CEM120 large guiding spikes (iOptron wasnt sure what this is)

[Noah Lambert]

Hello everyone,

I have been using a CEM120 for a few months now, and dialed in many of the quirks this mount has. However, just as I thought I had it down to a science. I started getting large spikes in RA and DEC. My guidescope is a Uniguide 50mm and 120MC-S for the guide camera (not the best). The main OTA is a GT81 and 294MM. 

These spikes seem to be random, but they are not related to wind, dont seem to be related to temps, and there no sangs in the cabling because I am using through the mount cables. These spikes ruin whatever frame is running and I would like to get them removed. Sometimes, it is RA, sometimes it is DEC, sometimes it is both. I am really scratching my head on this one. 

[Bruce Waddington]

Need to see both the guide and debug log files.

 

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

 

 

Bruce

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[Noah Lambert]

Here you go Bruce

https://openphdguiding.org/logs/dl/PHD2_logs_QKTF.zip

[Bruce Waddington]

Hi Noah.  It’s possible there are multiple causes for these problems.  One thing to remember is that through-the-mount cabling isn’t a magic fix for cable problems.  You usually can’t see what’s going on in there, and you might still have problems.  For example, the cables may intertwine and then their freedom of movement is constrained by the shortest cable in the assembly.  Setting that aside, I would look first at the guiding assembly itself since you’re using a separate guide scope.  Some of these sudden excursions are very large – here is one of the worst (RA in red, Dec in green):

 

 

In my experience, big errors like this are usually caused by something loose or moving around in the guide camera/guide scope/scope mounting area.  The 75 arc-sec deflection is equivalent to a shift of the guide camera by about 65 microns, close to the thickness of a human hair.  I don’t know what your guide assembly looks like, but if it has Delrin-tipped thumb-screws, 3-point mounting rings or a single stalk mount, it probably won’t be able to prevent these movements.  They can also arise from small movements of a Crayford-style focuser on the guide scope.  Unfortunately, there are many places to look.  The only way to rule these out is to create a test configuration where you are guiding for long stretches through the main scope – obviously without any imaging.  Or an OAG will eliminate them, obviously.

 

For the smaller excursions, I think you should temporarily disable PHD2 Dec backlash compensation.  It looks to me like the Dec axis may have some stiction and the axis movement doesn’t look symmetric in terms of north vs south.  The reversal delay that’s present doesn’t look particularly large so you may be able to live with it, at least until you’re done trouble-shooting other areas.  Also, if you’re in trouble-shooting mode, you should start each night’s session with a fresh calibration.

 

Good luck,

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[dpdal...@aol.com]

Gentlemen,

I read with interest Bruce's reply as I recently had issues with wind and saw firsthand what that does to guiding. However, after correcting those issues (and not trying to guide at 10-15mph winds), I still get the occasional 2-3" shift in RA with compensatory movement in the opposite direction to follow. It almost seems to occur at regular intervals of about 5 minutes however as the night progresses they disappear(I do use Periodic PEC guiding for RA). I have often wondered about my through the mount cables and the effects they have on each other. One question is: do you feel that providing slack at the entry points of the cables into the mount may help with this intermittent shift in RA?

The second question I have is: what do you feel is the most secure way to attach a light weight guide cam-guide scope combo to the OTA. I do have an accessory Vixen dovetail bar I can use or is it best to go straight to the OTA....and with what sort of secure attachment?
Thank you,

Dan

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[Brian Valente]

HI Dan

here's my response:

>>>One question is: do you feel that providing slack at the entry points of the cables into the mount may help with this intermittent shift in RA?

Maybe. The only way to know is to test it. This is a process of deduction, right? if you (or we) knew the answer, we would just do it! But we don't, so we have to look at what are the possible causes based on past experiences and go from there.

when you say "intermittent shift in RA", it would be helpful if you were more precise about this: what do you mean by "shift", is the period of intermittent and how large is it? How often does it happen and under what circumstances does it "go away"? 

If you have a periodic error that is from the mount, it will show up in the guidelogs (you may  want to do a longer guiding assistant run, maybe 20-30 minutes at meridian/ celestial equator intersection to get better data for your mount). 

>>>The second question I have is: what do you feel is the most secure way to attach a light weight guide cam-guide scope combo to the OTA. I do have an accessory Vixen dovetail bar I can use or is it best to go straight to the OTA....and with what sort of secure attachment?

Avoid rings, avoid the finderscope mount. Use at least two points of contact and make sure your guiding setup is attached using clamshell and not rings or other flimsy approaches. Better yet, an off axis guider would eliminate all those issues and more

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--

Brian 

Brian Valente

portfolio brianvalentephotography.com 

[Noah Lambert]

Bruce,

I probably should address these points in more detail than I did above. Cableing is 100% not an issue here. There are two cables, power and data, and each of them terminate at the saddle which is connected to separate ports at the base. Unless iOptron wired the two ports wrong, I do not see any way these could cause an issue or interference. 

The guiding assembly is very securely mounted. The guide scope is riding in a dovetail, and the camera is threaded into said guidescope, so little flexion there. I also have had this issue with my EdgeHD 9.25 and a OAG but it was less dramatic Here is a reference of the guidescope setup:

I have checked all the mounting points on the scope and mount and everything is secure. I made sure all the connections where threaded so there would be no slippage or flexion. These excursion are very random and really have me scratching my head because all the things you mentioned are what I also assumed (after 6 years of doing this, I learned from all the mistakes). 

The backlash compensation is an interesting idea, I will try that once my skies open up after 3 weeks of clouds. Some people report that the CEM120 can give some faulty backlash numbers and running various GA's runs can give varying results that can differ by over a second. As for the reversal delay, I do not know much about that or solutions for it.

Bruce hopefully this information helps because so far, I have been out of luck. 

[lmcl...@umich.edu]

Hi Noah,

Just be aware that there are cables inside the mount of the CEM120, actually quite a few cables.  And as the mount moves, those cables can move and shift which may cause the spikes you are seeing.

One thing you can do (even during the daytime) is unscrew the cap on the front of the RA and shine a flashlight in there while you move the mount in RA.  I have noticed that the internal cables can rub against each other and eventually slide past one another.  I have no way to tell for sure if this can cause a "spike" while PHD2 is guiding but it seems probable.

Lamar

[Noah Lambert]

Hey Lamar,

I am aware of the internal cabling and know there is going to be some movement, but at the slow rates we are tracking and guiding, I just dont see how that would be the issue here. 

Here is a new guidelog with a fresh calibration and 2 runs of GA. There was some wind last night which messed with things a little think, but it is a good 7 hours of guiding data. 

https://openphdguiding.org/logs/dl/PHD2_logs_yQZo.zip

[Henry McFly]

I have a slightly different question. I have a LX90EMC 8". It still has the original finder scope. I added a seperate scope for guiding. Is it possible to modify the original spotter scope to fit a 1.25" lens with illuminated graticule to help with setting polar alignment and general auto guiding setup? It has that wierd slider which doesn't seem to be available on other scopes and the eyepiece is not removable. Any ideas?

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[Noah Lambert]

Hopefully Bruce can look at this but I have still yet to figure out what these issues are with my CEM120. I am not sure if the DEC or RA needs to be adjusted, but some of these graphs get quite jumpy. Granted, my imaging scape is 2.03"/pixel, and most of these errors are below that. However, the peaks are what are getting me and really elongating my stars. My most recent attempt at fixing things (new cables to fix this strange camera timeout issue) and making sure there is zero flex, cable issues, and more, all have not done a thing. Here you can see how jumpy DEC gets at nearly 4" at certain places.  Also, I dont know why my graphs get all broken up. 

[bw_msgboard]

Sorry Noah, I can't look at anything when I don't have the log files.

 

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Noah Lambert

Sent: Thursday, January 20, 2022 6:09 PM
To: Open PHD Guiding

Subject: Re: [open-phd-guiding] CEM120 large guiding spikes (iOptron wasnt sure what this is)

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[Brian Valente]

>>>  Also, I dont know why my graphs get all broken up. 

a new log segment is generated when you stop and then restart the guiding. You or the app are doing this

Brian

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[Noah Lambert]

Bruce,

I am sorry, I didnt know you needed the specific log files. Here they are. https://openphdguiding.org/logs/dl/PHD2_logs_vNwg.zip

[bw_msgboard]

Your guiding sessions are being chopped up because whatever app you're using for automation is telling PHD2 to stop guiding.  Your mount has mechanical problems.  If you want to trouble-shoot that and figure out how to improve them, you need to run dedicated tests - so ditch the automation app temporarily and focus only on the mount and guiding until you have the mechanics sorted out.  The most obvious mechanical problems are in Dec which I think should have been fairly clear - your Dec guiding RMS is 2x worse than RA.  Here's a typical pattern:

 

 

 

This happens with every direction reversal in Dec guiding.  There is a long initial delay even taking into account that you were using a PHD2 Dec backlash compensation value of nearly 1 second.  After the axis finally gets moving, it nearly always over-shoots and you then repeat the same process in the opposite direction.  I think the drive system on the Dec axis is probably too tight, there is too much resistance to rotating the axis smoothly and consistently.  You probably have some combination of backlash and stiction.  I'm assuming, of course, that you aren't using one of the "EC" iOptron models. You should run the Guiding Assistant and let it measure the backlash.  Even if the measurement process "fails", you should look at the graph to see exactly what happened.  All of this will probably require a mechanical adjustment or correction of some kind, but the GA graph should make the situation pretty clear.

 

Good luck,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Noah Lambert

Sent: Wednesday, January 26, 2022 7:08 PM

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[Noah Lambert]

Bruce,

I believe the sections where it stops are where I have "stop guiding while autofocus runs" enabled in Nina. So when the focuser moves for filter offsets, I have guiding stopped but I might change that. I did speak with iOptron and they had me loosen the DEC and RA meshing to see if that helped. It seemed to help a bit, but not solve the problem. Looking at the DEC backlash graphs provide no clear insight as most of the time, they look pretty good. I have never had it fail or anything. I am just using the base model, but I am very frustrated with this mount because I purchased it to avoid the very issues I am facing. With my EQ6R-Pro running an Edge HD 9.25 and a OAG at the native focal length, I could guide at .7 on a bad night. This CEM120 with just a 80mm triplet and 50mm guide scope can barely guide at 1" on a good night. I do not under how this is possible given the cost premium. 

[Brian Valente]

>>> I believe the sections where it stops are where I have "stop guiding while autofocus runs" enabled in Nina.

Other apps (several of which i'm aware of) use pause autoguiding instead of stop. If you are of a mind, you can mention that to NINA developer if that is in fact what is going on

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[Noah Lambert]

Brian,

I have mentioned many things like that to the Nina devs, but they seem to not always appreciate it. I am not sure what the exact cause is for the separations, but at this point, I just want to figure out what these spikes are and why a $6,000 mount cannot guide an 80mm refractor better than even my AVX could. 

[Noah Lambert]

Bruce,

I loosened up the meshing again last night and just wanted to provide some more logs. On the 29th, guiding was pretty consistently bad, but that could be seeing. Last night, 1/30, ignore the graphs earlier in the night, like around 8-9:30pm. I was messing with backfocus on the OTA and guiding wasnt stopped. It seemed to get a lot better later during the night. 

Thank you Bruce for all that you do. You are really helping me out a ton! 

https://openphdguiding.org/logs/dl/PHD2_logs_pH3k.zip

[bw_msgboard]

Hi Noah.  It's hard to tell but I think you may have made a considerable improvement in Dec - several of the longer guiding sessions look quite good and I no longer see much evidence of the stiction/resistance in Dec.  The last 16-minute session showed a total guiding RMS of 0.6 arc-sec.  At that level, any further improvement is going to require good seeing, and I'm not sure you have that.  You are also getting too many lost-star events because of the mass-change detection feature, so you should disable star-mass checking (Advanced settings/Guiding tab). It's not doing you any good and is just muddying the water.   In order to get any better view of where you stand, you need to do what I recommended earlier: ditch the automation app and let PHD2 guide for multiple 15-20 minute sessions in various parts of the sky.  To be safe, you should probably precede those tests with a fresh calibration.  These little 4-5 minute sequences you're generating - which are riddled with various lost-star events - don't provide the right kind of information.

 

Regards,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Noah Lambert

Sent: Monday, January 31, 2022 6:06 AM

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[Noah Lambert]

Hey Bruce,

so I took your advice and got some really long guide logs (They are still broken up a little, but most are 2-3 hours long). I have continued to speak with iOptron and their solutions are not working. The gear meshing was heavily loosened, then tightened to removed backlash. Even with that, I am still getting some massive spikes at times. Here is one more log with a fresh calibration, GA, and long guiding sections. Any ideas? 

https://openphdguiding.org/logs/dl/PHD2_logs_9gDG.zip

[bw_msgboard]

I really don't see what you're worried about here.  Here are the statistics for the two long guiding sessions you had (1h27m and 2h33m):

 

 

 

 

These would have produced nice round stars essentially all night - look at how closely matched the RA and Dec RMS numbers are.  Are you saying this isn't good enough? 

 

To take this any further, you will need to become very aware of the seeing conditions from one night to the next and from hour to hour.  For example, most of the continental US has been buried under jet streams during early February.  There will almost always be guiding spikes of one sort or another during the night but you can't obsess about those.  What matters is the quality of your final images.  Really, I think you should just move on and do some imaging.

 

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Noah Lambert

Sent: Wednesday, February 09, 2022 6:44 AM

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