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Guiding after tuning and Rowan Belt mod to HEQ5

Astrophotography
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#1 Topographic

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Posted 04 May 2021 - 04:45 AM

After managing some pretty poor guiding results lately I decided it was time to (re) tune my HEQ5 and then went the whole hog and fitted the Rowan belt upgrade. Below are two runs (without imaging) with the ZWO mini guide scope and ZWO ASI290. The mount is on a pier. The 'target' was the Whirlpool Galaxy, so very in the sky. The first guidelog is using unsmoothed PEC data. The second using PEC data smoothed in PecPrep. The seeing on the first night was quite a bit better than the second. Polar alignment error for the first night recorded at 1.2', 2.1' on the second

 

The log shows  I was using 2000ms (2") exposures. Multi-star seemed to be working on the second night but not the first. Also the corrections don't show even though I saw corrections live on PHD2.

 

I am pleased with the results and the consistency, next will come a test with actually imaging with dithering. I am interested in the spikes in Dec in the second image, a couple of pairs seem to indicate a connection with the worm period of 638 seconds. Any ideas would be welcome.

 

The lack of the grinding sound from the old cogwheels is wonderful.

 

 

Attached Thumbnails

  • Screenshot (2).png
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Edited by Topographic, 04 May 2021 - 04:50 AM.


#2 michael8554

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Posted 04 May 2021 - 08:08 AM

To see the PHD2 corrections you need to expand the X and Y axis in Log Viewer.

 

Dec isn't driven the same as RA.

 

It is essentially stationary, particularly with good PA, which you had.

 

So IMO those spikes on the second night are a combination of Dec balance, and Stiction and Backlash suddenly being released.

 

With some mounts with very loose bearings, a move in RA can reflect into a Dec movement, sometimes seen in the Calibration.

 

But in your case I don't think those spikes are a function of the RA period.

 

As always, a PHD2 GuideLog would save making guesses.


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#3 Rasfahan

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Posted 05 May 2021 - 01:56 AM

The spikes look like something is sticking or lose on the DEC axis - recheck the belt tension and slew with the cover open to look for anything odd (both axes). I am using my modded Heq-5 without PEC and get reliable guiding between 0.6-0.8“ RMS. I found PEC worsened my results (to 0.8-1.0“), maybe because of elasticity in the belt which destroys the regularity of the PEC (my theory, not tested). There also seems to be some drift in DEC (which is not fully corrected in each step, see before) - did you check PA only with PHD? I found its drift-based alignment error less than reliable, differing greatly from the error given by ASIAIR, Sharpcap and Polemaster (which basically agree).


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#4 michael8554

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Posted 05 May 2021 - 05:03 AM

"Polar alignment error for the first night recorded at 1.2', 2.1' on the second"

 

The PHD2 gurus say that for guiding, 5arcmins or less PA error is good enough, no need to waste time getting arcsec readings.

 

Which are so tiny it is hard to get repeatable readings.


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#5 Rasfahan

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Posted 05 May 2021 - 05:08 AM

To agree with Michael8554: Also check DEC balance. It needs to be perfectly balanced with the belt mod, not east/west heavy. And perfect means to really obsess about it. It‘s the step that takes me longest to get right during setup.


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#6 Topographic

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Posted 05 May 2021 - 12:01 PM

I ran the belts with the cover off several times and they seemed fine. My set up is on a pier in an observatory so i have a pretty stable PA, done with Sharpcap a few days before running the test. 

 

The spikes look like something is sticking or lose on the DEC axis - recheck the belt tension and slew with the cover open to look for anything odd (both axes). I am using my modded Heq-5 without PEC and get reliable guiding between 0.6-0.8“ RMS. I found PEC worsened my results (to 0.8-1.0“), maybe because of elasticity in the belt which destroys the regularity of the PEC (my theory, not tested). There also seems to be some drift in DEC (which is not fully corrected in each step, see before) - did you check PA only with PHD? I found its drift-based alignment error less than reliable, differing greatly from the error given by ASIAIR, Sharpcap and Polemaster (which basically agree).

The drift stats were.

RA +0.04, +0.01 Dec -0.22, -0.04

and for the second night

RA -0.9, - 0.18, Dec -0.38, -0.18

 

Balance is one of those things that i need to get better at, that could be a good call as well.



#7 Rasfahan

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Posted 05 May 2021 - 04:02 PM

Yes, those stats look good. I think 1h is just not enough for PHD drift alignment, in my case. This basically leaves something flexing/sticking for the incorrectly errors and I think unbalance in DEC is the most likely culprit. I have a moveable 50g weight on my dovetail to get balance correct, positioning needs to be exact to 1 cm for best guiding/tracking. I do not find it difficult to get right with that weight but trying to move the OTA to the correct position was tedious. I am also quite marginal with 10 kg of weight and 1600mm FL.




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