Paramount MX+ Guiding Woes..

[gary]

I'm at a loss to figure out how to get decent guiding. Several days ago, I redid a careful balancing and Polar Alignment.  Last night I tested doing guiding calibrations and guide assistant two ways. My main OTA and camera image scale is 0.93" (Bin 2)with the guiding system at image scale of 2.35"(Bin 1).

First, I disabled the mount's PEC, Protrack TPoint sky model. Then I ran a fresh Calibration followed by the Guiding Assistant for 8 minutes. The GA suggested a PA error of 1.1' and suggested I set the RA MnMo to 0.10 and the Dec MnMo to 0.15.

Next, I ran 10 minutes of guiding leaving PEC, Protrack and TPoint 'off' as they had been during the calibration and GA runs. After 10 minutes of guiding the total=0.67".

Next, I ran 10 minutes of guiding, this time with PEC, Protrack and TPoint corrections all 'enabled'. After the 10 minutes of guiding, the total =0.40"

After all the above, I redid the Calibration and re-ran the Guide Assistant, this time with PEC, Protrack and TPoint corrections all 'enabled'. The GA suggested a PA error of 6.2' and made the same suggestions as above for the RA and Dec MnMo. 

Finally, I ran 10 minutes of guiding with PEC, Protrack and TPoint enabled, ending with a Total=0.59". 

After all that I ran a NINA sequence for several hours, ending with Guiding Total=0.74". For some applications, perhaps this performance isn't 'horrible', but for scientific imaging, it's not acceptable. The guide log(attached) shows consistent RA 'spikes' pretty much through the session. 

I would really appreciate any insights some of the experts here might offer on any aspect of the above that might lead to improved guiding. The mount is a decent one, recently regreased, balanced and polar aligned. The OTA is a Planewave CDK and Guidescope is a Stellarvue 50 mm, firmly mounted to the bottom rail. Thank you!

[Bruce Waddington]

Where are your guide and debug log files?  We can't help you much without being able to see the data.  Use the built-in Upload Logs feature built in to PHD2, don't use attachments in the forum interface:

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

You should also tell us what makes things unacceptable in terms of whatever your specific requirements are.  Right off the bat, I'm very suspicious of your separate guidescope arrangement in the context of working with a large CDK scope.

Bruce

[gary]

Hi Bruce : There is a link to the Logs below that relate to my initial post. Apparently the logs that I posted initially didn't get attached to the post. If the Guide-scope/image scale (50mm x 210mm fl with ASI 178mm camera) that I have for the CDK 12.5" OTA, is somehow a miss-match  I'd welcome input on another approach or criteria for creating a more appropriate guiding system for the CDK. I have begun considering switching to an ASI 2600 MM Duo to eliminate some sources of guiding errors. Thanks very much for helping out! Gary

https://openphdguiding.org/logs/dl/PHD2_logs_pPwD.zip

[Bruce Waddington]

I can see a few problems that will need to be sorted out.  First, look at the 2 back-back Guiding Assistant runs you did starting at 21:09 (RA in red, Dec in green):

The striking difference here is the amount of drift in Dec.  The first run was equivalent to a polar alignment error of about 1 arc-min while the second one was equivalent to an alignment error of over 6 arc-min.  What happened?  I can think of a few explanations keeping in mind that the scope pointing position didn't really change:

1.  You have a cable routing problem that is tugging on the guide camera.  The pointing position in the first session was Dec=8.8, HA=-1.42 hours and in the second session it was Dec=8.8, HA=-1.15.  So you were tracking closer to the central meridian from the west side of the pier, a position that often exposes cabling problems.  Keep in mind that through-the-mount cabling, if you're using that, doesn't exempt you from these kinds of problems.

2.  Something is loose in the guiding assembly

3.  You're using a bad sky model

Of course, trying to use a separate guide scope on a 12.5" CDK is not usually recommended for reasons of differential flexure and trying to do so often requires extreme measures to achieve rigidity in the guide scope assembly.  Whether you can get away with it will depend on how long the exposures are on the main camera.  You mentioned some kind of "scientific imaging" - are you talking about spectroscopy or photometry?  

The next problem is the sudden appearance of spikes in the Dec tracking in your last guiding session:

These are not associated with guiding, they simply "happen", and they appear semi-periodic.  Again, these could be coming from a bad sky model or from some mechanical behavior in the payload - things like filter changes, focus adjustments, fan operation, etc.  I think it's pretty unlikely they are coming from the basic mount mechanics because the Dec motor would be idle except for sky model corrections and guide pulses.

Overall, I think your mount needs higher-cadence guiding than what you're doing.  Since it doesn't have high-resolution encoders, it isn't a good candidate for variable-delay exposures, so I think you should stop trying to use that. Choose a fixed exposure time, perhaps something like 2 seconds, and run with that to see what changes.  With your current operation, the guiding is generally "falling behind" the tracking errors, particularly with the large Dec drift.

Hope this helps you sort things out,

Bruce