Re: Tracking Accuracy


Dave Schwartz
 

Another source of RA tracking error can be introduced by the (small, but never zero) backlash in the worm-to-ring-gear interface.

The reason is that, if you have west-heavy bias (and we consider frictionless bearings for now), the ring gear teeth will be pressed against the east side of the worm teeth. Sidereal tracking will be acting to lower the worm tooth allowing the scope to 'fall' as it would like to do. Now seeing that all most of the mounts we use will have a property known as stiction in the axis bearings, the axis will not actually move the fractions of an arcsec in the backlash until the west side of the worm tooth closes it up and then shoves the gear to the west. Then the scope's bias and/or inertia will rotate the gear through those backlash fractions of arcsecs, reopening the backlash, until it is stopped by again hitting the east side of the worm teeth.Then the process keeps repeating and the period is not necessarily constant or related to the periodic error frequency. This also happens when the RA axis is neutrally balanced due to the stiction alone.

To address this, your RA axis should always have a slight east-heavy bias. In this case, the ring-gear is always pressed against the west side of the worm teeth and the sidereal tracking motion of the motor is always acting to push the ring-gear against its natural 'fall' direction and stiction. The backlash therefore never opens up and the rotational error is strictly due to the periodic error.

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