Autoguiding setup

September 20, 2013
blog 2013_09_20 guiding errorblog 2013_09_20 guiding error
 
On my mobile setup I get better autoguiding in Maxim when I use different Minimum move and Maximum move values for the two mount axes. Here is how I am setting these parameters and why:
 
Autoguide speed in right ascension (RA) decreases with increasing declination (Dec). Hence a fixed Minimum move time in RA corresponds to a smaller linear movement threshold on the sensor with increasing Dec. On the other hand, autoguide speed in Dec and the corresponding linear movement threshold for a fixed Minimum move time remain the same.
 
So equal Minimum move times for both axes can lead to seeing chasing in RA at higher declinations since the linear movement threshold for autoguider correction grows smaller in RA.
 
To avoid this problem in Maxim I scale up the RA Minimum and Maximum move values. As an example, at 60 degrees declination I set RA Minimum and Maximum move values twice as large as the Dec values. In general, I scale the RA values by the factor 1 / cos(Dec). With this scaling the linear movement thresholds and limits for correction remains the same for both axes at all declinations.
 
The chart above shows guiding errors for a single 40 minute exposure captured in good seeing conditions. RMS guiding errors on my setup are usually in the range 0.5" to 1.0". Different seeing conditions account for most of the RMS guiding error variation. Wind gusts can also be a factor due to my wind unprotected, mobile setup. The larger errors in RA than in Dec due to RA axis periodic error are reflected by the asymmetric dispersion of the points in the chart. The non-zero average Dec error is due to slow Dec axis drift due to polar misalignment.  I purposely misalign the mount's polar axis by ~2 arcminutes in azmuith to avoid Dec axis backlash issues when imaging within ~2.5 hours of the meridian. The non-zero average RA error is not significant, as its sign and small magnitude vary randomly across exposures.

 


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