guiding constantly interrupted by "slewing", but I'm not

[brisguy foo]

Tonight was my first real attempt at guiding with an EXOS2 mount and SvBony 60mm guidescope with SV305 camera. I was able to get through a decent calibration and was excited to see a reasonable guide result, but it did not last long. I keep getting errors that guiding has stopped due to the mount slewing, but I have not asked the mount to slew and the guide star is staying perfectly centered. I simply hit the guide button and guiding starts again, but not for more than a few minutes and usually less. Then guiding stops again.

What is going on? I have attached the debug and guide logs. Help!

[Ken Self]

All but one of the errors were related to fairly large guide pulses on the Dec axis. I wonder if you have some sort of backlash compensation enabled in the driver that causes the mount to slew? 

[mj.w...@gmail.com]

I also noticed there wasn't a Calibration in that log for 19th Jan.

" I was able to get through a decent calibration"

Last Cal was on the 14th, and was not "decent" :   Last Cal Issue = Rates, Timestamp = 1/14/2022

Norm rates RA = 15.9"/s, Dec = 4.3"/s; ortho.err. = 7.0 deg  

With the mount set to RA Guide Speed = 15.0 a-s/s, Dec Guide Speed = 15.0 a-s/s,

Michael

Wiltshire UK

[Brian Valente]

You can disable the option to stop guiding when mount slews and see how it goes

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Brian 

Brian Valente

portfolio brianvalentephotography.com 

[brisguy foo]

thanks all for the quick replies. I do want to try again tonight since it is supposed to be clear and won't be again for a while.

@ Ken - no backlash comp that I am aware of

@Michael - Yes, I reused the calibration from the 14th, which I considered decent since there was only a 7deg orthogonality error. With this mount I have seen much  worse. I don't know how to interpret the other errors you listed, but I thought I read in the manual that unless the calibration fails, it is ok to proceed. Also, my understanding is that you would normally re-use a valid calibration since your target may not be within +/- 30 deg DEC. Is that not true?

@Brian - thanks for that tip

I was trying to guide at 100% of sidereal rate, which I thought was what other users of this mount used, but I found out today that is not correct, so I will try again tonight with something more normal like 0.5 or 0.4.

[bw_msgboard]

I don't think you're likely to see much improvement tonight because the problems probably lie in the mount and perhaps the mount driver.  The driver is rejecting guide commands because it thinks the mount is slewing.  So disabling the PHD2 option will simply mean you won't be guiding the mount when you should and the unguided tracking of the mount is probably fairly poor.   The failures are associated with very large Dec corrections and those are probably caused by the very poor calibration you are using.  That calibration produced an alert telling you the mesured RA and Dec rates were way out-of-whack, which they were.  You can't just judge the calibration by the orthogonality error, that's only one item.  Unless you get a calibration with no alerts, you're skating on thin ice and can expect poor guiding results.  In your case, the calibration measured a Dec guide rate of 4 arc-sec/sec when the value should have been much closer to 15, and that's why the Dec guide commands are so long.  The underlying problem is that your mount probably has a ton of backlash in Dec which means that reducing the guide speed in the mount will make it even worse.  To get a decent calibration, do this:

 

1.  Slew to the correct area of the sky

2.  Start PHD2 looping

3.  Use the hand-controller to move the mount north ('up' button) until you see the stars in the PHD2 display start moving

4.  Start the calibration

 

I think you probably need to accept that your mount isn't going to work very well right out of the box.  I think you should read the PHD2 manual and thoroughly understand calibration and the things that can make it go wrong - including Dec backlash.  You will probably be able to get this thing under control at least well enough to get some images - assuming the mount driver doesn't have bugs.  But it's likely to take some study time and effort.

 

Good luck,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of brisguy foo
Sent: Thursday, January 20, 2022 8:25 AM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] Re: guiding constantly interrupted by "slewing", but I'm not

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[Brian Valente]

Since Bruce was mentioning some basic tips on setting up calibration, you might look at following the baseline guidelog steps outlined in the link below for a full list of things to do and check:

https://www.dropbox.com/s/zrf0au7jaisvt85/How%20to%20create%20a%20baseline%20for%20guiding%20results%20using%20OpenPHD2.pdf?dl=0

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/938E4615BA4544F48620F51A17EBA896%40HomeDesktop.

[George Shoup]

I can give this "how to" for guiding baseline two thumbs up.  I just finished going through it multiple times.  

I learned:

• A polar alignment tool like PoleMaster will not give a good alignment if the Polemaster camera is not correctly seated in its fixture.  The PoleMaster will fake you out because all looks well, but this is not the case. 
• If the scope axes are not orthogonal, then guiding will be poor.

Both of these manifest themselves as poor polar alignment reported when one does the attached guiding baseline procedure.

George

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[brisguy foo]

Just to add a bit more info:

1. I know this mount is not a high-end mount and that DEC backlash is a big problem. However, it has an active user group and people seem to be able to get good guide results when the mount is well tuned and balanced.

2. Although this is my first attempt to use a real guide scope, I have played with PHD2 since last fall. At that time, I simply used my OSC mounted to my main scope. The image scale was pretty small (about 0.66 arcsec/px) so I think this lead to some problems. I rarely was able to complete the calibration step at all. Often when I did the orthogonality error would be more than 20 deg, so that is why I characterized my current results as decent.

3. Since I know that DEC backlash is a problem, I always slew North until I see star movement before calibrating.

4. I have read the PHD2 documentation several times and watched Bruce's video overview on YouTube. Not claiming I understand it all, in fact I know I don't, but now that I am starting to try to actually make this work, I am sure it will make more sense.

5. I use SharpCap for polar alignment and typically get it to Good or Excellent. When I ran GA after calibrating, it said my PA error was about 4min.

I do have a few questions after reading the document Brian suggested

1. Do have to be near the meridian in order to do the calibration? I thought I only needed to be near 0deg DEC. I can go near the meridian, but only have a small window at my location near O deg DEC.

2. Since Bruce is d=suggesting my main problem is with DEC, perhaps I should do some drift alignment after polar alignment and just guide in RA?

3. I have seen PHD tutorials where the user seems to imply that they always do a calibration AFTER slewing to their target.  I would think this is not needed or even proper. Is that correct?

Thanks!

[Brian Valente]

1. Do have to be near the meridian in order to do the calibration? I thought I only needed to be near 0deg DEC. I can go near the meridian, but only have a small window at my location near O deg DEC.

Ideally yes you want to be near the meridian. you only need a small window for a few minutes of calibrating and guiding

2. Since Bruce is d=suggesting my main problem is with DEC, perhaps I should do some drift alignment after polar alignment and just guide in RA?

If you want to take an approach like this, you'll want to guide DEC in one direction only by offsetting your polar alignment. I really don't recommend you not guide in Dec, your results will probably not be good

3. I have seen PHD tutorials where the user seems to imply that they always do a calibration AFTER slewing to their target.  I would think this is not needed or even proper. Is that correct?

This is old-school thinking when information about where the telescope is pointing was not available to PHD (aka ST-4 or "on camera" guiding). in that case, you had to calibrate for each target. 

however, this is not required when using modern pulse-guiding that includes mount connection to PHD via ascom or Indi, etc. 

In other words, it USED to be that way, but not any more. 

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/26f9ea9d-2697-4527-8d0c-88ea62c1f522n%40googlegroups.com.

[bw_msgboard]

Well this all sounds ok but there are two problems that require answers:

 

1.  Why are your calibrations so poor?   Instead of trying to just sidestep the problem, it would be better to understand it, and two things would help: 1) sending us a guide log containing a calibration and 2) letting the Guiding Assistant measure the Dec reversal delay.  Even if that test "fails", you can look at the resulting graph and understand what happened.  That information will also be available in the debug log file so you can send that along as well.  Use the Help/Upload Log feature to do those things.

2.  Why is the ASCOM mount driver claiming that the mount is slewing?  It shouldn't be doing that. You've added the further complication that you have POTH sitting in the middle of the mount connection.  If you can eliminate that, even temporarily, it will simplify things.

 

It's good to know that other people with this mount claim to get good results although you haven't said what your standard is for "good".  All that tells you is that their mounts work acceptably, not that yours is.  We've seen that the level of quality control on many of these inexpensive mounts is near-zero, so they often require end-user adjustment and fiddling to make them right.

 

The recommendation to calibrate near the meridian is just a shorthand way to say that you don't want the scope pointing near the horizon where seeing effects are worse.  If you can keep the scope pointing at least 45 degrees above the east or west horizon, it will be fine.

 

Regards,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of brisguy foo

Sent: Thursday, January 20, 2022 11:14 AM

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[brisguy foo]

Thanks Brian and Bruce. Your replies are very helpful and cleared up a LOT of questions. 

I live on the north side of a small mountain so actually, I am closer to the horizon when I am pointing south than east or west! Although, I believe, in my case, that's not really relevant if the issue is due to the atmosphere.

1.  Why are your calibrations so poor?  

Good question and one I have not been able to get an answer to yet. In the past, it may have been due to poor PA and possibly not clearing the backlash properly. I will try again tonight. My impression is that the two axes are not completely independent.

2. Why is the ASCOM mount driver claiming that the mount is slewing?  

This never happened when I tried PHD last fall, so I suspect it must be due to me setting the guide rate at 1x sidereal (which I had never done before). I am going to try a different rate tonight and may also try eliminating POTH if the problem continues. 

[Brian Valente]

>>> I suspect it must be due to me setting the guide rate at 1x sidereal (which I had never done before)

That may something to try, but FYI there are many mounts that guide at 1x sidereal, such as Astro Physics mounts. 

Your mount may have limitations on guidespeed, might be good to find out

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[brisguy foo]

Starting with a side note: Is there a way to prevent the endless quoting and requoting of all the previous messages when replying? I  manually removed it in this message.

I tried a new calibration last night after changing my mount guide speeds and got better results, but still a warning (too much variance in RA vs DEC speed).

I really wanted to see what kind of results I could get so just started guiding instead of trying to resolve the issue and was able to guide successfully for two different targets. One was near the celestial pole, so perhaps was 'easier', the other was near the meridian and about -5 deg DEC. Both tracked well with less than 1 arcsec RMS error although the second target error got as high as 1.5 arcsec.

I have uploaded the debug and guide logs, which include the new calibration. Since I am able to guide now (the slew error did not occur), I am basically happy, but wanted to follow-up with this info in case anyone spots anything that could help me get rid of the consistent warning about mismatched speeds. I think I have gotten this every time I have tried to calibrate.

Thanks again for all your help!

https://openphdguiding.org/logs/dl/PHD2_logs_tw7E.zip