Guiding Issue with Mesu 200 MKII mount

[Jose Ignacio Sanchez Rodriguez]

Hello,

I have a Mesu 200 MKII friction drive mount since October. I had been very happy with the guiding until two weekends ago, when I started experiencing problems. 

While imaging IC443 the mount responds to the guiding corrections in RA but it seems as if the RA error is on a slow divergence trend. At about 1 hour before the meridian the mount just not responds anymore to the guiding corrections in RA and the RA guiding error grows so much that my sequencing software kills the sequence.

I am using a 10kg rig. FSQ-85 refractor, Nitecrawler focuser and Atik 16200 camera with filter wheel and OAG. My guide camera is a Lodestar X2.

I have tried to reduce all the sources of flexture to a minimum and I have also checked that when the RA guiding error goes through the roof there is neither a pier crash nor tight cables pulling the rig nor any obvious physical explanation to the issue. It was a very clear night and there was no lost star either.

I am puzzled by this, cause the mount guided very well up until now. I am also sharing this in the SiTEch forum (the mount runs on a SiTech servo controller) but would like to know your opinion about this.

Here´s the log:

https://openphdguiding.org/logs/dl/PHD2_logs_9GFW.zip

looking forward to get your input. Best Regards,

Jose 

[bw_msgboard]

Hi Jose.  This is kind of an unusual situation – here’s a view of what’s going on showing only the RA in red:

 

 

 

About 10 minutes before you saw the big problem (point 2), the RA guide corrections were already not having any visible effect (point 1).  What’s important here is that the guide corrections are east with the scope on the west side of the pier.  With typical kinds of cable snags or static resistance or a loose clutch, you would expect the guide corrections would have to be west because something would be interfering with the ability of the mount to track west at the sidereal rate.  Your situation is just the opposite.  You haven’t told us whether you’re using a separate guide scope, but it looks like some part of the guiding assembly is moving around – you can think of it as having the guide camera “falling” too much to the west.  Of course, this assumes the mount is tracking at a constant sidereal rate.  One common source of this type of problem is a focuser that isn’t completely rigid, often because the focus tube is slightly loose in its housing -  but there’s really no way for us to guess.  The total amount of movement here is of course quite small – it was equivalent to a sag of the guide camera sensor by about 12 microns, ¼ the thickness of a human hair.

 

There is one other operational mistake you’ve made that is giving you trouble albeit nothing to do with this tracking issue.  You’re using a LodeStar guide camera which is a 16-bit camera, but you’ve specified a saturation level in PHD2 of 256.  So you’re crippling your ability to use the best stars in the field of view.  For 16-bit operation – which is what you want – you should specify a saturation level of 65000 or somewhat higher.  You will then find that the auto-select function will work much better for you.

 

Good luck – let us know when you’ve figured out the problem.

 

Bruce

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[Jose Ignacio Sanchez Rodriguez]

Hello Bruce,

Thanks a lot for your analysis. I am using an Off Axis Guider. It is the Atik one that comes with the camera so the setup is very rigid. I also moved from the stock focuser to the moonlite Nitecrawler which I am quite confident has reduced any sag present before. 

What I find very interesting from your analysis is that, given the type of corrections and the position of the scope, we can eliminate certain sources of error like cable snags, static resistance or loose clutch. This really helps me narrow the problem!!

You list the possible sources of error assuming that the mount is tracking at constant sidereal rate. This is important, because I have been analysing possible tracking issues at the Sidereal Technologies forum and there is some indication that this could be the problem.

Other owners of the same mount have had the servo motors failing (not many of-course, just a few cases), so I am going to check this issue together with the manufacturer of the mount at the end of this week. I´ll post here what we find.

I have been suggested at the Sidereal Technologies forum to run the Guide Assistant for a few hours in order to help me find out if there is any significant tracking issue. It seems to be a very sensible advice.

Thanks a lot for your help,

Jose 

p.s: Thanks for noticing the saturation level issue. I´ll change that

[bw_msgboard]

Hi Jose.  One other thing occurred to me after I sent the e-mail although it’s probably unlikely.  If for some reason the RA balance of the scope was significantly out of adjustment – in this case with the counterweights “too light” – it could be creating an unwanted torque to the west.  I don’t know anything about the mount and how sensitive it is to balance or what the clutch system is like.  Something like that would be consistent with the problem growing worse as you get closer to the meridian.  And of course this sort of blind analysis can never rule out cable problems.  With the scope on the west side of the pier, anything that tugs on the gear hanging on the back of the scope can create unwanted motion to the west – I have encountered that myself.  

 

Running the Guiding Assistant for extended times is definitely a good idea.  You might want to do that in multiple pointing positions – both sides of the pier and maybe two separate distances from the meridian on each side.

 

Hope you can track it down,

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/9d6f97a0-3e67-4f66-8717-fc8386bbc8ben%40googlegroups.com.

[Jose Ignacio Sanchez Rodriguez]

Hello Bruce,

Thanks a lot for your answer. After your first response I started thinking that the most likely explanations could be either:

1.- The guiding camera falling to the west.

2.- Balance issue: scope heavy. This is a friction drive system which can slip if too much out of balance.

3.- Mount tracking problem

I have used this same setup many times before and it performs very well. No signs of flexure. I also tried the same setup with a CEM 60 mount and the guiding was very good. The only problem is that I did not try it at the same position in the sky due to time constraints.

Although the mount is sensible to lack of balance, I have not changed the way I balance it and I had never had this kind of issue before.

I don´t rule out options number 1 and 2, but I think they are unlikely for the reasons stated above.

So I started thinking about option number 3. As I said before, some owners of this mount have had issues with the servo motors. 

I happen to have recorded the session of the log I shared in my opening post. The idea was to analyse side by side the guiding behaviour and the RA reading (based on the servo motor ticks) provided by the mount´s Ascom driver. What I see is that the RA reading on the mount´s Ascom driver is decreasing. As I understand this, RA increases EASTward, so the mount´s driver is telling me that the mount is actually moving WESTward faster than sidereal. This cannot be due to the guiding commands cause they are EASTward. So it has to be either option 2 or 3. That would also explain why the mount does not respond to the guide commands. 

What do you think about this?

As I said, balance is quite easy to achieve, I have done it many times with this mount, and have never had issues like this. So I favour the possibility that the RA servo motor is not working well. 

Here´s a link to the videos I refer above. The shorter one is the last part of the session and the longer one a portion of the rest of the session:

session videos

And this is how the driver window looks with the RA reading circled in red:

Let´s see if we can identify any servo issues on Friday´s Skype call with the manufacturer of the mount. 

Thanks again,

Jose

[bw_msgboard]

This sounds promising, let us know what you find out.

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/5ab5d993-ef37-4153-a8c3-737f24e501ebn%40googlegroups.com.

[Jose Ignacio Sanchez Rodriguez]

Hello,

I wrote a while ago about issues guiding with a new friction drive mount. The manufacturer asked me to send back the mount to him and re adjusted it. Last weekend I had the chance to try it under the stars and noticed some issues:

1.- The mount would stop tracking close to the equator: It does not reflect on the phd2 logs nor is completely repeatable, but at some positions close to the equator the mount stops tracking. It has happened 3 times during the weekend. I just close the program I use to control the mount and open it again, re initialise the mount and set it to tracking and it works. It is also not related to horizon limits.

2.- Mount does not respond to RA corrections while performing Drift Alignment with the PHD wizard: This is reflected in the first segment of Saturday April 3rd´s log.

https://openphdguiding.org/logs/dl/PHD2_logs_HnXK.zip

This was using the last calibration from the night before (section 9 on Friday´s April 2nd log) which was performed at high DEC. I include that log in the link above for reference. I have "Use DEC compensation" selected.

Is it possible that using a calibration performed at high DEC results in the mount not responding at all to RA corrections while drift aligning close to the equator? 

I also noticed that, while the drift alignment sections (6 and 7 on Saturdays log) show very little polar alignment error, the PA error during normal guiding (which was very good with RMS of 0.3-0.4 arctic) is very big (5-7 arcmin). Is that normal?

Even if the guiding is quite good I noticed that sometimes I have spikes in RA and DEC and the RA spikes take 2 or even 3 pulses to correct, even when I use 100 aggression.

I would greatly appreciate if you could shed some light on this. I find this behaviour odd and wonder if there is still something wrong with my RA motor encoders. I know that it seems odd to complain when I am getting 0.3-0.4 RMS, but I am planning to approach that limit soon with a smaller scale system. 

Thanks in advance for your help,

Jose

[Bruce Waddington]

Hi Jose, see below.

 

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Jose Ignacio Sanchez Rodriguez
Sent: Sunday, April 04, 2021 8:32 AM
To: Open PHD Guiding <open-phd...@googlegroups.com>
Subject: Re: [open-phd-guiding] Guiding Issue with Mesu 200 MKII mount

 

Hello,

 

I wrote a while ago about issues guiding with a new friction drive mount. The manufacturer asked me to send back the mount to him and re adjusted it. Last weekend I had the chance to try it under the stars and noticed some issues:

 

1.- The mount would stop tracking close to the equator: It does not reflect on the phd2 logs nor is completely repeatable, but at some positions close to the equator the mount stops tracking. It has happened 3 times during the weekend. I just close the program I use to control the mount and open it again, re initialise the mount and set it to tracking and it works. It is also not related to horizon limits.

 

As you know, this has nothing to do with PHD2 – PHD2 doesn’t start and stop tracking.  In addition to horizon limits, you should probably check for any meridian limits in your app or in the mount driver.  Some of these software components can be configured to stop tracking at the meridian in order to avoid a counterweights-up situation.

 

2.- Mount does not respond to RA corrections while performing Drift Alignment with the PHD wizard: This is reflected in the first segment of Saturday April 3rd´s log.

 

https://openphdguiding.org/logs/dl/PHD2_logs_HnXK.zip

This was using the last calibration from the night before (section 9 on Friday´s April 2nd log) which was performed at high DEC. I include that log in the link above for reference. I have "Use DEC compensation" selected.

 

This is because your mount wasn’t tracking at anywhere near the sidereal rate.  PHD2 was pounding away with constant 2500ms guide pulses to the west but these weren’t big enough to overcome the huge tracking error.  Here’s what it looked like:

 

 

There’s no way to know why this happened but it doesn’t have anything to do with guiding.  That said, don’t calibrate at such high Dec positions, that serves no purpose and can produce inferior calibration accuracy.

 

Is it possible that using a calibration performed at high DEC results in the mount not responding at all to RA corrections while drift aligning close to the equator? 

 

I also noticed that, while the drift alignment sections (6 and 7 on Saturdays log) show very little polar alignment error, the PA error during normal guiding (which was very good with RMS of 0.3-0.4 arctic) is very big (5-7 arcmin). Is that normal?

 

The amount of Dec drift changes as a function of the pointing position so that can affect the estimate.  The other thing to consider is whether the mount axes are moving at all after you’ve completed the polar alignment.  In your case, none of it appears to matter unless you’re using a gigantic imaging camera and see field rotation – which is pretty unlikely.

 

Even if the guiding is quite good I noticed that sometimes I have spikes in RA and DEC and the RA spikes take 2 or even 3 pulses to correct, even when I use 100 aggression.

 

It’s not worth pursuing, these are tiny guide star deflections, your guiding is fine.  I don’t advise leaving the aggression at 100 however.

 

I would greatly appreciate if you could shed some light on this. I find this behaviour odd and wonder if there is still something wrong with my RA motor encoders. I know that it seems odd to complain when I am getting 0.3-0.4 RMS, but I am planning to approach that limit soon with a smaller scale system. 

 

Your guiding is fine.  If you’re getting a total guiding RMS of 0.3 – 0.4 arc-sec, you’ll get the same results regardless of image scale.   You’re not likely to get better with any off-the-shelf mount and PHD2-style guiding and probably don’t need to unless you’re in a mountaintop observatory with 1 arc-sec seeing.   In that case, you might want to look at an AO solution.

 

Cheers,

Bruce

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/b0430da4-4c12-458c-a488-b1ff57cfa3e6n%40googlegroups.com.

[Jose Ignacio Sanchez Rodriguez]

Thanks a lot for your quick reply Bruce!! I actually took a snapshot of the ASCOM controller during the section where the mount is not responding to the PHD2 pulses, and it says it is tracking:

So that is my problem, when the mount tracks well it guides very well, but sometimes it does not track well. At the point above I was quite above the  horizon limits and passed the meridian, so not the usual suspects. I will do some more testing, but given the behaviour I am observing  I suspect of a problem with the RA motor encoders. 

Thanks again for your help and have a nice week,

Jose