Improving the Celestron Advanced Series Go-To mount (AS-GT)
Improving the Celestron Advanced Series Go-To mount (AS-GT)

DISCLAIMER: I accept no responsibility if you should try this and cause damage to your equipment.

Why do this anyways?

Living in New Brunswick the temperature in the winter months can be quite chilly -40C during cold snaps (not that I would be out in this cold -15C is tops for me). During the last three years I have used the AS-GT mount on average 3 hours per observing sesion. After a couple of hours of usage in cold weather I noticed that the motor would drag and axis became quite stiff. Also I found it nearly impossible to balance the scope. This became more apparent a week ago when I entered the observatory and turned on the mount at around -10C. The axis barely turned and the mount sounded like a dying cat. I knew it was time to do something about it.

I decided to perform the procedure from AstronomyBoy.com to improve the mounts performance. This tutorial is based on my nottes and the AstronomyBoy tutorial.

Tools you will need:

  • Set of metric hex keys the smallest being - 1.5mm
  • Phillips screwdriver with long shaft.
  • A form of degreaser - I used Varsor (paint thinner)
  • A good synthetic grease - I used MotoMaster sythetic grease. Whatever you use make sure it's not temperature sensitive.
  • Small piece of wood with two small nails or small drill bits. This will be used to make a spanner tool to remove the large retaining nut on each axis.
  • Lots of lint free cloths.
  • A few pairs of thin latex gloves helps.
  • Fine sand paper
  • Small sharpening stone or file.
  • Toothbrush
Disassemble the DEC Axis
  1. Remove the counterweight bar
  2. Separate the DEC axis from the RA axis. Remove the two HEX screws located near the polar scope hole.

  3. Open the casing by removing the two 3 Phillips head screws holding the case together.

  4. Remove the motor (still attached to the casing) by unscrewing the top hex screw.

  5. Remove the top cover by removing the other two hex screws.

  6. Turn over the axis and remove the retaining ring and the small plastic washer underneath then the setting circle. Make sure to keep track of this washer and how it was oriented.

  7. Loosen but don't remove the two small hex set screws (1.5mm) around the side of the ring nut. You may have to rotate the axis in order to see the screws.

  8. Now the ring nut can be removed. The easiest way to do this is to make a spanner tool using a piece of wood and two nails. The two nails should be spaced apart to fit into the two holes on the top of the ring nut.

  9. The DEC shaft can now be removed from the housing. After removing the shaft, look inside the housing for a small plastic button that is part of the DEC lock lever. It does fall out easily, so put it aside. There is a washer under the lock nut.

  10. Remove four screws holding the worm assembly to the DEC shaft. Make sure to note which holes the 4 screws came from. They may be of different lengths (mine were not, but I kept track of them anyways). There is a small hex set screw in the middle of the two large hex screws. Do not remove this set screw. NOTE: In this image the top plate is still attached. This is because my original image was corrupted and had to retake this one. Put the worm assembly aside.

  11. Now the worm gear should slide off the DEC shaft. There is a plastic washer on each end of the gear. Keep track of these because they differ slightly. On mine it seemed like one was thicker than the other.

  12. To take apart the worm assembly, simply remove the silver nut at the end of the shaft. A threaded sleeve is now visible and can be unscrewed. The worm can then slide out through the assembly. You will find three o-ring rings at each end. I ended up re-greasing these as well. One improvement here is to replace the o-rings with plastic bushings see http://d54h.tamu.edu/rdb/CG5/wormgear.htm.

Here is a photo of all the parts in the DEC axis.



Cleaning

The step is to clean all the parts with the grease remover, and dry them off. Check for any rough edges on the washes and try and smooth them out using an sharp knife. Check for rough spots in the housing. There were not many rough edges in my mount. Use the toothbrush to clean the worm gear teeth and worm assembly.

Grease

Place a small thin coat of grease on all surfaces that touch. This includes, washers, inside of the shaft, worm assembly, screws inside the worm assembly and anywhere surfaces come into contact with each other.

Adjustments

Re assembling and adjusting takes a bit of patience, but is critical to how smooth the mount performs.

  1. Ring Nut
  2. Worm shaft end play
  3. Worm gear mesh

Ring Nut

Place the worm gear back on shaft (making sure to put the washers back). Slide the shaft back into the DEC housing and put the washer and ring nut back on. Re-tighten the nut with the spanner tool. The trick here is to tighten just enough so that the shaft moves easily, but does not feel loose or wobbly. Once you have found the right spot, tighten the two set screws on the side of the ring nut.


Worm Shaft End Play

The threaded sleeve at one end of the worm shaft allows the shaft to move back and forth in the bracket. If it moves, you will have slop in the mount. The idea is to tighten the threaded sleeve just enough to stop the shaft from moving back and forth, but not too tight or the worm will not turn freely. Once you find the correct balance, re-attach the silver nut on the end. This locks the shaft into place. You should be able to spin the shaft with your thumb and index finger and it should make a couple of turns before stopping. If it feels rough or does not spin easily then remove the nut and loosen the threaded sleeve at bit, re-attach the silver nut.

Worm Gear Mesh

Here is the most important and time consuming step. The idea is to adjust the worm so that it meshes into the teeth of the worm gear.

Here is how to do it:

  1. Attach the worm assembly to the DEC housing by tightening the four hex screws. Tighten the two screws next to small set screw first (tighten them the same amount).
  2. Tighten the top two screws.
  3. Now try and turn the shaft.
  4. If the axis is too tight and does not move easily.
    • Loosen the 4 hex screws a bit and turn the middle set screw a tiny bit clockwise. This pushes the worm assembly further from the gear, loosening it.
    • Go back to step #1
  5. If the axis is too loose
    • Loosen the 4 hex screws a bit and turn the middle set screw a tiny bit counter-clockwise. This will allow the worm to get closer to the gear, loosening it.
    • Go back to step #1
DEC Re-assembly

Now re-attach the washer, retaining ring and setting circle.

Now test the axis with the HC. If the motor sounds like it's struggling, you may have to make adjustments to the worm gear mesh by following the procedure above.


Disassemble the RA Axis

This procedure is similar to the DEC axis. The main difference is that there are sealed bearings inside the RA housing.

  1. Open the casing
      2. a. Unscrew the plastic screw on the top of the casing
      b. Remove the plastic clip
      c. Remove philips screw
      d. Remove hex screw near the Altitude adjustment.
      e. Open the casing and disconnect the motor from the circuit board and put the electronics aside.

  2. Loosen the three Phillips screws holding the motor and slide it out of the way, then tighten them back again. This will allow the worm assembly to be removed easier in the next step.

  3. Remove four screws holding the worm assembly to the DEC shaft. Make sure to note which holes the 4 screws came from. They may be of different lengths (mine were not, but I kept track of them anyways). There is a small hex set screw in the middle of the two large hex screws. Do not remove this set screw. This is not the best image but hopefully you will be able to figure it out. You can also disassemble the worm assembly (see DEC worm assembly).

  4. Remove the polar scope (if you have one), RA setting circle screw and setting circle.

  5. Now you want to remove the ring nut by first loosening the three (DEC has only two) hex screws. Rotate the RA shaft until you see the set screw through the RA setting circle screw hole.

  6. You will probably have to increase the size of your spanner tool by about 1/4 inch. Use the spanner tool to remove the ring nut.

  7. Once the ring nut is removed you will find the top casing to the bearing assembly, the bearings, and then the bottom part of the casing. The top part of the bearing case was stuck to the bottom of the ring nut after I removed it. I could not easily remove the bearing assembly, but when I slid the shaft form the housing the bearings came out easily.

    Again look in the housing for a small plastic button that is part of the RA lock lever. It does fall out easily, so put it aside. There is a washer under the lock nut.

  8. The worm should now slide of the shaft. There should be a washer on each end of the worm gear. Note where the washers were found and in what orientation they were placed.

Cleaning

Perform the cleaning as you did with the DEC axis. Clean all the parts with the grease remover. Check for any rough edges on the washes and try and smooth them out. Check for rough spots in the housing. There were not many rough edges in my mount. Use the toothbrush to clean the worm gear and worm assembly.

Grease

Place a small thin coat of grease on all surfaces that touch. This includes, washers, inside of the shaft, bearings, worm assembly, screws inside the worm assembly and anywhere surfaces come into contact with each other.

Asjustment

As with the DEC axis, re-assemble and adjust.

  1. Ring Nut
  2. Worm shaft end play
  3. Worm gear mesh

Issues I ran into

Loose RA worm mesh - One issue that I ran into after doing the re-greasing and re-assembling of the RA axis was while doing the ‘worm gear mesh’ adjustment. Seems even though the set screw was all the way out (which means the worm should have been meshing tightly to the mesh), there was still a tiny bit of wobble. This does not seem to cause any problems, but I’d like to find out why it’s not meshing as close as it should be.

RA Motor runaway & '16 No Response' - Another problem which scared the crap out of me, was after I attached the casing, hooked up the hand controller, turned on the mount and pressed one of the RA buttons (right, left), the mount to move in RA and would not stop. It sounded really loud and moved faster than rate 9. The HC reported ’16 No Response’. I spent the next hour trying to see if I accidentally unhooked a cable or broke a wire. Turns out it was my Celestron Power Tank that was dead. Phew… Changing the battery put things right.

Dec set screw stripped - One of the small set screws on the DEC ring nut was stripped. The other was fine. Maybe over time the ring nut may come loose, but I'm not too concerned.

RA plastic housing screw replaced - One last thing I encountered when re-assebling the RA casing was that the small plastic screw stripped and I ended up having to cut it off and replace it. I think this screw/clip idea could be improved.

Results

So what has all this accomplished. Well in -14C, the mount performs smoothly and balances with as little as 1 lbs. I have tried some preliminary tests on how well it tracks. I used GuideDog (free tracking PC software) with my webcam attached to my Orion 900mm f/11.4 guidescope. Before the re-grease after about 30 seconds of tracking the dx/dy values would jump to on average +-4.5. Now after 5 minutes the dx/dy only reaches +-3.1. Mission accomplished !

Conclusion

I hope that you have found this tutorial to be useful. I would like to thank all who have helped with this process. If you have any questions or would like to leave me some feedback please e-mail me: tim@deepskyeye.com.