Sudden loss of guidance ... incomprehensible
Dominique DANIEL
Bruce Waddington
Something external to PHD2 caused the mount/scope to suddenly lurch by a huge amount to the south:
A common reason for this is a cable that hasn’t been routed correctly and starts to pull or drag at various pointing positions. At the point of the red arrow above, the Dec motor on the mount wasn’t running at all, which is the normal case during basic sidereal tracking. But anything that touches the scope or interferes with the tracking by even a tiny amount can cause these large excursions. Guiding is a very precise business – on your setup, a measured movement of 10 arc-sec corresponds to 20 microns of movement at the guide camera sensor, less than ½ the thickness of a human hair.
You will probably need to start using the LogViewer tool to sort through basic problems like this:
https://openphdguiding.org/phd2-log-viewer/
https://openphdguiding.org/tutorial-analyzing-phd2-guiding-results/
Imaging and guiding is a difficult undertaking and most people go through a lot of trouble-shooting to get things working correctly.
Good luck,
Bruce
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Dominique DANIEL
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Bruce Waddington
I think you’re in denial about the problem and aren’t remembering/characterizing your experience accurately. I have no doubt you made a good attempt at cable management and you’re convinced it’s exactly right. That’s a very common beginner mistake because it’s hard to get this right. What often happens is that people use a lot of cable ties, get a nice cable loom that’s fastened on the mount somewhere and it’s very tidy, looks very professional. And it works fine 95% of the time. But if one of the cables in the bundle is slightly shorter, the scope can move into a position where it pulls because it can’t move enough to create slack. That’s just one of dozens of possibilities – maybe there’s a projection or a knob on the mount that the cable is looped around. The fact that your problems emerge after a meridian flip is a strong clue. You probably need to invest at least a full night’s work on this – forget imaging, forget using NINA, forget anything but eliminating this problem. You have to do a whole sequence of manual meridian flips working through a full range of pointing positions. In your case, you can start at a known problem point – a meridian flip at Dec = 21 degrees, then let the mount track for about 5 minutes. You can’t do this during the daytime and only guiding will show the problem – as I said, you aren’t going to see movements of 20 microns. Of course, you won’t want to do any of this until you get past the notion that it’s a PHD2 problem – IT ISN’T.
Now I will walk you through the exact sequence of things that happened in the log you sent for April 4:
1. You did a calibration at 23:04 that looked ok. THIS WAS THE LAST CALIBRATION OF THE SESSION.
2. At 23:05, you started a 15 minute Guiding Assistant session at a pointing position of RA = 10.33 hr, Dec = 21.8 deg, side-of-pier = west.
3. You stayed in that location for another 15 minutes letting the system guide, and the guiding was fine.
4. At 23:47, you did a meridian flip and started another guiding session in the same pointing position but with the side-of-pier now east. PHD2 adjusted the calibration data correctly and things went fine for about 5 minutes. Then something caused the guiding assembly to make an abrupt move to the south. That is what I explained in the previous message. Did you look at the arrow? There were no guide commands issued when this occurred and the Dec motor wasn’t running. This move was not caused by PHD2.
5. Staying in the same pointing position, you then attempted four short guiding sessions in which the guide star continued to make huge excursions to the south. There were zero south guiding commands issued during this time – these moves were simply a continuation of your original mechanical problem in step 4.
6. At 00:01, you started another guiding session and the guiding went fine for about 3 minutes. During this 3-minute period, some south guide commands were issued and the mount responded correctly. YOU DID NOT RECALIBRATE. THE PRIOR CALIBRATION WAS APPARENTLY FINE AND ADJUSTMENT FOR THE MERIDIAN FLIP WAS FINE.
7. After about 3 minutes, the guiding assembly made another huge excursion in Dec, but this time to the north. Again, the guide star was never restored to its lock position.
I don’t see how I can help you any further. I’m not there to see what you’ve done with your setup or why it’s misbehaving. I think you need to roll up your sleeves and do the hard and annoying work to determine the cause. If you have any doubt about the correct setting of ‘reverse-dec-after-meridian-flip’, you should re-run the ‘Calibrate meridian flip’ wizard, being extremely careful to do exactly what is says.
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Dominique DANIEL
thank you again for all the attention and advice you give me. It is very very precious. In particular the analysis on the behavior of PHD2. I sent you the log of only one evening, but the observations made relate to several evenings (in particular for the recalibrations). After the overhaul of the wiring, I focused on the setting of PHD2. You have ruled out that it could be the cause of the failure. I will focus on the wiring again. I understood that the cause may seem ridiculous. A question remains: why does the failure occur after a few minutes and not immediately after the turnaround? Maybe there is a link with the elasticity of a cable? However if I recalibrate PHD2 the guidance becomes normal again.
It's a shame that the weather is ticking..I'm hurried to carry out these new tests .. I'll keep you informed if I find it ..
It is very precious to have you in support in this way. Thanks again.
Dominique
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Bruce Waddington
The delay can be random depending on the scope’s pointing position. Elasticity in the cable loom is definitely a possibility but it could also happen if the loom is getting blocked by some mechanical projection on the mount like a knob or some other fixture. If you still think that the problem is always resolved by re-calibrating – I haven’t seen any data to support that – you should by all means re-run the ‘Calibrate Meridian Flip’ wizard doing exactly as it says. Meridian flip adjustments in PHD2 have been around for as long as PHD2 has, it’s used by thousands of people every month so it works.
Hope you can track it down,
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Dominique DANIEL
Bruce Waddington
Have you run the Meridian flip calibration wizard or are you just guessing at the setting? It looks like the CPWI software has lots of settings dealing with meridian flips so I suggest you take this up with one of the Celestron forums. There are 100’s of users who use CPWI and don’t have this problem with PHD2, so I think you’ll get better help elsewhere. Sorry, I don’t have anything else to suggest.
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Dominique DANIEL
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