AM5N Guiding/Tracking Issues??

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.

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?

I calibrate the guide every night, it cost 100 seconds, in EST of course.
PA was done fine ?

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.

Focuser sag, I'd reckon.

@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.

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.

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

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.

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.

Jeramie:
I'm assuming that you have dew heaters on your guide setup?

Yes. Dew heater straps on guidescope and main scope.