I’m using my mount with an autoguider and getting mixed results. What are the best settings to use with Celestron mounts?
June 16, 2010
An autoguider works with the mount in a feedback loop. Signals from its imaging chip circuitry represent drift of a star off of the center or other location in the imaging field. In technical terms, gains are set with the autoguider’s controlling firmware and or software to control the amounts of proportional and integral feedback to the drive. Proportional feedback will adjust the drive by a percentage of the measured drift. Integral feedback will adjust the drive by a percentage of averaged tracking over time. Most programs will call these settings by other names. For example, the popular autoguider program PHD Guiding uses aggressiveness and hysteresis as feedback control names.
Using low proportional settings is usually recommended. Try either 50% (default in many programs) or a lower value like 25%. Set the other control value (for example, hysteresis) to zero and see if that works. It usually will.
If seeing conditions are unsteady, turn proportional feedback down to smooth quick correction movements. If the autoguider is correcting too slowly, increase its value.
If your mount has lots of periodic error and you are not using periodic error correction (PEC) or there are wind gusts, then try slowly increase the integral (hysteresis setting) by factors of two until oscillations set in and then back off.
Another factor is the autoguider’s camera exposure time/correction interval. Under poor seeing conditions, stars will appear to rapidly move around. If the exposure/correction time is too short, this will cause jumpy corrections by the autoguider. It will be “chasing the seeing.” A typical setting avoiding this problem with most guide scopes will be from 1-4 seconds.
Guiding problems will generally be more exaggerated with longer focal length guide scopes and or imaging scopes.
Other issues that are not caused by the autoguider but that it may be blamed for include field rotation (cause: poor polar alignment) and oblong stars (cause: differential tube flexure between guide scope and imaging scope).