AP1600GTOE RA drift

Craig Young

I have polar aligned the mount and without using a pointing model I get good pointing accuracy, always close to the center of the ccd sensor.  I have used PEMPRO to check the alignment and it also shows the mount is well polar aligned.

To check tracking I point the telescope at 0 DEC and HA=-0 10 00 (ie, 10 minutes east of the meridian).  At this location there should be no RA drift and only a DEC drift as a result of any remaining polar alignment error.  I then take a 10 second image, wait 5 minutes and take another 10 second image.  I plate solve both images and get this:

  Rho (arc-sec): 4.3
  Rho (pixels ): 7.6
  Theta (degrees): 77.3
  RA (arc-sec): 4.2
  RA (pixels): 7.4
  DEC (arc-sec): 0.9
  DEC (pixels): 1.7
Duration (seconds): 317
Drift Rate (arcsec/sec): 0.0136 @ 77.3 degrees
Drift Rate RA (arcsec/sec): 0.0133
Drift Rate DEC (arcsec/sec): 0.0030

Rho is the arc second displacement from the plate solve of image 1 to image 2.  The plate scale is 0.57 arcsec/pixel.

Theta is the direction of the drift.

Notice the drift in RA is 4.2 arcsec and DEC is 0.9 arcsec.  This is where I am puzzled, the DEC drift I would expect because of a slight polar alignment error, but why the very large RA error when this mount has absolute encoders?

My first thought is the encoders or clock is off, or there is a Southern Hemisphere correction being applied that is opposite to what it should be.


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