Troubleshooting Celestron Nexstar11GPS

Right ascension (RA) - drive-slippage troubleshooting. (Common problem).

For a long time, whenever I put my C11 up on the wedge on 35 degrees for Canberra, I had problems with the RA drive getting to a point where the motor could be heard slowing down, changing in tone like it was labouring, then the telescope base would give and almighty “click” whilst visibly slipping.

I never took it apart for fear of ruining it, but as the warranty had run out anyway, it was time to seek advice and act.

I rang a reliable source, and was directed to a very basic photo tutorial on disassembly. This looked a lot easier than I had imagined but I noted there are a few things to watch out for. So, this page is to cover it in a little bit more depth.

Here’s how to do it.

Preparation.

Prepare a place to put your telescope down once you take it off its base. Take all the accessories off your scope and fit the plastic caps over the openings. This thing is heavy and cumbersome, but also easy to break if dropped. A second person to help at the critical times would be good. A safe and stable place to do this is while still mounted to your tripod or pier.

Make sure that before you take things apart, you write down where things go. If you take a connector off, make sure it goes back on the right way, as some can fit on upside down. If you get it wrong, you can burn out all your electronics when you power up again.

Tools.

1. Open-end spanners, ranging from small to large.
2. Allen keys that will fit the socket head cap screws. Being made in the U.S., the sizes are strange. I bought a set of metric Allen keys and a set of imperial, but neither fitted. Its one of those sets that has all the funny in-between sizes of imperial.
3. Medium Phillips-head screwdriver.
4. White petroleum jelly and a cleaning rag.

Step 1.

Remove the cover off the base. Once the 5 cap screws have been removed, place them where you won’t lose them. Lift the cover, making sure you don’t pull forcefully on the wires connected underneath. Slide the cover out of the way. Note the twin black and red wire coming out of the centre stud and unplug it from the top contact plate. Make sure you note which way up it was. Note the other twin black and red wire coming out from underneath the bottom contact plate and do the same with it. Step 2.

Remove the contact plate retaining bolt and lift off the top contact plate. Undo the four silver Phillips-head screws holding the bottom contact plate and lift it off, exposing the bearing and lock nut underneath.

Step 3.

Undo the RA bearing lock-nut. At this point, the telescope is just sitting on three roller-bearings inside the base. Make sure you support the telescope from overbalancing and falling. Lift the telescope off the base and gently place it down somewhere safe. The bearing may fall out at this point, as it is just sitting in the middle hole. While you have the telescope off, look underneath and observe how the RA stepper and worm assembly move in and out toward and away from the centre of the scope. This will give you a better understanding of how the whole thing works and moves.

Step 4.

Undo and remove the RA gear lock-nut and washer. Release your clutch and lift the brass RA gear off.

Inspection and action.

Clean off and inspect the teeth on the RA gear for any wear or damage. Curved indents in the middle of each tooth would indicate that the stepper and worm drive assembly is loose. See below for how to fix that. If the teeth are badly damaged, you may have to source a new one. If Celestron is unable to supply, then there are plenty of metal fabricators in the phone book that will make you one copied from yours.

Turn over the RA gear and inspect the security of the black hub underneath. Check the three screws. If they are loose, tighten them and as a preventative measure, place a drop of Loctite in the end of each one.

Put the RA gear back and lock the clutch. Try hard to turn the gear in the clutch. If it still moves, undo the clutch and adjust the clutch adjustment bolts. Try again until it feels right. Do not over-tighten.

Put the lot back together again, tightening the RA gear lock-nut to allow the scope to rotate freely, but with no lateral wobble. Apply some white petroleum jelly to the teeth. Place the telescope back on and tighten the RA bearing lock-nut. Nice and firm but do not over tighten. Check rotation of the scope by grasping the fork arms and turning. Do this first with the clutch unlocked. The scope should rotate freely without undue force or sloppiness.

Lock the clutch. Grasp the fork arms and exert a little rotation each way. Watch the RA stepper and worm assembly to see if they are moving in or out. If there is too much movement, adjust the stepper and worm tension adjustment nuts. This is trial and error. Reassemble the rest of the telescope in reverse order, connect the wires but leave the cover off. Power on the telescope, set the slew rate to max and try to rotate the telescope using the keypad. If the RA motor sounds like it is struggling, back off the adjustment nuts slightly and try again. Do this until it sounds and feels right.

Once the telescope is fully reassembled, place it on a wedge, at least 45 degrees, and conduct a full rotation each way using the keypad, with the outer tube assembly (OTA) parallel with the fork arms, then perpendicular.

Happy tracking!