Oldham Coupler PerformanceBelow I have plotted the track log recorded on August 16, 2006 while imaging NGC6978 aka Hickson88. The mount had been tuned earlier in the summer and was performing quite well. At this time the stock Oldham coupler was used between the gearbox and worm gear. This object is at declination -5 deg so the contribution of the mount tracking error to the RA component is maximized. The RMS error on the DEC axis is usually a good indication of the local seeing. From the nearly equal RMS DEC and RA tracking residuals we can conclude that the main limitation to tracking is seeing in this case.
Helical Coupler PerformanceThe Oldham couplers were replaced by Helical couplers. Below is the tracking error for both axes recorded on Jan 2nd, 2007, half way through the run I turned on tracking for the DEC axis. Unfortunately I did not note the declination of the star used for the log, but I am certain it was a good distance from the equator, perhaps 30-45deg, therefore the RA errors are underestimated. The RA worm peak to peak error is about 8". I removed the linear component along a small section of the RA curve to calculate the RMS error, I found 1.1". The random DEC error was also calculated under tracking and non-tracking conditions, I found 0.75" and ~1" respectively.
Plotted below is the track log acquired on Dec 30, 2006 while imaging at declination +48deg, the seeing that night was not particularly good.
Plotted below is the track log acquired March 23, 2007 while imaging at declination +16deg, the seeing that night was very good as witnessed by the low noise on the DEC axis tracking. However I have been plagued recently by sudden large (> 2") jumps in DEC. I have not identified the source (wind gusts should affect both axes?). I the spikes in DEC larger than 1" are ignored the RMS error drops to 0.4".