Kipp & Zonen Brewer Mk III Operator's Manual page 65

5. Return to the Computer and follow the instructions for saving the new siting information.
Before saving new values, recheck that the Date and Time are correct.
For accurate tracking to take place the Tracker must be quite level.
Levelling can be checked by alternating the times of day when siting is done - check the
siting in the early morning on one day, and in the late afternoon on another.
If it is found that the North Correction numbers alternate between higher and lower numbers
consistently from morning to afternoon then this is an indication that the levelling is not
accurate.
If there are very inconsistent North Correction values from one siting to another, this could
be an indication of the Tracker drive mechanism slipping.
If there are inconsistent Horizon Correction values, this could be an indication of Zenith
Prism slippage.
If there is a constant drift in both Horizon and North corrections, then this could be an
indication that the site co-ordinates could be incorrect.
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MKIII OPERATOR'S MANUAL