Horizontal Heavens Observatory "...From  A Galaxy Far, Far Away"
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This page was last updated on 03/30/11.

Adjusting the RA Drive Worm

When my MI-250 started to show an increased periodic error, Larry Myers talked me through the process of adjusting the tension on the worm shaft bearing using the three Allen screws (shown below left).  While going through this process, I was introduced to the exacting parameters to which each mount is adjusted.  Under the worm assembly are a series of shims (below right), the left one is 0.004" and the two stacked shims on the right are 0.005" and ).0015" thick/thin, all required to square the worm with the worm gear!  WOW!

(Below) Using a piece of .25" key stock, the worm assembly can be reinstalled very close to the needed position.  This is best done under a load, but can be accomplished without the load.  Final adjustments under load are more accurate than without the load.  Thus far I have adjusted the worm WITHOUT a load and have managed to attain just 5.7 arc. sec. of total P-T-P periodic error (MI-250 PEC Page).  When the weather clears and I again get motivated, I may go back and readjust the worm under a load and see just how small I can make the periodic error.