600EGTO - Adjustments to Remove Worm Gear Backlash

Backlash (as defined by Astro-Physics):
Backlash is the result of a gap between the worm and the worm wheel. If movement cannot be felt or seen in the eyepiece when making the following tests, the adjustments described here will not be needed.
Play (as defined by Astro-Physics):
The movement in an axis where there was none before. It can be felt as a back and forth movement of a gearing setup or seen as an image shift in the eyepiece where there should be none.
Background Information

Both the right ascension and declination axes of the 600E GTO are driven through the motors and spur gear reduction gearing. The R.A. worm, worm wheel and the spur gear on the end of the worm shaft are shown here.

The remaining gears in the mount are on the motor mount plate. The brass idler gear is shown on the top.

Test for Right Ascension (R.A.) Backlash and Correction

Step 1

Place your hand near the end of the counterweight shaft and move the shaft up and down as if to rotate the R.A. axis. This test can be done with the mount alone on its pier or with the telescope and counterweights attached.

If no play is felt or seen, then you do not have an R.A. backlash problem.

If play is felt or seen in the telescope eyepiece, proceed with the following adjustment.

Step 2

Rotate the mount to the position shown. Using a 5 mm long-arm hex key, loosen the M6 socket head cap screw on the right hand side of the mount ONLY. Re-tighten enough to provide moderate tension.

The second screw should remain tight to provide a pivot point.

Step 3

Using a rubber mallet (or steel hammer with a block of wood or plastic), gently tap the worm housing near the corner. This action moves the worm gear closer to the worm wheel, which removes the gap that you felt earlier.

Removing the space between the gears is a delicate adjustment. The distance between too much backlash and a jammed gear is only a few thousandths of an inch. The tighter the screw, the harder you have to tap with the hammer. The looser the screw, the gentler you have to tap.


Step 4

Once the backlash is gone, re-tighten the screw and try a manual slew at 1200x for about 5 minutes of run-time at the sidereal rate.

If the motor does not labor or stall, you are done.

If the motor does labor or stall, please go to step 5.

On the GTOCP2 control box,(shipped after February 16, 2000) a motor stall is shown by the LED on the box glowing amber.

Step 5

If the engagement has been made too tight, back it off by tapping on the opposite side as shown.

Note: only the one screw used previously should be loosened for this action as in step 2.

Return to step 3.

Test for Declination (Dec.) Backlash and correction

Step 1

Grab the end of the cradle plate or telescope and move it back and forth as if to rotate the Dec. axis. This test can be done with the mount alone on its pier or with the telescope and counterweights attached.

If no play is felt or seen, then you do not have a Dec. backlash problem.

If play is felt or seen in the telescope eyepiece, proceed with the following adjustment.

Step 2

Rotate the mount to the position shown. Using a 5 mm long-arm hex key loosen the M6 socket head cap screw on the left hand side of the mount ONLY. Re-tighten enough to provide moderate tension.

The second screw should remain tight to provide a pivot point.

Step 3

Using a rubber mallet (or steel hammer with a block of wood or plastic), gently tap the worm housing near the corner. This action moves the worm gear closer to the worm wheel, which removes the gap that you felt earlier.

Removing the space between the gears is a delicate adjustment. The distance between too much backlash and a jammed gear is only a few thousands of an inch. The tighter the screw, the harder you have to tap with the hammer. The looser the screw, the gentler you have to tap.


Step 4

Once the backlash is gone, re-tighten the screw and try a manual slew at 1200x and about 5 minutes of run-time at the sidereal rate.

If the motor does not labor or stall, you are done.

If the motor does labor or stall, please go to step 5.

On the GTOCP2 control box,(shipped after February 16, 2000) a motor stall is shown by the LED on the box glowing amber.

Step 5

If the engagement has been made too tight, back it off by tapping on the opposite side as shown.

Note: only the one screw used before should be loosened for this action as in step 2.

Return to step 3.


If the previous backlash adjustments are made according to the instructions, no change to the spur gear adjustment should occur.
Should spur gear adjustment be required, please contact
Astro-Physics.

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This page was last modified: April 28, 2004

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