Nexstar mount backlash?

[Seoras]

Hi, I’ve been having problems lately with my Nexstar 8SE. There seems to be an awful lot of play in the alt axis; I got in touch with Celestron support and they suggested it might be fixed with the backlash settings but I’ve had no luck so far. There is probably about 4-6mm of “loose” movement and the scope will slip downwards at the slightest touch. When trying to adjust the backlash settings I could hear the motor engage to compensate for backlash but it still leaves the looseness. I looked online and found a video that shows how to tighten the clutch with a big nut on the mount but mine is already tight.

I’be attached a couple of videos to help illustrate what I’m talking about but they don’t seem to give a good representation of the extent of the movement. Any ideas?

thanks, Geo 

[DeanCJ]

Hi Seoras 

looking at your first two videos there seems to be a lot of sideways play on the geared shaft. I would suspect that a bearing has perhaps failed, or there is possibly excessive wear to the shaft. If you have a small inspection mirror, such as what a Dentist uses, I would try and look behind the plate with the three cap head Allen screws, you may get a better idea of what is happening. Small bearings are widely available online. Perhaps it would be worth searching YouTube.

[Seoras]

9 minutes ago, DeanCJ said:

Hi Seoras 

looking at your first two videos there seems to be a lot of sideways play on the geared shaft. I would suspect that a bearing has perhaps failed, or there is possibly excessive wear to the shaft. If you have a small inspection mirror, such as what a Dentist uses, I would try and look behind the plate with the three cap head Allen screws, you may get a better idea of what is happening. Small bearings are widely available online. Perhaps it would be worth searching YouTube.

Thanks for your help. I did think the sideways movement was excessive but I’m new to telescopes so wasn’t sure if it was something that is built in as part of the backlash compensation. The gears seems quite small and flimsy considering the weight they’re moving so it wouldn’t be surprising if the bearings failed 😞. I’ll try to see what I can as per your suggestion with the mirror, it would be great if I could fix it myself rather than send it off to Celestron for a repair. Thanks again 👍

[Cosmic Geoff]

44 minutes ago, Seoras said:

There seems to be an awful lot of play in the alt axis; I got in touch with Celestron support and they suggested it might be fixed with the backlash settings but I’ve had no luck so far. There is probably about 4-6mm of “loose” movement and the scope will slip downwards at the slightest touch. When trying to adjust the backlash settings I could hear the motor engage to compensate for backlash but it still leaves the looseness. I looked online and found a video that shows how to tighten the clutch with a big nut on the mount but mine is already tight.

I have the same experience with mine.  I could not adjust out the vertical backlash with the settings, and the scope once slipped downwards while in use. I tightened a large nut but it was not very slack anyway.  The 'slipping downwards' has not recurred.

Your videos show a lot of play, but I do not recognise that part so cannot comment. 

My advice is to just live with it unless it is making the outfit unusable for visual use. 

[Knighty2112]

Yes, looks like an issue with a bearing. Just took some photos of my 8SE goto mount and you can clearly see the bearing if you take the other covers off the other side. 

 

If you take the gears off and the collar with the grub screws, you should be able to see if the bearing has totally gone. With the amount of movement I suspect it has broken.

 

 

[Seoras]

2 hours ago, Knighty2112 said:

Yes, looks like an issue with a bearing. Just took some photos of my 8SE goto mount and you can clearly see the bearing if you take the other covers off the other side. 

 

If you take the gears off and the collar with the grub screws, you should be able to see if the bearing has totally gone. With the amount of movement I suspect it has broken.

 

 

Thanks very much, that makes things a lot clearer. I’ll take the other cover off and see what’s going on 👀

[Grogfish]

This thread on CN might be worth a read, you may just need a little tightening on the correct nut. I needed to slightly tighten the altitude on my Nexstar 4 after piggybacking heavy loads for some time, and this worked for me:

https://www.cloudynights.com/topic/258333-altitude-nut-adjustment-nexstar-se-series/

[EarthLife]

If it's the same as the 6SE mount then it will use a cheap chinese plastic toy gearbox that sits between the brushed motor and those output metal gears. We stripped down these plastic gearboxes to see how well made they were due to the high level of backlash on these celestron mounts.

The plastic gears in these toy gearboxes don't mesh properly, so much looseness/backlash in them, it's terrible that they are using them on a telescope mount.

What the Nexstar SE mounts need is someone to manufacture a replacement metal geared gearbox ensuring that the internal gears actually mesh properly.

But whether or not you also have backlash in those external metal gears you've imaged I'm not sure of, the ones in our SE mount are fine, it's the plastic toy gearbox that drive those metal gears that is the real problem with ours

[Peter Drew]

On my 8se I noticed any backlash problem was caused by the shaft holding the brass gears being unsupported at the outer end.  Despite the gears meshing accurately, the brass gear unit could be pressured sideways enough to allow flexure of the Dec axis.  To remedy this I turned a diameter register on part of the brass gear and made a bracket with two small ballraces spaced radially to form a rolling support under the brass gear.  The only downside was having to cut an aperture in the cover to clear the modification.    🙂

Edited February 6, 2023 by Peter Drew

[Seoras]

Well I’m almost positive that Knightly2112 is correct with their diagnosis. I removed the casing from the other side of the mount and had a look at the gears from that side; there’s definitely a lot of play in the tiny gear shaft and I think the bearings look a bit squint. I’ve sent an email to Celestron support to ask them to fix it for me as I’m not dexterous enough to work with such small components (shaky hands 🙌). Not sure how long it will take to get the mount back from them but I’ll report back here once I know more. Meanwhile thanks very much to everyone who contributed to this thread, I really appreciate your help,

take care, Seoras 👍