Occasional erratic behaviour when slewing EXOS2 #EXOS2

Leigh Caldwell

Hi all

Thanks for the always-interesting discussions in this forum! I am a relative beginner but starting to get the hang of the PMC-8 EXOS2 mount and my imaging setup.

Occasionally, I will press the slew button (in Stellarium, connected to ASCOM Device Hub) and the mount seems to go off on a journey to nowhere. Most recently, I tried to slew from M42 to Mars and the mount started turning on DEC axis, no movement at all in RA, and kept going and going, I'd guess 240 degrees of motion until the scope was almost pointing at the floor. At first I thought it was just going around the meridian in a roundabout way, but at some point I realised it was all going wrong and managed to stop it manually. (The objects may have been on opposite sides of the meridian - I didn't notice at the time but it's plausible given when this happened). The RA clutch was tightened normally and slewing had been working fine a little earlier (and tracking correctly too).

On two or three earlier occasions, I started out with an approximate slew to a target, then identified the actual location I ended up pointing at, and used the Sync button in Stellarium to sync the mount. I then attempted to slew by no more than 5 degrees to get to my original target, and the mount went on a trek halfway across the sky until I stopped it manually. On that occasion I didn't notice whether it was the DEC motor, RA or both that were moving.

I do have a challenge with a very restricted viewing angle in my location (a south-facing balcony in London) which means I can't do an accurate polar alignment nor a 2- or 3-star alignment, so I have to rely on either the sync in Stellarium or plate solving in Sharpcap. I have previously managed to do a polar alignment from a different location and locked the Alt levers in a good position, and by carefully measuring out my Az angle and mount position I think I have got an acceptable PA - my tracking is good enough for 30-second shots at 1000mm focal length. But is it possible that not doing a proper star alignment has confused either the ASCOM driver or the mount electronics?

Most of the time everything works fine (especially now that I got Sharpcap plate solve working), it's only occasionally that it shows this bizarre behaviour. Has anyone seen something like this happening?

I am not sure if I have any ASCOM or Stellarium logs from these events, but if that would be useful I'll make sure logging is turned on and post them if this happens again.

Other info:
 - Mount is on AC power
 - Connected to PC via serial through an unpowered USB hub
 - Sky-Watcher 200PDS scope (f/4.9)
 - ZWO ASI294MC-Pro
 - PHD2 + Mini Astro Essentials guide scope with Altair GP-CAM 290C guide camera (although I was not guiding when this happened)
Total equipment weight is somewhere around 10kg/22lbs which I think should be OK on this mount?

(I have a different problem with guiding and a few mm of wiggling in RA but I'll leave that for another post!)

Thanks for any advice you can give (even if it's just "don't worry about it, these things happen")

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