Poor guide performance with Ioptron CEM60 and CEM70 mounts

[Lou B]

There has been a lot of talk on CN and other sites about oscillations and setting for running PHD2 on these mounts. 

Seems that one persons setting are fine.  Do the same on another mount and get erratic and or oscillating behaviors.  default settings might as well be poison to my mount.

I'm one of the later. 

Has there been much discussion about what is causing these issues with the CEM type mounts?  Any one come up with answers or solution or causes?

LouB

[bw_m...@earthlink.net]

We don’t really deal in generic behaviors of various mounts, we only look at results from individual users, one mount at a time.  In general, there are two sources of major problems: 1) cockpit errors by the users and 2) mechanical defects in the mounts.  If you do a search on this forum, you will probably find dozens of analyses of performance on these mounts.  If you want to understand what’s going on with your particular setup, you should submit your log files, a general description of your setup, and what you’re unhappy about and we will take a look.

 

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

 

Regards,

Bruce

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[Lou B]

will do next log .  I was looking for generic info re mount mechanical or programing. 

"cockpit error" i am going to use that. 

[Bruce Waddington]

If you want to do a thorough job of measuring your mount’s performance and avoiding a lot of back-and-forth in the analysis process, the best approach is to follow this procedure:

 

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

 

I recommend that you first upgrade to the latest PHD2 Dev release, 2.6.11dev5:

https://openphdguiding.org/development-snapshots/

 

This will allow you to use the new Calibration Assistant feature which will maximize your chances of getting a good calibration.  The first part of the baseline procedure involves creating a new PHD2 configuration profile – this is important because it will revert the guiding parameters to their defaults.  Many users have a tendency to screw these up in a futile attempt to overcome mechanical problems with the gear and this just makes it harder to us to diagnose the real issues.

 

Good luck,

Bruce

[weegieb]

https://openphdguiding.org/logs/dl/PHD2_logs_a9vJ.zip

I managed to get home and do a baseline and follow what was in the document.

The seeing conditions were not great.  I did notice significant changes and i'll call it improvement from what I have been experiencing.

Probably should have done this long ago. 

Any analysis would be appreciated.

thank you

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Lou Weegie B

[Brian Valente]

Hi weegieb

I am looking through your logs right now, but one thing jumps out at me

you switched the Dec algorithm from resist switch to lowpass2 

Is there a reason for this?

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Brian 

Brian Valente

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[Bruce Waddington]

Yes, this doesn’t look too bad.  Of course, you did commit a couple of cockpit errors by not following my advice to use the Calibration Assistant tool.  As a consequence, it wasn’t able to correct the other mistake you made when completing the new-profile-wizard.  Your mount is configured to guide at 0.9x sidereal but you said it was 0.5x in the new-profile-wizard.  The result is that your calibration used a fairly small number of steps to complete.  Fortunately, that didn’t cause any problems.  If you will use the Calibration Assistant the next time (under the Tools menu), it will fix the problem for you.

 In one of the earlier guiding sessions, it looks like you may you have stiction on the Dec axis, quite possibly caused by an overly-tight mesh.  We have seen this fairly frequently on iOptron mounts.  Here's what that looked like (Dec in green):

Paradoxically, this didn't show up at all in a later guide session.  That can happen during periods of bad seeing when the initial guide star excursions are fairly large.  You can combat this by increasing the min-move values during periods of bad seeing but I think you should take a look to see if the Dec mesh is too tight.  i know you were happy with the great result in the backlash test but it's better to have somewhat more backlash if you can eliminate the stiction.  Simple backlash can be fairly well controlled by PHD2 but stiction is much harder.

Your RA tracking results show fairly typical periodic error but there are some unusual contributors at higher frequencies that will limit your tracking accuracy: (RA in red)

The largest error at the right probably corresponds to the worm period of the mount, estimated at about 295 seconds here.  The other two contributors to the left are roughly at 78 and 98 seconds, keeping in mind that the guiding sessions were not long enough to get a very accurate frequency analysis.  All of this says that you may need to use shorter guide camera exposure times in order to guide more aggressively in RA.  Since you are using multi-star guiding, you can try using guide exposures down to 1-sec.  If you want to try using PHD2 PPEC in RA, you could try "locking" it on the highest frequency component rather than the largest one in this case.  I don't want to wander into the swamp of whether your iOptron mount can deal with periodic error correction coupled with guiding. 

As you can see, your limitations are coming from the mount, nothing to do with the PHD2 settings or not knowing the "magic recipe" for guiding an iOptron mount.  They are all different, they all have different limitations.

Regards,

Bruce

[Lou B]

Bruce,

I'll start saying thank you for your time and effort.

I did as much as i could and tried to be as efficient as possible to collect useful data.
Unfortunately, Clouds, work, and family all consume my time so spending any on my hobbies is a luxury.  

I updated to the PHD 2 build you suggested.  

I did try the Calibration Assistant tool.  I looked at it, got a bit confused and moved on figuring to get done what i could.  

When configuring and connecting to the mount, PHD gave an error "mount is not reporting guide rate."  
This never happened before.  I run the mount from a NUC through a USB/Serial adapter.  I checked connections, etc. and found nothing.   I did open commander software and verified it was set to 0.5x.  Why it is reporting 0.9x, i do not know.  It is something i need to check further.

You asked why i changed from resist switch to Lowpass 2.  Guide assistant made the suggestions so I followed.  I typically used resist switch in the past on my Atlas and EQ6 mounts where backlash is more of an issue.  Not knowing how guide assistant makes its decision or much about the Lowpass algorithm, I can't comment why Guide assistant made the recommendation.    

The DEC stiction is noted.  I find the control to engage the worm on the CEM60 a bit of a challenge.  You turn a knob until it is fully engaged and then back off.  I'm likely not backing off enough and will increase.

 I did guide assistant for 20 minutes.  guided for 20 min.  then ran unguided for another 20.  That graph looks like the last unguided run.  I noticed the build up of error near the end of the run and stopped.  The guiding run looked acceptable.  

Thank you again.

LouB

[Bruce Waddington]

Hi Lou, I'll try to respond to each of these points.  It's disappointing to hear that you were confused by the Calibration Assistant.  It's basically a wizard-type interface that only does two things: slew the scope to a good location for calibration, then start and calibration and evaluate the results.  There is a large block of instructions at the top of each dialog that tells you what to do and what to expect.  I wonder if you could take a look at the documentation and tell me what you found confusing:

https://openphdguiding.org/man-dev/Tools.htm#Calibration_Assistant

Here are the debug log entries showing what the mount was reporting for its guide speed - notice the timestamps:

19:57:09.421 00.000 8036 ScopeASCOM::GetGuideRates returns 1 0.000 0.000 a-s/sec

21:21:59.280 00.250 5816 ScopeASCOM::GetGuideRates returns 0 13.500 13.500 a-s/sec

So, why is there a difference?  At 19:57, the scope was pointing at the north celestial pole so it may have been parked or not even fully initialized.  By 21:51, it had been slewed away from the pole and was able to report sensible values.  The value of 13.5 arc-sec/sec is 0.9x sidereal which is consistent with what PHD2 eventually measured - so that is the correct value.  I think you should ask iOptron about this, it looks like one or more bugs in their software.

I understand why you switched to LowPass2, that was a question asked by someone else.  That recommendation came from the very low backlash result from the GA and, as I said, I'm very skeptical of that result.  I think the axis is probably meshed too tightly.  I think a more credible result would be a backlash of 1-2 seconds and no recommendation to switch from the default Resist-Switch algorithm.

I'm sympathetic to your lack of time to invest in these diagnostic efforts.  On the other hand, if you are trying to extract the best possible results from gear whose out-of-the-box performance is not very good, it is going to take time.  This hobby is typically a large sink of both time and money for anyone who is serious about it.  From what I've seen during the past year or so, iOptron has come to understand what the PHD2 data is showing them and has often  helped people improve their results. So I think you should seek help from them as well.

Good luck,

Bruce

[babaganush1234]

Bruce et al,

Can I thank you all as the new calibration assistant feature is fantastic. Many thank yous as I was having difficulty with my calibration and then saw the new feature...it worked perfectly.

Marc Whitsett 

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[Bruce Waddington]

Thanks for the comment, Marc, it's nice to know that the Calibration Assistant worked well for you.  It's often hard for us to know whether people are trying the new features or not...

Regards,

Bruce

[Lou B]

Just an update. 

Tracking and guiding has been great since I took your advice. 

I'm limited by seeing. 

I made one additional change regarding my CEM60 by losening the clutches a bit more.

This reduced stiction and working fine.