I have a new CEM25P mount and whatever I try, I cannot get rid of the backlash in DEC. It guides phenomenally well in RA, but not so in DEC. I have tightened the belt, there is no discernible play in the gears, I have tried guiding in perfect balance and moderately unbalanced (to preload the gears), nothing makes a difference.

Any suggestions how to improve on this are most welcome!



Jo

Jo,

My girlfriend has a CEM25P as well and we get MUCH better guiding with it than what you show here. What telescope is mounted on the CEM25P? What guide scope and camera do you use?


Wouter

Hi Wouter,

Yes, I know, and the fact that this mount works so well for you was a big factor for me getting it to replace my iOptron SmartEQ mount that I had been using with the same WhiteCat 250mm and 120mm/30mm ZWO guide scope. The weight of that scope is well within the limits of the SmartEQ (11lbs) and very easily within those of the CEM25P (27 lbs).
The guide scope is not the factor in the DEC deviation. If it were, it should show the same deviation in RA, however there the guiding is nearly perfect.



I suspect this is a mechanical problem in the DEC array only. When you look at the guide graph, these excessive deflections happen suddenly, then the guider sends repeated guide pulses to the mount that, however, have no effect whatsoever, until suddenly again, the scope swings over into the other direction. It looks to me that there is a lot of backlash, while a tension builds that, when passing a threshold, sends the mount into the opposite direction, where the same then plays out again.
I tightened the drive belt in DEC to the iOptron specifications (~3 mm play), but that had no effect at all. There is no other play I can detect when tightening the tension screw. The only thing left is the little gear wheel on the shaft of the stepper. If that is not centered and tightened, I think that could slip just enough to explain this behavior.

If anyone else has experience with that, please post here, before I tear the mount apart in vein.

Greatly appreciate any suggestions.

Jo


PS: I see the exact same behavior whether the mount is perfectly balanced or with an (intentional) slight imbalance in DEC to preload the gears and take out slack that way. Doesn't seem to make any difference whatsoever and I would not have expected it, since there is no discernible movement when I manually try to move the mount in DEC. No mechanical play whatsoever. That's why I think the problem is at the level of the force transfer from the stepper to the belt.

Yes that telescope should be well within specs. As a matter of fact, I ordered a RedCat for my girlfriend and she intends to use it on the CEM25P as well. Great that you get such good RA guiding with such a small focal length for the guide scope!

Could the DEC problems be related to plastic gears or shafts somewhere? Perhaps they flex when put under pressure until they cannot flex anymore and then rotate. Just a guess, I don't know the internals of the mount well enough.


Wouter

As far as I can see, there are no plastic gears in that mount. Everything seems to be metal, which was a main reason why I wanted to switch from the plastic gear SmartEQ to the CEM25P, to avoid exactly that.
The only weak point I can see is the the belt, but that cannot be the problem as the mount is driven by exactly the same principle in RA and there is no variability. I would expect to see the same behavior then.
This must be something specific to the DEC force transfer that is causing that excessive amount of backlash.
I am still hoping to hear from someone who had the same problem and found a fix for it. If not, I'll disassemble my - brandnew - mount and see if I can find the source of the backlash.
Not what I was planning to do....

Jo

If no solutions come then I'd advice you to contact iOptron Tech Support. They generally respond VERY quickly and adequately. But if I recall correctly, you have had the same experience in the past.

Wouter

Yes, iOptron already sent me their instructions for taking the mount apart and putting it back together to eliminate the play/slack. I am still trying to avoid making the matter more complicated than it has to be, so if anyone else has experienced that and has a suggestion what the most likely cause is, I am all ears.

I've found this thread on CN that might be of an interest.

I have the same issue. How did you solve the problem? Thanks.

You have to take the DEC cover off, straighten the belt and then tighten it so it has no more than 3 mm slack when you press it down in the middle. Having the proper tension and it running straight is key.
Nonetheless, this remains finicky. For instance, I can now get outstanding DEC guiding with an RMS of 0.5" early in the evening, only to see it fall apart and disintegrate to >1" after the meridian flip. That shows the mechanical limitations of the mount.
You will have to get a feel for how much tension the belt should have and whether the axle is running tight, but without excessive resistance, etc. A real problem IMO is that the DEC motor is drawing about 40% less power than the RA motor, which results in it losing steps whenever there is too much resistance.
Also, make sure that the mount is perfectly balance in DEC and that there is no cable drag. With those weak motor, you have to strive to eliminate any potential mechanical interference.
Hope that helps,
Jo

Thank you for your input. Did DEC belt tensioning eliminate the sawtooth pattern in the DEC guiding graph?

This is the DEC guiding graph I got.