Newbie questions from a PhD2 autoguiding and PEC training session

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hhzhang
Posts: 730
Joined: Sun May 10, 2020 4:11 pm
Favourite scope: 115mm APO

Newbie questions from a PhD2 autoguiding and PEC training session

Post by hhzhang »

Rig: 115mmF7 APO, CEM40, ASIM183MM Pro. Pixel scale: 0.61"/px

A little bit struggling with my learning auto-guiding using the main OTA+cam. Well, the purpose was actually to learn how to train PEC and to fine tune the system so that autoguiding will become unnecessary for my targeted subexposure lengths at 60s.

The guiding star was an auto-selected one next to Arcturus. Near meridian.

Initially I could not get the PhD2 calibration right. And the reported dec-ra angle error went over 40deg! Playing with various parameters and setting, and the result was not satisfactory. Maybe the bad thin high clouds and unsteady atmosphere and the terrible pollution are all to blame?

When the sky got a little better, recalibration seemed to work -- the reported dec-ra axis error was still 3.3deg. Something I need to look into.

Q1. What would be deemed a good range for the error? What could be the cause of the error? (Later I measured the polar alignment error at 4' -- how much would that affect this calibration error?)

The next autoguiding test saw the guiding lines in a better shape but from time to time deviated more than 2", with RMS at .9" in both dec and ra.

Q2. Could it be due to the poor seeing? I don't know how to measure the seeing. Anyway, the sky did not look very good at the time.

In the autoguiding test, I also noticed that, during the first couple of minutes, the guiding lines were rough (for the said RMS above). But after a while the autoguiding become more steady, and most of the time slowly drifting within 1.5" and -1.5" -- I think the RMS would be much smaller but I did not have time for a new measurement session. I repeated the test and observed the same phenomenon.

Q3. Does PhD2 need to take time to adapt and stabilize the autoguiding?

I was trying PEC calibration on the mount. The initial few tests (when it was still a mess -- before the Q1 point) did not see any signs of improvement -- sometimes the result seemed to point the opposite way.

Learning from Q3, I waited for the autoguiding to become seemingly stabilized (and thanks to be better sky then), and ran the 400-sec calibration again.

With PEC correction turned on, I then spent the next few minutes (194sec) on testing PhD2's guide assistant and this is the screenshot of the result.

Image
Click here for the full org-size image. https://photos.app.goo.gl/2YxEiLyMAfCrJm7C8

The PEC does seem helpful. Without autoguiding (until the last few seconds), the drifting in both DEC and RA was relatively steady.

The first section of the error lines seems to be significantly better in terms of smaller deviations from the trend lines.

Q4a: the main concern here is about the RA drift. It should not have happened, right?

Q4b: Does it mean that this PEC calibration worked better for the then first part of worms engaged, but was not trained well for the 2nd half? How would I better train the PEC so that it could perform consistently as in the first 1/3 of the session here (within 1" deviation from the trend lines)?
Last edited by hhzhang on Thu May 28, 2020 1:14 pm, edited 3 times in total.
hhzhang
Posts: 730
Joined: Sun May 10, 2020 4:11 pm
Favourite scope: 115mm APO

Re: Newbie questions from an PhD2 autoguiding and PEC training session

Post by hhzhang »

Here is the link to the photo of the factory report on PE. Max deviation at approx. 2.5" only. (Specifications say +-7")

https://photos.app.goo.gl/p7BbhdRdoWidkhEm6

Q5. Has the factory test auto-corrected the slow drifting in RA?
Last edited by hhzhang on Thu May 28, 2020 1:00 pm, edited 1 time in total.
hhzhang
Posts: 730
Joined: Sun May 10, 2020 4:11 pm
Favourite scope: 115mm APO

Re: Newbie questions from an PhD2 autoguiding and PEC training session

Post by hhzhang »

Q4c. If that RA drift is confirmed consistent, shall I fine tune my tracking rate to compensate for it? According to the graph, I would have to tune the tracking rate to 1.0020888888888888888888888888889X. On CEM40, the rate could be set to a number between 0.9900X to 1.0100X.
Last edited by hhzhang on Thu May 28, 2020 1:01 pm, edited 1 time in total.
kochu
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Re: Newbie questions from an PhD2 autoguiding and PEC training session

Post by kochu »

1 First do a Drift alignment.
2.Then Fine tune Drift alignment using Drift alignment utility in PHD
Kochu
hhzhang
Posts: 730
Joined: Sun May 10, 2020 4:11 pm
Favourite scope: 115mm APO

Re: Newbie questions from an PhD2 autoguiding and PEC training session

Post by hhzhang »

kochu wrote: Thu May 28, 2020 11:12 am 1 First do a Drift alignment.
2.Then Fine tune Drift alignment using Drift alignment utility in PHD
Kochu
Thanks. I think most of the questions are not really about this error -- although it contributed some little part.

After all, a 4' PA error would be contributing 1.01arcsec/min drift in DEC only to this star tracking. And all the questions are not about this small steady drift.
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