount of correction done by 

the autoguider.  However, it is possible that these two independent control systems can 

work against each other.  You will want to experiment with some of the PEC options 

described below to find the best settings for using PEC and an autoguider together. 

All PEC data is stored permanently in the Gemini SRAM memory.  Gemini also stores 

the position of the worm whenever it is powered down, and as long as the motor is not 

loosened or disconnected from the worm, all acquired and stored PEC data will remain 

available and valid. 

For users who are satisfied with the existing periodic error of their mount, Gemini 

s PEC 

function need not be used.  PEC is needed when the mount 

s periodic error causes 

unacceptable tracking, especially noticeable when imaging. 

4.3.2  The PEC Submenu 

The PEC submenu (  Setup   Mount Parameters   PEC  ) has these options: 

Train PEC 

Clear Data 

PEC On/Off 

Delay Correction 

Drift Correction 

Smooth Data 

Average Data 

Restore Data 

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