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Novice question about PHD guiding

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#1 snoraste

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Posted 09 December 2018 - 01:46 PM

Using PHD2 guiding does not give me round stars even on short (4min) exposures. I know it could have multiple causes, but I was wondering if my PHD log would be able to indicate an issue that I'm missing. Thank you. 

 

equipments: NEQ6/ES102 guided by 60mm/f4 guider. Camera QHY 5L IIc 

 

Polar Alignment error was about 10as

 

Guiding Assist Parameters:

 

 

INFO: Guiding parameter change, MountGuidingEnabled = true
INFO: GA Result - Backlash= 4603 ± 226 ms (36.0 ± 1.8 arc-sec)
INFO: GA Result - SNR=20.4, Samples=98, Elapsed Time=146s, RA RMS=  0.19 px (  0.60 arc-sec ), Dec RMS=  0.17 px (  0.55 arc-sec ), Total RMS=  0.25 px (  0.82 arc-sec )
INFO: GA Result - RA Peak=  1.32 px (  4.25 arc-sec ), RA Peak-Peak   2.59 px (  8.34 arc-sec ), RA Drift Rate=  0.12 px/min (  0.38 arc-sec/min ), Max RA Drift Rate=  0.18 px/sec (  0.57 arc-sec/sec ), Drift-Limiting Exp=   1.4 s
INFO: GA Result - Dec Drift Rate= -0.26 px/min ( -0.85 arc-sec/min ), Dec Peak=  1.24 px (  4.00 arc-sec ), PA Error= 3.3 arc-min
INFO: GA Result - Recommendation: Try setting RA min-move to 0.25
INFO: GA Result - Recommendation: Try setting Dec min-move to 0.30
INFO: GA Result - Recommendation: Backlash is >= 4603 ms; you may need to guide in only one Dec direction

 

 

 

 

Guiding Begins at 2018-12-08 22:34:46
Dither = both axes, Dither scale = 1.000, Image noise reduction = none, Guide-frame time lapse = 0, Server disabled
Pixel scale = 3.22 arc-sec/px, Binning = 1, Focal length = 240 mm
Search region = 15 px, Star mass tolerance = 50.0%
Equipment Profile = mine
Camera = QHY5LII-C-6177e210415141466, gain = 40, full size = 1280 x 960, have dark, dark dur = 1500, no defect map, pixel size = 3.8 um
Exposure = 1500 ms
Mount = EQMOD HEQ5/6,  connected, guiding enabled, xAngle = -65.0, xRate = 2.147, yAngle = -155.1, yRate = 2.444, parity = +/-,
X guide algorithm = Hysteresis, Hysteresis = 0.100, Aggression = 0.700, Minimum move = 0.250
Y guide algorithm = Resist Switch, Minimum move = 0.300 Aggression = 100% FastSwitch = enabled
Backlash comp = disabled, pulse = 20 ms
Calibration step = phdlab_placeholder, Max RA duration = 2500, Max DEC duration = 2500, DEC guide mode = Auto
RA Guide Speed = 7.5 a-s/s, Dec Guide Speed = 7.5 a-s/s, Cal Dec = 7.4, Last Cal Issue = None, Timestamp = 12/8/2018 10:13:33 PM
RA = 5.59 hr, Dec = -5.4 deg, Hour angle = -1.65 hr, Pier side = West, Rotator pos = N/A, Alt = 38.5 deg, Az = 147.8 deg
Lock position = 716.369, 179.532, Star position = 716.369, 179.532, HFD = 5.29 px
Frame,Time,mount,dx,dy,RARawDistance,DECRawDistance,RAGuideDistance,DECGuideDistance,RADuration,RADirection,DECDuration,DECDirection,XStep,YStep,StarMass,SNR,ErrorCode
1,1.789,"Mount",-0.065,-0.503,0.428,0.272,0.270,0.000,126,W,0,,,,3861,43.32,0
2,2.976,"Mount",-0.214,-0.345,0.222,0.340,0.000,0.000,0,,0,,,,4480,46.68,0
3,4.756,"Mount",-0.422,-0.590,0.356,0.633,0.225,0.000,105,W,0,,,,4231,45.44,0
4,6.428,"Mount",-0.352,-0.087,-0.070,0.356,0.000,0.356,0,,145,S,,,4090,44.53,0
5,8.791,"Mount",-1.070,0.770,-1.149,0.642,-0.724,0.642,337,E,263,S,,,4444,46.65,0
6,9.463,"Mount",-0.423,-0.108,-0.080,0.429,0.000,0.429,0,,175,S,,,4180,45.12,0

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#2 einarin

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Posted 09 December 2018 - 01:56 PM

It could be that you have so much backlash.

Have you tried DEC guiding in one direction only (like GA suggested) ?

Also it could be flex from guide scope.

What kind of attachment do you have for it ?



#3 geothomas

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Posted 09 December 2018 - 04:32 PM

I would suspect flexure from the camera. Also it appears focus is a bit soft.



#4 Stelios

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Posted 09 December 2018 - 05:15 PM

What was your RMS for the *guiding?* (not the GA run). In RA? in DEC? Total? 

 

Are you balancing East-Heavy? 

 

You should dither in RA only. You should also try to guide North or South only, especially as you've disabled backlash comp.

 

The recommended minimum moves seem high to me. These are more like OAG minimum moves. Your exposure interval seems low. And I've never believed in 100% aggression. Still, none of those parameters is definitely wrong--just something I'd play with after I'd balanced east-heavy and set dither and DEC guiding as per above.

 

Finally, a frequent cause of oblong stars is differential flexure, the only sure cure of which is an OAG. You can't easily diagnose that from a single image,  but if you take a series, and some images have round stars and some don't--with similar guiding RMS--then it's very likely differential flexure.



#5 snoraste

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Posted 09 December 2018 - 07:38 PM

Thank you all for your suggestions and comments. The guidescope itself is bolted to the handle of the ES102. I don't feel there's any movement in the scope itself. What I need to do is to make sure the guidescope ringscrews are tight. I can imagine the QHY guidesamera may be flexing a bit. So my first order of business will be to double check all the nuts and screws.

 

For focusing, I usually focus on a very bright star (because of UHC filter, I cannot see any dim starts thru my LiveView), use APT's focus aid and then lock down the focuser . Then I slew to the target and pray all works.  

 

My total RMS was around 0.75 (forgot what DEC and RA numbers were, but both below 1). What I dont fully understand is guiding in one direction - specially DEC guiding. Isn't a backlashed-DEC guiding better than no guiding at all (sort of like vision and hearing?)

 

I will also play around with minMoves and  dithering to see what happens. PHD2 is connected to my APT app, Would that have impact on guiding?



#6 Mike7Mak

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Posted 09 December 2018 - 08:27 PM

Just going by the image (PHD logs make me dizzy) that looks like a DEC guide issue, not flexure. A screenshot of the graph and RMS numbers would be easier (for me) to diagnose.

 

If you can truly achieve 10 arc/sec polar alignment (which at your guiding scale is 3 pixels) you should be able to turn off DEC guiding altogether for 4 minute exposures, for test purposes at least. Then note the direction of drift in DEC and set PHD to correct that direction only. Lower the agression to 70 or so.

 

What is the pixel scale of the imaging scope? At 3.22 a/s/p in the guider a minimum move of .3 is nearly a full arc second. Might want to lower that as well depending on your imaging scale.

 

Increase guide exposure to 2-4 seconds. This is probably little more than DEC backlash combined with chasing the seeing.


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#7 StephenW

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Posted 09 December 2018 - 09:12 PM

Hey Snoraste,

 

Can you attach the full log?  It will really help diagnose what's going on.

 

But as was already pointed out, the 4.6s of DEC backlash is problematic

 

>What I dont fully understand is guiding in one direction - specially DEC guiding. Isn't a backlashed-DEC guiding better than no guiding at all (sort of like vision and hearing?)

 

No, backlash in not better than no-guiding.  But the suggestion here is to guide DEC in one direction, which is not the same as no guiding.

 

To guide in one direction you have to deliberately miss-align your polar alignment, causing stars to continuously drift in DEC in (mostly) one direction only.  This means PHD2 only has to correct the DEC drift of the guide star in one direction, and as it doesn't have to change guide direction it avoids backlash

 

Steve



#8 snoraste

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Posted 09 December 2018 - 09:54 PM

Hi Steve,

 

Please see attached the log file. Is there an easy way to fix the backlash on NEQ6? I tried the Astro-Baby method of cleaning up the DEC

 

 

Attached Files



#9 KenS

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Posted 10 December 2018 - 02:40 AM

In addition to backlash you've got issues on your RA axis as well. There's a couple of worrying jumps during the RA calibration and its moving back and forth around 5" 

I'm guessing clouds came over during your guiding session based on the number of Star Lost errors so there's not a lot of data to work with.

Looks like you need to adjust the meshing of the various gears and possibly also the end float. Worm mesh is the simplest but you might need to open up the mount and make sure the motor is also meshing properly.

 

Then run the guiding assistant for at least 15 minutes to check how that went.



#10 Der_Pit

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Posted 10 December 2018 - 06:40 AM

The elongation is in RA.  So my assumption would be a (too) steep gradient either from the normal periodic error (you should try the PEC mode as guide algorithm), or a higher frequency gear box oscillation (around 13s).  The guide assistant derives a drift limited exposure time of 1.4s, a bit shorter than what you use.

My mount has similar issues, in my case it's the gear box.  I can only properly catch it with 0.5s exposure time :o

But whether you can go that short mostly depends on your seeing conditions.



#11 snoraste

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Posted 10 December 2018 - 07:46 AM

Thanks you all again for feedback on this. I did clean up the DEC axis recently, but apparently did not work. Perhaps I should look into ceramic bearings and redo the process for RA as well. Just curious, can you actually get NEQ6 gears to run smooth (no backlash or gear box issues) or am I better off playing around with the software (one direction DEC, shorter exposure time, etc)?




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