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Azimuth Error Case
In the diagrams below, crosses mark the position on the sky of the true North Celestial Pole (NCP) and the mount's "pole". A dotted line circle shows the path of a star around the NCP, while a solid line circle shows the tracking path of the mount.
Case 1: Mount points east of true pole
First consider the situation of an azimuth alignment error, and in particular, let's see what happens if the mount's polar axis is pointed east of the true pole:
If we sight on a star straight up from our mount's north axis (i.e. along our meridian, higher than the pole) and track for some time, the mount will track along the solid circular path. In the mean time, the star will actually travel along a different circle, centered on the true pole. The final position of the star ends up further away from the pole than we expect -- i.e. the star "drifts" southward. It also will shift slightly east, but the main motion is the N-S drift.
What happens if we select a star east of the pole instead of on the meridian?
Azimuth error - mount axis to the east of true pole, tracking star to east.
The result is that the tracking error is hardly noticeable. So if we are interested in aligning our mount quickly, a star on the meridian is a better choice as it will show the mount's pointing error quickly.
Case 1: Mount points west of true pole
In the case where our mount's axis is off to the west of the pole and we again sight on a star on our meridian line, the situation is as shown below:
Azimuth error - mount axis to the west of true pole, tracking star on meridian.
Now the star is apparently drifting northwards (towards the pole) compared to where we expect the star to be based on the mount's tracking. In the diagram above, the view through our scope's crosshair eyepiece has been superimposed to make it clear what we are seeing. However, note that the image could be flipped depending on your scope's optical setup.
Another point to note is that the closer to the pole you select the star for tracking, the smaller the drift movements will be. The largest drift for a given time interval will be along the plane of the equator. In all drift observations, the N-S drift is the important factor to watch. RA (E-W) drift may be ignored.
Copyright 2001 by David A. Kodama