Periodic huge dither

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Paul Patterson

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Sep 14, 2021, 9:21:30 AMSep 14
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I have an issue where I get a huge dither and then PHD2 has see-saw attempts to settle. I'm embarrassed to say I've suffered through this for months. At times it can take phd2, 45+ seconds to settle.
I've attached a folder that has my last night log file and a screen shot of what it looks like in PHD2. As you can see not all my dithers behave this way but I have seen a night where 80% of my dithers look this way. 

-Mount is a Ioptron CEM 60 EC with zwo asi1600mm pro with zwo filter wheel and OAG.
-guide camera is the Lodestar X2.
-PHD2 version is 2.6.9dev5. 

I first encountered the error after updating my ASCOM platform to satisfy NINA. This prompted me to update my firmware for the mount to coincide with the newer ASCOM platform.

I redid calibration last night but not guiding assistant as I had clouds that would roll in soon. Side note, PHD2 tells me there is considerable difference in my RA and Dec calibration step size which may indicate a large DEC backlash. I see differences as well during calibration that my East & West steps are very small but north/south ones complete in the normal 8 - 12 steps. However, my guiding seems good (screen shot).

Ignore the last part of guiding as clouds did finally roll in and PHD2 was giving me star mass loss, etc.
Thanks
James Patterson
20210913_dither.jpg
PHD2_GuideLog_2021-09-13_105553.txt

Brian Valente

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Sep 14, 2021, 2:53:56 PMSep 14
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Paul

i suspect this could be in your calibration 

you need to be calibrating at the intersection of meridian and celestial equator (dec 0) - it looks like you are calibrating far from that

also your # of calibration steps should roughly be the same, but you have a monster number of RA steps and a relatively small number of DEC steps

I suggest you focus on getting a good calibration and see how it goes from there

you can also follow the baseline guiding document to ensure you have everything configured properly 
 

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Brian Valente

Paul Patterson

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Sep 14, 2021, 4:07:17 PMSep 14
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The small RA step size could very well be how I calibrated last night. I have a very small swatch of sky at my house.

However, my random large dither and see-saw behavior happens even when I am out in an open field and I calibrate at Meridian and Equator. So, there is still something going on there. I was hoping the log file might shed some light on why it goes past the center, where the guide star is. It's as if it can't reacquire the guide star.

I'll take a look at the document to see if I have any settings set incorrectly.

Thanks
James

Brian Valente

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Sep 14, 2021, 4:17:03 PMSep 14
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Hi Paul

>>>However, my random large dither and see-saw behavior happens even when I am out in an open field and I calibrate at Meridian and Equator. 

I can only respond to the data you sent ;) 

if you have other logs that demonstrate that, happy to look at it as well




bw_msgboard

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Sep 14, 2021, 11:57:04 PMSep 14
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As Brian said, your problems are coming from a really bad calibration.  You should be getting an alert message telling you the RA and Dec guide rates computed in PHD2 don't make sense. In your case, the Dec guide rate was 12.2 arc-sec/sec which lines up well with the reported guide speed in the mount of 12.0 arc-sec/sec.  But the RA guide rate from the calibration was just 1.8 arc-sec/sec - and that was corrected for your pointing position.  More importantly,  the east moves look really bizarre - look at the rightmost column in this sequence of east moves - the number just shows the star's distance from the starting point in units of pixels:
 
East,23,5.685,-24.860,244.113,123.726,25.502
East,22,4.099,-18.142,245.699,117.008,18.599
East,21,4.033,-17.945,245.764,116.811,18.392
East,20,4.351,-18.470,245.447,117.336,18.975
East,19,2.347,-11.248,247.450,110.114,11.490
East,18,2.332,-11.221,247.466,110.088,11.461
East,17,2.503,-11.684,247.295,110.550,11.949
East,16,0.579,-4.401,249.219,103.267,4.439
East,15,0.507,-4.539,249.291,103.405,4.567
East,14,0.733,-4.452,249.065,103.318,4.512
East,13,-0.854,2.576,250.652,96.291,2.714
East,12,-0.861,2.697,250.659,96.170,2.831
East,11,-0.774,3.106,250.572,95.760,3.201
East,10,-2.847,10.549,252.645,88.317,10.927
East,9,-2.710,11.081,252.508,87.785,11.408
East,8,-2.920,11.161,252.718,87.705,11.536
East,7,-4.719,18.338,254.517,80.528,18.936
East,6,-4.768,19.030,254.566,79.837,19.618
East,5,-5.089,19.621,254.887,79.245,20.271
East,4,-6.695,26.822,256.493,72.044,27.645
East,3,-6.875,27.596,256.673,71.270,28.440
East,2,-7.074,27.531,256.872,71.335,28.425
East,1,-8.962,34.849,258.760,64.017,35.983
East,0,-9.351,35.559,259.148,63.307,36.768
This should show a smooth, continuous movement in the same direction.  But it doesn't at all - it starts out moving ok, then takes an abrupt jump of over 7 pixels, then stays at a plateau value with little movement, then suddenly over-shoots by a large amount!  I think there is something wrong with the RA mechanics - something like binding - or how the mount controller is working.  It's also possible you have a cable that's binding and preventing the RA axis from moving freely.  Going forward,  don't use exposure times less than 1 second for calibration and be absolutely sure there's nothing interfering with the mount's ability to move in RA.  When you do a calibration, you should immediately look at the results using the Tools/Review Calibration menu.  Look at the computed guide rates and see how they compare to the 'expected' rates - they should be reasonably close, not off by a factor of 10x.  I can't imagine why you would have had to upgrade your mount firmware but I would be concerned about that.  You may need to do some kind of factory reset to get things back in order.  If this behavior continues, I think you'll need to get help from iOptron.  You can do a PHD2 star-cross test to show a clear example of how the mount is responding to these guide commands.
 
Good luck,
Bruce


From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Paul Patterson
Sent: Tuesday, September 14, 2021 1:07 PM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] Periodic huge dither

Paul Patterson

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Sep 16, 2021, 6:05:39 AMSep 16
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Thanks Bruce. I'll employ those tools next clear night at Meridian and equator and see if it behaves the same.

James.

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