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Altitude Error Case
Case 1: Mount points above true pole
Altitude error can be examined in the same way as azimuth error. The case of the mount's polar axis pointing above (higher in altitude) than the true pole as illustrated below:
Altitude error - mount axis above true pole, tracking star to the east.
In the case above, if we select a star on the eastern horizon and track it with the mount, the star will appear to drift towards the pole (northwards).
Altitude error - mount axis above true pole, tracking star on meridian.
But if we select a star on the meridian, the drift error is small and is thus not very useful for determining if the mount is misaligned in altitude.
Case 2: Mount points below true pole
For the case where the mount's polar axis is below the true pole,
Altitude error - mount axis below true pole, tracking star to the east.
As in the case of an azimuth error, a 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 Dec (N-S) drift is the important factor to watch. RA (E-W) drift may be ignored.
Copyright 2001 by David A. Kodama