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Peje

Bizarre Dec Guiding Blip

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As title,

Most nights I get a spot of weird guiding error, normally I only notice it once but it may happen again when I'm in bed. Basically the Dec drifts off really far and then after maybe 5mins it seems to just snap back to what it should be.

The attached image shows the event in PHD and the scope position. Looking at this position I doubt it is related to balance, which is set to be camera heavy.

Anyone ever seen this before? My mount was stripped down during the summer, belt-modded and regreased; no problems with the worm were observed (I did ask the chap to check). This blip has been present for a couple of years so it isn't related to the rework.

Keen to hear any thoughts.
Pete

Guiding - Weird Drift.png

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Two things I'd check for wold be something sticking or a cable dragging.
 

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I am not expert in any way shape or form but this has happened to me and it was a cable catching on, my laptop, though after two years I would have thought you would have noticed a cable problem, good luck.

Alan 

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That's interesting, I have my cabling bundled in a loom with spiral wrap. The plus side is that I have RA and Dec as separate pieces so I can look specifically at one piece. 

Not totally convinced this is the issue but I don't have another ideas :)

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You may have 2 separate 'looms', but have you left enough slack to ensure that there is free movement all around.

I also used to use spiral wrap, but found it a pain when changing things around. I now just use the cables bound together with small Velcro ty-wraps.

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The 'camera heavy' weighting will reduce to zero as you point at the zenith. (The heavy end will be directly below the light end.) Does position in the sky seem to have any connection with the problem? I think it's unlikely that this is connected with your issue but if your camera-heavy weighting is intended to stop the Dec axis oscillating across any backlash there is a better way to achieve that. You can deliberately polar misalign (not enough to trouble your longest subs) and then disable whichever Dec guide direction is not needed to correct for this misalignment. Unfortunately you need to reverse the disabling after a flip. 

Your trace is pretty surprising. Firstly, as it's drifting off, it remains smooth and doesn't look as if it's struggling with a snag (though it might be.) Secondly, when it goes back, it does so without overshooting or oscillating. If you disable Dec guiding what does the Dec trace do? Does it in any way resemble the errant trace?

Olly

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I am no expert at all but I may have an idea. To me it looks like the dec is suddenly not responding to the guide pulses and just slowly drifts off on its own (probably due to a slight polar missalignment), and then suddenly it responds and gets back to where it should be. Would not a lose connection / bad plug be able to do this? Maybe you could try to change the ST4 cable or what ever is connecting your computer to the mount.

You could also wiggle the cable a bit an see if you can induce this behaviour.

Edited by gorann

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PS. If you are present next time this happens you could listen to the mount. You should be able to hear if the motor is responding to the guide signals. If it is then there must be some mechanical disconnection inside (you did mention you had taken it apart). If the motor remains silent while PHD2 is sending signals then it is likely to be a loose connection.

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I would suggest cabling as well.  The feature looks like something is increasingly resisting motion until it snaps back.  That would suggest something is getting caught until such point as it is stretched to far and whatever is resisting it slips over and it releases the pressure.  I'm thinking maybe the spiral wrap is snagging on something (e.g. polar scope cover etc) and it stretches until it is pulled so taught that it slips against whatever is holding it back.

 

 

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Your image reports you were at Dec 39, not that high, so your camera heavy setup should not have allowed the scope to get south at all. 

So maybe it wasn't as camera heavy as you thought. 

And as Olly suggested, a slight PA error that causes the Dec trend to be south will help, about 5arcmins. 

I agree with Olly that the stiction or cable snag behaviour, but without an overshoot on the recovery, is hard to explain, so listen for Dec activity next time as Goran suggests. 

Michael 

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20 hours ago, ollypenrice said:

The 'camera heavy' weighting will reduce to zero as you point at the zenith. (The heavy end will be directly below the light end.) Does position in the sky seem to have any connection with the problem? I think it's unlikely that this is connected with your issue but if your camera-heavy weighting is intended to stop the Dec axis oscillating across any backlash there is a better way to achieve that. You can deliberately polar misalign (not enough to trouble your longest subs) and then disable whichever Dec guide direction is not needed to correct for this misalignment. Unfortunately you need to reverse the disabling after a flip. 

Your trace is pretty surprising. Firstly, as it's drifting off, it remains smooth and doesn't look as if it's struggling with a snag (though it might be.) Secondly, when it goes back, it does so without overshooting or oscillating. If you disable Dec guiding what does the Dec trace do? Does it in any way resemble the errant trace?

Olly

I will have to give that a try, one slight issue will be that this often happens when I'm not watching it so I only actually see it from time to time.

18 hours ago, gorann said:

I am no expert at all but I may have an idea. To me it looks like the dec is suddenly not responding to the guide pulses and just slowly drifts off on its own (probably due to a slight polar missalignment), and then suddenly it responds and gets back to where it should be. Would not a lose connection / bad plug be able to do this? Maybe you could try to change the ST4 cable or what ever is connecting your computer to the mount.

You could also wiggle the cable a bit an see if you can induce this behaviour.

Don't think it could be a loose cable, the comms are single Rx & Tx so a loose connection would knock out RA & Dec.

12 hours ago, Whirlwind said:

I would suggest cabling as well.  The feature looks like something is increasingly resisting motion until it snaps back.  That would suggest something is getting caught until such point as it is stretched to far and whatever is resisting it slips over and it releases the pressure.  I'm thinking maybe the spiral wrap is snagging on something (e.g. polar scope cover etc) and it stretches until it is pulled so taught that it slips against whatever is holding it back.

 

 

This does seem reasonable, just need it to stop raining long enough to have a look :)

7 hours ago, michael8554 said:

Your image reports you were at Dec 39, not that high, so your camera heavy setup should not have allowed the scope to get south at all. 

So maybe it wasn't as camera heavy as you thought. 

And as Olly suggested, a slight PA error that causes the Dec trend to be south will help, about 5arcmins. 

I agree with Olly that the stiction or cable snag behaviour, but without an overshoot on the recovery, is hard to explain, so listen for Dec activity next time as Goran suggests. 

Michael 

It could potentially be explained by a slight slip in the dec but it's hard to explain why this would be happening as it doesn't happy when I extremely unbalance the OTA.

I don't think this can be explained by a neutrally balanced OTA, it's too consistent. It drifts off then snaps back. As you say, it's not very high so I would expect this would happen more readily pointing towards zenith (which it didnt as that where my nights imaging began)

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So is this Dec drift

a) the result of the Dec guiding simply failing to work for a spell. (Seems a bit extreme for that?)

b) the result of errant guiding inputs driving the mount the wrong way for some reason.

c) the result of something mechanical like snagging or slippage.

The test to eliminate 'a' would simply be to see how fast it drifts in Dec without any guide correction at all.

Instinct keeps me fixated on that totally smooth return to the right position without any overshoot afterwards. That doesn't have the look of mechanical origin to me. It smells of electronics/software, but watch me be wrong...

Olly

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Might be worth updating to the latest version of PHD2 - one of the improvements is better detection of runaway guiding, excessive backlash and calibration problems etc.

https://openphdguiding.org/downloads/

v2.6.6

24 March 2019

  • Lots of improvements to the Guiding Assistant
  • Updated camera support: Altair, QHY, SBIG, SSAG (Mac), ZWO ASI
  • New ToupTek camera support for Windows
  • New MallinCam SkyRaider camera support for Mac
  • INDI SBIG AO support
  • Better detection of problems like runaway guiding, excessive backlash, and calibration problems
  • Improved backlash compensation
  • New Meridian flip calibration tool
  • New ZFilter guide algorithm
  • Guide algorithm None has been removed
  • INDI improvements
  • Various minor user interface fixes
  • Updated Help file
  • Updated translations
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4 hours ago, tooth_dr said:

It’s slips back to an unnaturally flat line

I also noticed that. I would like to see more data points after the dec drift is restored. My suggestion: run the guiding assistant. 

Edited by wimvb
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Updating PHD2 seems like a good idea, I hadn't noticed the flat portion of the line. I can grab the PHD2 debug logs are replot with some time after to see what it later does.

It certainly could be a software glitch, the issue with that is it's unlikely I'll be able to fix it!

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On 12/09/2019 at 16:13, glowingturnip said:

you could post your guiding log to the phd forum, they're pretty helpful over there.  It's not excessive Dec backlash is it ?

Good suggestion, done :)

 

Have attached a screenshot of the output from guiding assistant, other than huge dec backlash I don't see anything interesting.

Guiding Assistant.PNG

Edited by Peje

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The original guide graph shows a much stronger dec drift than the backlash measured by the guiding assistant. Nevertheless, first order of business would be to reduce that backlash.

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happened to me once in a while which took me about 1 month to fix. Mount is sticking due to overtightened worm gear assembly. DEC will run away and comes back. By loosening the allen screws and tuning it, it was gone. The saddle will actually wiggle if overtightened and tight when loosened..I have a CGX mount

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1 hour ago, retret66 said:

happened to me once in a while which took me about 1 month to fix. Mount is sticking due to overtightened worm gear assembly. DEC will run away and comes back. By loosening the allen screws and tuning it, it was gone. The saddle will actually wiggle if overtightened and tight when loosened..I have a CGX mount

I don't think it can be the same issue for me, I have quite a large amount of dec backlash (on purpose). 

That's said, I plan to reduce that significantly this week.

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8 hours ago, Peje said:

I don't think it can be the same issue for me, I have quite a large amount of dec backlash (on purpose). 

That's said, I plan to reduce that significantly this week.

I just checked mine I have CGX, yeah your backlash is big. Here is what I have and my mount has been sitting on my wooden deck for a week and I am surprise the drift rate is still good.

EA5E7583-E00B-4811-840C-648C89E6A211.jpeg

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