RA and Declination guiding rates differ by an unexpected amount

[Neil Martin]

I need some help interpreting the calibration results and the output of the Guide Assistant.  I've used PHD2 in the past (for shorter exposures) with mixed results.  I put that down to user error, but now I'm starting to think that my AVX mount maybe a big factor.  On Sunday I tried taking some 5 minute subs with a Ha filter.  I started with a fresh PHD profile and got a lot of warnings when it calibrated.  I went ahead anyway and no surprise, the resulting images were appalling - larger stars were crescent shaped.  The warnings had me take a look at my mount and it made me realize how sticky the declination axis is.  Additionally, with the clutch released, and turning through a large angle, there were definite "lumpy" spots.  These seemed to clear with repeated exercise, but not a good sign.  

Last night I went through the same process, but with only my guide scope mounted and the counterbalance weight and spindle removed.  I've captured the PHD output with a few snapshots using my phone, which I have attached along with my log.

https://openphdguiding.org/logs/dl/PHD2_logs_4DK1.zip

[Bruce Waddington]

Hi Neil.  These logs are virtually empty.  When you use the Uploader tool,  you need to look at the timestamp and duration columns to be sure you're covering the time period when you had problems.  PHD2 creates log files whenever it is run, whether you actually do anything or not.  If you can't figure it out any other way, you can do a text search on 'Calibration Begins' in the PHD2_GuideLog*.txt files to see when you attempted calibrations.

Bruce

[Neil Martin]

Hi Bruce,

Sorry about that, I trimmed the log file (which was huge, but also wanted it to only contain the one night's data) and saved it to another file.  I must have uploaded the wrong one..

https://openphdguiding.org/logs/dl/PHD2_logs_wF0t.zip

Neil

[bw_msgboard]

Hi Neil.  We don’t seem to be making much progress, so I’ll ask you to carefully review the instructions here:

 

https://openphdguiding.org/getting-help/

 

In particular:

1. Don’t trim or modify the log files in any way.  What may seem extraneous to you is often important to us, and edits can break custom tools we use for support.  And yes, the log files can be large, that’s why we provide the upload service
2. When you’re having trouble with guiding, we need to see the *guide* log.  If you’re having equipment connection problems or you think there’s a bug in PHD2, we need the *debug* log.  If you can’t decide, it’s ok to upload both.
3. If you can’t get the upload tool to work for you, it’s ok to post the files to a shared service like DropBox, but you have to be sure the files are public or accessible to anyone who has the link.  We can’t handle being “invited” to share all these log files.

 

Good luck,

Bruce

 

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[Neil Martin]

https://openphdguiding.org/logs/dl/PHD2_logs_5RXj.zip

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[bw_msgboard]

Thanks Neil, now we can get into the problems.  Two of the problems are expected for this class of mount, a fairly large uncorrected periodic error in RA and a lot of Dec backlash.  You can see the RA periodic error in the GA results:

 

 

 

This is a peak-to-peak periodic error of about 30 arc-sec and looks like it’s occurring at the native worm period of the mount, about 480 seconds.  If you can find a way to apply a usable periodic error correction to the mount, that would be the best approach.  If not, you can try using the PPEC guide algorithm for RA, giving it an initial period value of 480 sec.

 

The backlash test showed that it’s taking nearly 7 seconds for the Dec axis to reverse direction at your current guide speed of 0.5x sidereal.  If you increase the guide speed setting in the mount up to, say, 0.9x sidereal, that backlash recovery time will decrease proportionately.  Beyond the basic backlash, we can see the “lumpy” movement in Dec you were talking about.  Here’s an example during a calibration:

 

 

This is probably a mechanical problem of some kind that you’ll want to eliminate.  It may be something as simple as grit or dried grease in the Dec drive train.  Sometimes slewing the axis at high speed in both directions a few times can clear this stuff.  Otherwise, you’ll probably have to open up the drive assembly to see what’s going on.  I assume, of course, that you didn’t have some external that was interfering with the movement like a tugging guide cable.

 

Most of your calibration alerts were coming from the large Dec backlash.  With your mount, you’ll need to precede *every* calibration with a forced north movement of the mount.  You can do this with a short slew or by using the hand-controller.  That will clear the backlash and get you better calibration results.  You may see advisory messages that the mount didn’t move south very much at the end of the calibration but you can disregard those.  

 

With all of that said, you actually got some good guiding during the 6-hour session on 8/4.  The total guiding RMS was only 0.8 arc-sec and your should have been getting nice round stars.  So if this is the period when you saw the bad imaging results, it doesn’t look like guiding.  What is the main scope you’re using?  If it’s a long focal length SCT or something like that, you probably won’t be able to guide effectively with a separate guide scope.

 

Hope this helps,

Bruce

---

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Neil Martin
Sent: Thursday, August 08, 2019 9:28 AM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] Re: RA and Declination guiding rates differ by an unexpected amount

 

https://openphdguiding.org/logs/dl/PHD2_logs_5RXj.zip

On Thursday, August 8, 2019 at 9:21:45 AM UTC-5, bw_msgboard wrote:

Hi Neil.  We don’t seem to be making much progress, so I’ll ask you to carefully review the instructions here:

 

https://openphdguiding.org/getting-help/

 

In particular:

1.      Don’t trim or modify the log files in any way.  What may seem extraneous to you is often important to us, and edits can break custom tools we use for support.  And yes, the log files can be large, that’s why we provide the upload service

2.      When you’re having trouble with guiding, we need to see the *guide* log.  If you’re having equipment connection problems or you think there’s a bug in PHD2, we need the *debug* log.  If you can’t decide, it’s ok to upload both.

3.      If you can’t get the upload tool to work for you, it’s ok to post the files to a shared service like DropBox, but you have to be sure the files are public or accessible to anyone who has the link.  We can’t handle being “invited” to share all these log files.

 

Good luck,

Bruce

 

---

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Neil Martin
Sent: Thursday, August 08, 2019 4:40 AM
To: Open PHD Guiding
Subject: [open-phd-guiding] Re: RA and Declination guiding rates differ by an unexpected amount

 

Hi Bruce,

Sorry about that, I trimmed the log file (which was huge, but also wanted it to only contain the one night's data) and saved it to another file.  I must have uploaded the wrong one..

 

https://openphdguiding.org/logs/dl/PHD2_logs_wF0t.zip

 

Neil

On Wednesday, August 7, 2019 at 10:54:04 PM UTC-5, Bruce Waddington wrote:

Hi Neil.  These logs are virtually empty.  When you use the Uploader tool,  you need to look at the timestamp and duration columns to be sure you're covering the time period when you had problems.  PHD2 creates log files whenever it is run, whether you actually do anything or not.  If you can't figure it out any other way, you can do a text search on 'Calibration Begins' in the PHD2_GuideLog*.txt files to see when you attempted calibrations.

 

Bruce

 

On Wednesday, August 7, 2019 at 12:54:06 PM UTC-7, Neil Martin wrote:

I need some help interpreting the calibration results and the output of the Guide Assistant.  I've used PHD2 in the past (for shorter exposures) with mixed results.  I put that down to user error, but now I'm starting to think that my AVX mount maybe a big factor.  On Sunday I tried taking some 5 minute subs with a Ha filter.  I started with a fresh PHD profile and got a lot of warnings when it calibrated.  I went ahead anyway and no surprise, the resulting images were appalling - larger stars were crescent shaped.  The warnings had me take a look at my mount and it made me realize how sticky the declination axis is.  Additionally, with the clutch released, and turning through a large angle, there were definite "lumpy" spots.  These seemed to clear with repeated exercise, but not a good sign.  

 

Last night I went through the same process, but with only my guide scope mounted and the counterbalance weight and spindle removed.  I've captured the PHD output with a few snapshots using my phone, which I have attached along with my log.

 

https://openphdguiding.org/logs/dl/PHD2_logs_4DK1.zip

 

 

 

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[Neil Martin]

Hi Bruce, Thanks for the detailed analysis.

The imaging session was only using a Canon 70-200 F2.8 lens attached to a ZWO 183 camera.  Interesting to know that I actually got good guiding.  I think I'll try another session with the same set up to see if I can figure out the cause of the poor imaging.

Can I generate that period error graph myself?  I didn't see that in the GA, or am I missing something?  Or is this produced by another tool that you use to analyze log files?  It would be great to be able to generate the graph myself to see if I'm able to make any progress addressing the issue.

A clear sky is a possibility tonight, so I'll hopefully be able to see what that yields.  This weekend I'll take a look at the Declination backlash issue and see if I can clean up the stickiness in that axis.  There's a lot of posts on CloudyNights for that issue with AVX mounts.  PEC is new to me, so I'll need to research that one.

Thanks for the help, 

-- Neil

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[Andy Galasso]

On Thu, Aug 8, 2019 at 3:49 PM Neil wrote:

Can I generate that period error graph myself?  I didn't see that in the GA, or am I missing something?  Or is this produced by another tool that you use to analyze log files?

Hi Neil,

The plot was done with PHD2 Log Viewer (https://adgsoftware.com/phd2utils/)

Andy

[Brian Valente]

Neil

 

>>> Interesting to know that I actually got good guiding. 

 

Keep in mind although the graphs look pretty grumpy, the pixel movement was only 0.37 average for the entire 6 hour session, so we’re talking minimal guide- or mount-related issues on the final imaging.

 

If you aren’t happy with your images, you could also share some raw files via dropbox or other file sharing? It could be just about anything: seeing, focus, etc. but happy to take a look

 

 

 

 

 

 

Thanks

 

Brian

 

portfolio https://www.brianvalentephotography.com/

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[Neil Martin]

Hi Brian,

I've uploaded one of the raw files to my Google Drive.  I think I jumped to the conclusion that guiding was the issue, simply because I had all the warnings etc.  If I'd looked at the images a little closer, I might have considered other possibilities, although it wasn't really obvious at the time on my 13 inch laptop in the dark!  The distortion of the stars is consistent across all the images, which makes me wonder if this is an optical issue?  Although that's not terribly prophetic once we've ruled out guiding :)  

 Also, there are a number of "firsts" with this session.  I have a new EF adapter which can drive the focus motors in the camera lens.  First time using that and first time using auto focus in SGP.  Also this is the first time I've used the filter wheel.  This image was taken using the Ha filter, but tonight I'm going to try other filters, as well as imaging some much shorter subs.

I'd value any opinions on what might be causing the distortion.

https://drive.google.com/file/d/1V8iDVUuYpiqblYo51WZDeJEOZFBoLJR4/view?usp=sharing

-- Neil

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[Brian Valente]

Hi Neil

That helps a lot. I see there's quite a bit of optical aberrations on this image, particularly on the right side. 

this is a 100% crop of upper right - the stars have distortion to the point they aren't really stars, they are... well, i don't know lol. 

and in the lower left, it's similar but noticeably different.

to me this looks like a combination of optical problems with the lens, not quite accurate focusing, and possibly some of your imaging elements being slightly off axis. you might check each of these. If possible, can you stop down your lens to maybe 5.6 and see if that at least helps? i know you just quadrupled your imaging time, but slow good images may be a better option than fast disorted images ;)

have you had good experience with this setup before? still photographic lenses can exhibit many of these behaviors 

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--

Brian 

Brian Valente

portfolio brianvalentephotography.com 

[Neil Martin]

Hi Brian,

I've only had limited experience using this lens for astrophotography - but the Canon 70-200 is a well regarded lens and I've seen some good astro images using it.  Previously I've used it with a DSLR and the main issue with that was coma distortion with the lens wide open.  One of the reasons I bought the new EF adapter was that it will let me set the aperture when connected to a ZWO camera.  I have stopped it down to 5.6 and will try that when it gets dark.  I do have a  manual EF adapter and other lenses I can try,  so I think through a process of elimination I ought to be able to narrow this down... but I won't get to all of that tonight.  Hopefully our clear skies will hold for tomorrow.

-- Neil

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[Brian Valente]

Good luck, i hope you make some progress Neil

i looked up some articles on that lens for ap work, and the results seems consistent with your experience:

http://www.astropix.com/html/i_astrop/eq_tests/c70_200.html 

 

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[steve]

On 9/8/19 3:26, Neil Martin wrote:

 the Canon 70-200

Hi Neil, everyone

If stopping to 5.6 doesn't fix it -I gave up on my Canon zoom for the same reason-, you may want to have a look at an old Takumar 200mm. Over APS-C, this gives you sharp stars corner to corner and costs around $50. Complete your wide field armory with a 135mm from the same source.

HTH and clear skies,

Steve

[Neil Martin]

At f/5.6, I did get slightly better images, although in a different configuration -  I removed the filter wheel and the Astromechanics focuser.  It's possible the images were slightly better because I manually focused.  It's also possible that the optical distortion was sabotaging SGP's autofocus.  Still the images weren't acceptable, which is disappointing for an expensive Canon lens (although it is several years old and has traveled a lot).  Last night I tried a 50mm lens which definitely gave me rounder stars, but the whole night I had high clouds that made getting good images almost impossible.  One conclusion I did come to is that manual  focus with a camera lens is painful and that means I'd prefer to find another lens that has AF.  I won't have any clear skies this week, so I'll have to come back to this another time.  In the meantime, it might give me an opportunity to take a look at the declination backlash issue on my mount!

-- Neil

[Brian Valente]

Hi Neil

the canon is a great photographic lens. But like many photographic lenses, that greatness doesn't always translate into astronomy, where the lens designs and demands are much different. I wouldn't blame the lens so much as realize it's probaly not well suited for astronomy

For the $1,000+ that lens goes for you could get a really nice 70-80mm refractor and have a ball with your auto focuser and astro camera, just a thought ;)

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[Neil Martin]

Hi Brian,

Unfortunately, the Astromechanics device is a means of  focussing by driving the focus motors in the lens.  It also allows you to set the aperture externally.  It wasn't a huge investment, but I thought it might enable me to leverage my existing photography lens.  I'm going to revisit some of the threads on CloudyNights where some other users have been reporting their experience with this device.  Might be interesting to see what lenses people are using.  Otherwise, as you say, a small refractor might be the way to  go.  I quite like the new Williams Optics Redcat.  51.

-- Neil

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[Brian Valente]

ah yes, i get it now.Clever device

Although in my experience for astrophotography it seems the lenses that are better suited for astrophotography that are DSLR-mount (canon, nikon, etc.)  are made by third parties who do manual focus. (Rokinon, irix, laowa come to mind). Sigma has some good stuff and would work with that, but that's some fairly big investment

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[Neil Martin]

The 50mm lens I used last night is a Sigma Art f/1.4... really nice lens, but I'd like something a little longer for astrophotography.

--  Neil

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