4.3.2.5  Drift Correction 
When you train the PEC, it is assumed that the only error being corrected is the periodic 
error in the worm.  Because of the periodic nature of this error, the star will appear to 
drift back and forth in RA during the period of the worm's rotation, but will always end 
up in the same spot at the end of every rotation.  Theoretically then, at the end of a 
training run, the total amount of correction to the west should exactly equal the total 
amount of correction to the east.  If, however, there was systematic drift of the object 
caused by something other than periodic error (i.e. misalignment of the mount, 
differential refraction, etc.), then the corrections made during the training run would 
reflect this error also. 
This is not a problem if (1) the object being imaged is the same as (or very near) the 
object used to train the PEC and (2) the mount has not been moved since the training.  In 
that case, the same systematic errors that were present during the training run will also be 
present during playback, and both the periodic and systematic errors will have been 
corrected.  However, in many cases, you will want to use the same PEC data for widely 
scattered objects over several observing sessions when the telescope has likely been 
moved between the sessions.  In this case, the amount of systematic error will likely be 
different than during the training run, so just playing back the corrections made during 
training will actually introduce errors. 
Drift Correction solves this problem by effectively removing the systematic drift 
component from the recorded PEC data.   If everything were perfect and there were no 
systematic drift, corrections in both directions would cancel each other out, and this 
feature would not be needed.  If, however, they do not cancel out after a worm period, the 
existence of systematic drift is assumed.  
Selecting this menu item removes this non periodic component.  Gemini sums up the 
corrections (negative and positive) made in both directions, and spreads them evenly 
throughout the worm period. It does not, however, change the tracking rate itself, because 
to do so would lead to imprecise tracking in other regions of the sky.  
Because you may choose to train the PEC on the same object as the one that is to be 
imaged, Drift Correction of the PEC data is not done automatically after each training 
run; this helps minimize the guiding corrections necessary. You should only select the 
  Drift Correction   function if you want to use the PEC in a region of the sky other than 
the one where you trained it, or if you plan to use the PEC data across multiple sessions 
where the mount will be moved between sessions. 
  
After you have selected this function, 
the drift correction will remain active until a new training session overwrites the data. 
Using this function multiple times is not necessary, but also is not harmful. The amount 
of drift compensation is displayed, and should become zero after subsequent calls. 
4.3.2.6  Smooth Data 
In addition to periodic error and systematic drift, the corrections recorded during PEC 
training runs can also contain random errors caused by scintillations (  seeing  ) and over 
corrections.   The Smooth Data function helps eliminate these random influences by 
Gemini Users Manual 
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