Dec backlash problems with PHD2

[glowingturnip]

So on my last guiding session, I was having awful problems with my Dec guiding and backlash with PHD2.

During calibration, RA would calibrate fine with about 15-20 steps (about 600 calibration step), but then after that, it would run a few steps of 'clear backlash', then start stepping north with no discernable movement of the guidestar on the screen, until presumably the dec backlash properly cleared and the guidestar would leap out of the box in a single bound and calbration would fail.

Eventually I got a calibration run to complete, though I knew it would be a poor Dec calibration since it only took about one step each in N and S, and they were big ones, only just staying inside the box.  Sure enough, I lost the guide star several times during the imaging session - the Dec graph would be nice and stable, not showing any significant drift, until out of the blue the guide star jumps and it fails.

Eventually, after much hardship I managed to settle it on a dynamically stable setting - there was a small PA drift, so I put it on N-only (or was it S, I forget), and reduced the max move such that it could never quite catch up but was constantly pushing it in the same direction, and finally I was getting a nice straight line in Dec.  Mind you, it was about 3am at that time.

I've not had any signifcant problems with Dec calibration before, so was quite surprised I was having such problems this time.   I've recently added guidescope rings to the rig (the guidescope was just screwed into the piggyback screw before ) so there's a little bit of extra weight there and the guidescope is a little further from the mount axis, but it's all still well within the mount's max rated weight.  I'm using a QHY5Liic as guidecam instead of a Meade LPI but I can't see that's relevant, and using latest version of PHD2.  Guidescope is 400mm focal length, 70mm aperture.  Mount is a CG4 with after-market Synscan GOTO kit on it, connected via ASCOM through the handset.

Reading around on the subject, I found a very good tip that says you should clear the Dec backlash manually before calibrating by moving the mount North with the manual controls until you get definite star movement, and then calibrate, so I shall definitely try that.  Also going to double-check that my scope is still well balanced on its Dec axis.

Anyone else had similar problems, and is there anything else I can try ?

[Earl]

i have seen it i just close it restart and calibrate, normally fixes it

[Leveye]

In my experience it's almost always balance issues. When i ever lose the guide star it's because of cloud cover.

[Thalestris24]

Hi

I have backlash problems with my heq5. I probably need to do some adjustments but that means taking everything off my more-or-less permanent set-up.

Anyway, just to check, have you used the PHD2 built-in calculator to work out your calibration step duration? PHD2 says for a 400mm guide scope, qhy5l-ii and 10 calibration steps,  it should be around 3250ms if guide speed is 0.1 x sidereal, or 650ms if guide speed is 0.5 x sidereal, or 350ms at 1 x sidereal. 

Louise

[glowingturnip]

yep, I did those calcs.  I actually find I usually need to go about 100 or so longer than it recommends.  My guidespeed currently set at 0.5x, and I don't think I can differentiate between RA and Dec guidespeed

[Thalestris24]

yep, I did those calcs.  I actually find I usually need to go about 100 or so longer than it recommends.  My guidespeed currently set at 0.5x, and I don't think I can differentiate between RA and Dec guidespeed

Hi

Ok, though 15-20 steps sounds a lot! I usually aim for about 10 - there doesn't seem to be any point in doing more than necessary! I think PHD needs a minimum of 7 steps? It doesn't sound like you're calibrating properly anyway... It's certainly worth checking you're balanced ok in all 3 axes.

Craig Stark's troubleshooting guide is a useful resource:

http://www.stark-labs.com/craig/resources/Articles-&-Reviews/DiagnosingPHDGuiding.pdf

Louise

[glowingturnip]

Thanks Louise, I'll have a read through that, not seen it before

[glowingturnip]

3 axes ?

[Thalestris24]

3 axes ?

Yep, x,y and z! edit: that's my shorthand for RA, DEC and vertical!

Astronomyshed have some good vids:

https://www.youtube.com/watch?v=hGduG2jB9ec

With a follow up for imagers:

https://www.youtube.com/watch?v=uK2bXfVNoQU

Louise

[glowingturnip]

vertical !  hmm, I bet i'm not balanced in that properly, especially with the camera on.  Will have to have a fiddle