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NexStar 8SE Mount Stopped Working


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I was out the other night with my Celestron Nexstar 8Se and all of a sudden the left / right azimuth motor packed in.....it moves very very very slowly and eventually stops turning altogether. The Altitude motor works fine - its just turning on the azimuth that's gone wonky and not much happens.

The sounds emerging from the device seem to suggest that its not getting enough power to turn the motor.

Since its out of warranty and cant afford another mount at the moment, I took the mount apart in the hope that there might be a loose wire flapping about but as far as I can tell, everything seems ok and plugged in where it should be. I didn't go as far as looking under the big metal plate where I guess theres lots of cogs,

Here's a highly exciting Youtube video of it not working

I've never turned the base by hand and have always kept it in its box, plus its relatively unused due.

Any ideas - anything that can be fixed?

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Difficult to make a remote diagnosis but...

Most of the noise does not come from the motor directly but from the drive gears.

All the gears in the 8Se are metal so it is unlikely you have broken one.

Looking at your video there is no error code displayed, this means that the RA motor is turning with the correct speed and direction as determined by the encoder bolted to one end of the motor.

However there is no noise which suggests the motor is not engaged with the rest of the gear train.

The opposite end of the motor from the encoder has a pinion gear meshed with the rest of the drive train and this pinion gear is held to the drive motor shaft with a single grub-screw.

The most likely thing is that the grub screw has worked loose and the pinion gear has either slid off the motor shaft or is in place but not engaging with the flat surface of the motor shaft.

Here is a link to a picture of the motor, you can see the flat area of the motor shaft that the pinion gear engages with on the right side, encoder is to the left side.

(slow link, may take a while before it opens)


Since you have not yet looked under the metal plate I guess you are not into DIY however there is not very much to be worried by as long as you take your time to disassemble the mount.

Take loads of pictures as you strip the base down, or make sketches as you go, take a large sheet of cardboard and stick double-sided sticky-tape in rows to the card, as you remove the various screws then stick them to the tape and write under them where the screws came from.

You should be able to reach the RA motor fairly easily and then you can check that the pinion gear grub-screw is tight.

If you really don't feel this is something you could try yourself then possibly contact your local astronomy club and see if anyone there can help you.


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Thanks for all the advice. :grin:

Sadly my tools aren't big enough to undo that big bolt in the middle of the metal plate - I guess that's what I need to undo to have a look at my insect based screw?

What size spanner would I need to get?

I learnt on the first viewing using this scope that batteries were a waste of time and found it amusing that they couldn't provide enough ummph to sustain moving the scope around - so it gets part way there and has a bit of a "senior moment" and forgets what its doing and goes off on bit of a random tangent.

I'd actually far prefer being given a computer program to reverse engineer in assembly language for vulnerability analysis and I have taken a few computers to pieces but doing stuff with motors and cogs gives me the willies.

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Hello again

So once you've got the right sort of spanner and removed the big bolt in the middle of the silver plate - how do you go about separating it and getting to see all the cogs?

At the moment it just sort of wobbles about a bit and doesn't come apart as if something else is holding it all together


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