AP1200 Ra gear train.

The image shows the AP1200 RA gearbox with the cover removed. (The 20T gear is underneath the 70T gear.)

The gear reduction ratio is (96/20) x (70/21) x (72/20) = 57.6 : 1
When tracking at sidereal rate the motor turns at 9 rpm or 54 degrees per sec.
Working this through the grar train results in 0.375 degrees per sec rotation of the worm.
The worm wheel reduction is 225:1 so the final rotational rate of the RA axis is 0.375/225 = 0.004166 degrees per sec or 15 arc sec per sec.

The encoder resolution is 0.05 arc sec per 'tick'. Therefore the encoder gives one tick every (0.05" x 225 x 57.6) = 0.18 degrees of the motor.
360/0.18 = 2000 ppr encoder is used. (500 lines in quadrature)

If you are experiencing less than perfect tracking and have checked the worm mesh as per the AstroPhysics procedure it could be worth taking the cover off the gearbox had having a look.
Check the grub screw that holds the 72T gear to the end of the worm shaft is tight while your there !
Otherwise run the Pulseguide test and send the images to AP for evaluation and advice.

Dave Rose
22th October 2004
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