Re: [APML] Autoguider Calibration/Guiding Rates

[Aplanatic@aol.com]

I agree with Chuck.  The most likely mechanism is an oscillation 
in the system that is driven by the ST-4 corrections.  Note that it is 
the angular inertia of the telescope and a torsional spring that 
comprises the oscillator.  The weakest torsional spring in the system 
will cause the lowest frequency of oscillation, and the lower the 
oscillation frequency the higher the amplitude of the oscillation (for
 the same applied impulse) and the more likely the autoguider 
corrections will excite this oscillation.

Undercorrected guiding reduces the impulses that excite the 
oscillation.  In addition, the closed loop response function will 
have a much higher damping term when undercorrected guiding 
is used.  The only problem with undercorrecting is that the telescope 
and mount will take longer to correct for fast errors, especially those 
errors induced by a worm gear that is not perfect, or by external 
influences like wind.  If the gear is smooth this should not be much 
of a problem and if there is enough wind to disturb the scope then 
the tracking will be bad anyway.  So, I see nothing wrong, in principle, 
with setting the autoguider to undercorrect for errors.  In this mode it 
will be less likely to chase seeing, a side benefit.

Dave Rowe




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