Nexstar SLT azimuth slip problem

[Cosmic Geoff]

I have had a Nexstar SLT Goto mount with Maksutov 127mm for nearly two years. It has always had a tendency to lose alignment after a period of use.  It's been hard to put a finger on the problem, but AFAIR it never happens during daylight use (solar system align), and mostly happens after an hour or two in the dark, which I take as a signal that it's time to end the session.

I just acquired a Starsense which has rather pointed up the problem - great when it works, but last night got this horizontal slippage again. I thought there might be an azimuth clutch I could adjust, but some sources say there isn't one. Any ideas?

Or, how much would it cost to get the thing serviced? Or get another used head? 

[jambouk]

If it is anything like the Skywatcher version, there is a nut for each of the two axes which can be nipped up. The one for azimuth is underneath a bar at the base of the mount arm (have to take it out of the tripod and undo a screw on the bar to get to the nut); and the one for the altitude is inside the arm, under where the dovetail for the scope would sit. Nip these up, not too tight else the motors may stall and burn out, then just do a 360 in both axes to make sure it's not too tight.

James

 

[Cosmic Geoff]

Thanks. I have looked at stripdown pictures for an earlier version of the mount, in which the lower nut apparently adjusts slippage. But there is no slippage in my mount when I try to move the base in azimuth by hand.

[jambouk]

Have you checked the altitude for slippage too, and tightened if appropriate? 

These are pretty basic mounts, and do an amazing job, but you can't expect it to retain bang on alignment with a long focal length scope for hours on end. I wonder if you are expecting too much from it maybe? 

Do you get it nice and level before doing the initial alignment? Do you change eye pieces much during the session? The merest movement of the axes manually (or the tripod) will have a cumulative effect of making the GOTO and tracking less accurate throughout the night.

I hope you get it sorted.

James

 

[Cosmic Geoff]

FWIW I took off the covers and had a look inside the mount. There are two small motor/gearbox units with encoders, one for altitude and one for azimuth. These units look identical.  There is a small PCB with components, and another PCB for the connectors. 

On the upper side of the base is a small locknut which adjusts the preload between the azimuth rotating parts.  On the lower side of the base is a locknut done up tight.  When I slacked it off nothing came loose and the  azimuth did not loosen up so I tightened it up again.

I removed the dovetail clamp screw and tightened the altitude slip clutch up a bit.

[jambouk]

Do you think this will help?

[Cosmic Geoff]

I have used the mount since the last post and did not get the slippage problem, so at least I have not broken it.

[jambouk]

Sounds like you might have fixed it. Well done.