Hi All, I have an AM5N mount with no counterweight and a payload of 17 pounds. I have attached a screenshot of guiding RMS error versus subframe number from last night. Each sub is 600 seconds. The target (SH2-114) starts low in the NE and rises toward the meridian. As you can see from the plot, the guiding errors steadily increase until frame 31, which is the first sub after the meridian flip. My guiding is usually 0.5" - 0.6". Any ideas what is going on? I should also mention that the majority of the error is in the RA axis. I have never seen this behavior before and it has only appeared in the last few sessions. My first thought is some sort of mass imbalance, but it is hard for me to verify that since my scope is at a remote observatory.  Jim
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When (and more importantly, where) are you calibrating guiding? I had a similar issue with my AM5 and was educated that you should calibrate within +/- 30 degrees of Celestial equator. Started following this guidance and I average ~.4 total RMS with my C11 on my AM5 now.
Which guiding software/platform are you using?
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Hi Jeramie,
I am using Phd2 with NINA. I ran the calibration and guiding assistant two nights ago. I follow the Phd2 assistant recommendations and slew the scope to near the meridian and +/- 5 degrees of equator.
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From the graph you provided it does seem that something has come out of alignment. If it were me, I'd try guiding on a target in a different part of the sky and see if the behavior replicates. Of course, I'd also want to visually inspect the rig to make sure that the guide scope hadn't been moved (think flexure and how that can impact guiding).
I've used PHD2 in the past, and it is pretty straight forward. Outside of recalibrating, I can't think of anything else settings-wise inside of the app that would cause this behavior.
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Additionally, the counterweight shouldn't be an issue at that payload weight. The CW was only recommended to help the mount from tipping over IIRC. I have a steel pier and run a CW just because I'm running 27 pounds on my AM5. I also assume that you're on a pier in your RO.
For others that may read this thread, can you describe your physical environment a bit? Are you on a pier? Guide setup details?
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Yep, I am on a pier. I can try guiding on a target on the western side of the meridian and see what kind of behavior I get.
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I calibrate the guide every night, it cost 100 seconds, in EST of course.
PA was done fine ?
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Yes, I calibrated and ran the guiding assistant. The guiding assistant ran for 10 minutes and reported a PA error of 0.3 arcmin. So everything good there.
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AstroCapture325:
Yes, I calibrated and ran the guiding assistant. The guiding assistant ran for 10 minutes and reported a PA error of 0.3 arcmin. So everything good there. 10 minutes? I would be interested in that behavior. I can manually calibrate in a fraction of that time. My ASIAir will do it in about a minute. I have seen times when my ASIAir takes a long time to complete a task, and even though it completes things are not correct. Sounds suspicious to me.
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I'm use asiair too, the time for the calibration depends to the steps. Minor time in steps = major time to calibrate.
Default step time is good, but if u use very short focal lenght (es 135 mm) or very long (es 2000 mm) u need to change it to perform the best calibration.
Of course, i'm using an oag. If u have an external guide tube u don't need change steps in u change the main ota focal lenght.
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It is not PA error - if PHD says it is less than 1 arc minute you should be fine
For now let's discard pier rigidity too - it is a light setup on a pier so mount/pier flex is not a concern nor should be inbalance.
Now this graph going up then back to normal after meridian flip intrigues me. It is behaving like if there was a rubber band stretching and making it harder and harder to keep up then if flips and it gets back to normal. Which could mean there is a cable stuck or been squeezed between the RA and DEC "boxes" of the mount.
It looks like a physical issue to me. I would have it checked by advanced support requesting them to see what is happening when the mount slews up slowly. Who knows if a velcro strap disattached and moved to where it shouldn't move.
At this point Starfront is running 300+ setups so if there is something snagging on the crowd they'll never find by themselves.
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@andrea tasselli The staff checked the focuser draw tube of my WO ZenithStar 81 and it was a little loose, but that was the night before I had these guiding issues, so although the focuser was loose and then tightened up, it was not contributing to the problem. I wish that was the problem.
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Jeramie:
10 minutes? I would be interested in that behavior. I can manually calibrate in a fraction of that time. My ASIAir will do it in about a minute. @Jeramie and @Michele Campini I should clarify that the Phd2 calibration assistant runs in about two minutes, but I choose to run the guiding assistant for ten minutes to get a more accurate measure of the PA error and any uncorrected periodic error in RA.
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Rafael Amarins:
It is behaving like if there was a rubber band stretching and making it harder and harder to keep up then if flips and it gets back to normal. Which could mean there is a cable stuck or been squeezed between the RA and DEC "boxes" of the mount.
It looks like a physical issue to me. I would have it checked by advanced support requesting them to see what is happening when the mount slews up slowly. Who knows if a velcro strap disattached and moved to where it shouldn't move. @Rafael Amarins A very interesting hypothesis. Thank you. I am going to try an experiment tonight and guide on a target that is west of the meridian (probably M51) and see what the performance looks like. But I also want to have the support staff watch the mount when it is slewing to check if anything is snagging or binding.
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AstroCapture325:
@andrea tasselli The staff checked the focuser draw tube of my WO ZenithStar 81 and it was a little loose, but that was the night before I had these guiding issues, so although the focuser was loose and then tightened up, it was not contributing to the problem. I wish that was the problem. That graph is typical of either focuser sag or cable drag. Which would be a much better outcome than anything relevant to the mount's performance.
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To refresh everyone. The hypothesis is that there is some physical something, such as a cable snag or bundling issue, that is causing the RA tracking accuracy of the mount to degrade as the target approaches the meridian. Once the target crosses the meridian tracking returns to expected values (for my setup 0.4" to 0.7"). I ran an experiment last night. First I tracked M51. I selected this target because it already had crossed the meridian by astronomical dark. As expected, the mount tracked normally (<0.6"). The plot shows guiding RMS vs. subframe number for M51. Each subframe is 10 minutes, so nearly four hours of tracking data.  Next I imaged NGC 7380, which was still rising in the east and had not crossed the meridian. I figured that once again the guiding errors would increase as the target approached the meridian, but that is not what happened! The mount tracked just fine with errors typical of what I have experienced in the past (<0.5"). The plot below shows this. This about 2.5 hours of tracking data.  My rig is at Starfront and yesterday I used the my pier cam and slewed the mount in various directions to see if I could identify any issues with cables, based on @Rafael Amarins and @andrea tasselli thoughts. I could not see anything that looked suspicious, but I did ask the staff to take a look to see if they could spot anything that may have been snagged or could lead to a snag. So after a couple of nights of crummy guiding, last night's guiding was great. Not sure where to go from here other than to monitor the situation to see if it re-occurs. Any thoughts on possible causes or resolution appreciated. Thanks, Jim
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Glad to hear that things are working, but I don't like not having RCA on these issues.
Is it possible that you have a bad cable or worse, a bad guide cam? This intermittent behavior can be troublesome. Any major differences in the environment from one night to the next? High(er) humidity? Big temperature shift? Any recent updates to any of your software or drivers (ASCOM or other)?
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There definitely were changes in the environment. On the nights with poor guiding the humidity was quite high. Several people had issues with dew on their sensor. Last night with good guiding the humidity was lower.
No recent software updates and I don't see anything in the Phd2 guide logs that suggests camera issues.
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Tony Gondola:
What was the wind doing? Winds were pretty calm. Less than 7 mph. My scope (Zenithstar 81) has a pretty small profile, it's not a wind sail.
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AstroCapture325:
There definitely were changes in the environment. On the nights with poor guiding the humidity was quite high. Several people had issues with dew on their sensor. Last night with good guiding the humidity was lower.
No recent software updates and I don't see anything in the Phd2 guide logs that suggests camera issues. I'm assuming that you have dew heaters on your guide setup?
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Jeramie:
I'm assuming that you have dew heaters on your guide setup? Yes. Dew heater straps on guidescope and main scope.
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I'd continue to monitor at this point. I've had everything from the random bug/spider setup shop to odd lights hitting my guide scope that decreased accuracy.
I'd think that if things guided well that the setup is at least capable of properly guiding. This could just be some random cause that may not show up again, but hopefully you get more data to help solve this issue.
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I would have expected that a cable snag would have sorted itself once you flipped the scope the first time around. Unless you are there when it happens it is difficult to figure out what happened retroactively. Even more so remotely. S**** happens…
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