Should I bother with PEC correction?

[sarg314 sarge]

After repositioning the drive motors on my Celestron AVX in an attempt to reduce backlash, I ran guiding assistant for  21 minutes.  That's 2 whole rotations of the AVX worm gear. See the G.A. report at 21:49.

Dec. is not great, but it is much better than it was. Before repositioning the motors G.A. would never even complete a backlash measurement. Now it says I have 22.9 arcsec of Dec backlash.

The RA perodic error is 21.79 arcsec peak to peak, if I am interpreting the report correctly.

So, my question is, given the rather long 10 minute period of the worm, the effect of Periodic Error is slow - 1 arcsec in roughly 14 seconds.  Is it worth trying to correct for, or should I expect PHD2 to just guide it out?

[Andy Galasso]

Hi Tom,

Nice job getting the dec backlash under control.

Slowly varying RA PE is not a problem, that will guide out easily.   The bigger concern would be these rapid jumps where the RA axis moves by up to 2-3 arc-seconds per second:

Those are going to be tough to guide out.   The moves appear to be at 60s intervals, so that is probably a clue about which component of the gear train is causing them.

Andy

[sarg314 sarge]

Andy:

Thanks for that analysis.   I can't think of what would cause the RA jumps.  At best my RA guiding is in the 0.8 to 1.2 arcsec rms range. 

I normally observe with a slight east heavy pre-load.  As I recall this G.A. run was done without pre-load.  Do you think a pre-load would improve these results?  Maybe smooth things out a bit?

May I ask how you produced the graph you made?  The normal log viewer can't do that, can it?  Looks like you extracted the RA data and remove the linear drift.

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Tom Sargent

[Brian Valente]

Slightly biasing your weight could help, it's definitely worth a try

regarding the graph, you can right-click on the graph in PHD2 Log Viewer and select "analyze selected raw RA" and then uncheck the DEC box in the lower left 

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Brian 

Brian Valente

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[Andy Galasso]

On Sun, May 26, 2019 at 12:33 PM Brian wrote:

regarding the graph, you can right-click on the graph in PHD2 Log Viewer and select "analyze selected raw RA" and then uncheck the DEC box in the lower left 

that's right, though in this case I used "Analyze Unguided Section" so it would just take the data from the guiding assistant run

 

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[sarg314 sarge]

That's great, thanks.

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Tom Sargent

[sarg314 sarge]

I showed this log to Celestron support to see if they could suggest  a cause for the jumps in RA.  They were just about to have me RMA my mount back to them, but changed their mind. They asked first for  me to use the mount's PEC correction. Once they see the PEC corrected results, they'll (probably) ask to have the mount back for repair.  

My problem is doing the PEC analysis. I'm using the PECTool program. It gives no indication of success or failure and Celestron support is hard communicate with about this.  There's about a 3 or 4 day lag in communication.  So if any one on the list is familiar with their PECTool can you tell me if I am doing it right?

1- configured a PHD2 profile to use ST4 only.

2- chose a bright guide star at 5 deg. dec.

3- oriented guide camera's X & Y pretty accurately with RA and Dec

4- did a calibration and started PHD2 guiding

5- started the PECTool "training"  (first time it lurched to find the worm index, then I started the whole process over again)

6- when training finished, I uploaded the PEC correction curve to the mount.

7- using hand controller I turned on PEC playback.

8- started using PHD2 Guiding Assistant to measure the PEC behavior as I have done many times in the past.

The problem is that the PEC corrected curve was no different from the uncorrected curve. So I question if I actually accomplished anything.  Can any one verify for me if this is the right sequence of steps?  Thanks.

[Brian Valente]

If you are using PHD to generate the input file, you should use it no differently than you would normally. i.e., set up a profile using the profile wizard, bump north prior to calibrating, let it auto select the guide star, etc. the calibration will align camera x/y with the mount's RA and DEC

Thew rest of it seems very specific to your mount and PEC.

Perhaps you would have better luck in a forum dedicated to PECTool and/or your mount? 

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[peter wolsley]

Tom,
I own a Celestron CGEM and I have used the PECtool app.  I looked at your list of steps when you ran PECTool and they seem correct to me.   I don't think you should have to oriented the guide camera any special way.  PHD2 figures everything out during calibration and your mount will not care what the camera orientation is. If you turned on PEC playback and then ran the guiding assistant, the RA deviations should be closer to a straight line.  So I think you didn't accomplish anything.

One test you could perform would be to get PECTool running again and use the "Download Data From Mount" method to get a copy of the stored PEC curve in the mount.  The curve will be displayed on the PECTools screen so you can verify that there is a valid curve.  You can also use the "File" menu to save or load PEC data.  If you saved PEC data when you were training your mount you will have a way to call up the stored PEC data from your hard disk and then Upload the Data to your mount.  If all you see on the PECTools screen is a straight line or a curve that looks like garbage then you need to try again.  Be sure to use the Remove Drift button...this makes sure the beginning and end of the curve goes to zero.  I would like to see some actual guiding from your mount...ideally at least 30 minutes.  Maybe you could upload the Guidelog from when you were training your mount.  If you do get PEC working I would also like to see a Guidelog with PEC running.

Good Luck

Peter
P.S. 22.9 arc-seconds of DEC backlash is very very good for your AVX.  My CGEM has 50 arc-seconds.

[sarg314 sarge]

Peter:

Thanks for the feedback.  I took the trouble to align the guide camera X-Y axes with RA -DEC because the Celestron AVX mount manual says to do that when you do PEC measurement.  I've never bothered with it before.

I tried again last night and I was able to get some results. The problem is PEC training is erratic. That's probably why I was so confused in my first attempt. More than half the time it produced completely weird, impossible results (like virtually all zeroes).  I used both the PECtool and just the hand controller to record PEC combined with PHD2 using either ST4 connection or ASCOM. On 2 occasions when using the hand controller, the recording just stopped half way through the 10 minute worm period.  The saving grace of the PECtool is it lets you view the results so you know when it worked. I was able to get 4 reasonable runs, averaged them and played them back (it took half the night).  

I did 4 Guiding Assistant runs with that PEC playback running and it did decrease the amplitude of the Periodic Error by more than a factor of 2.  The mount still displayed the quick 5 or 6 arcsecond jumps which was the motivation for this whole exercise.  I don't have enough experience to know if PEC will consistently improve my guiding performance.  For some test periods last night the guiding looked quite good, but sometimes it does that anyway.  I suspect it won't help, since the periodic error has such a low frequency with this mount that I think it is not a big contributor to guiding error.

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Tom Sargent

[Brian Valente]

Hi Tom

 

>>> The mount still displayed the quick 5 or 6 arcsecond jumps which was the motivation for this whole exercise. 

 

If the rapid jumps are more like spikes, PEC is unlikely to address that, and PHD would probably not help there either.

 

 

 

>>> I showed this log to Celestron support to see if they could suggest  a cause for the jumps in RA.  They were just about to have me RMA my mount back to them, but changed their mind. They asked first for  me to use the mount's PEC correction. Once they see the PEC corrected results, they'll (probably) ask to have the mount back for repair.  

 

 

Seems like you have the data you need to send that mount back for repair. I was looking at the earlier messages in this thread, partcularly Andy’s illustration of the jumps. That should be enough for them to RMA it, I think

 

 

 

 

Thanks

 

Brian

 

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[peter wolsley]

Tom,
I'm glad you finally got some PEC data and that the data did reduce the "residual periodic error".  I used PECtool a few years ago and was able to get a pretty detailed curve.  You certainly are dedicated and capable of getting to the bottom of your guiding woes.  Brian and Andy and myself all agree that those fast spikes are a showstopper for good guiding.  The AVX mount simply should not be exhibiting those spikes.  Celestron should be satisfied that the mount needs to be returned.

If you ignore the fast spikes you can say that your mount's PEC curve is pretty smooth.  Combining that with the fact that it takes 10 minutes for one cycle, it becomes reasonable to find that you can have periods of very good guiding.  Astrophotography requires much more than periods of very good guiding.  Hopefully Celestron will act quickly on your behalf.

Best of Luck,

Peter

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Tom Sargent