Paramount MX+ Poor Guiding

[Philip Keyser]

I recently bought a Paramount MX+ and I have having a lot of trouble getting consistent guiding. I've gotten good guiding before <0.4" total RMS but lately and especially last night I  could barely get under 1" and maybe 0.7-0.8" on a good stretch. I have tried to adjust my aggressiveness and Min Mo settings all over and haven't gotten anywhere. Here is a sample graph and attached is my Guide Log.

[Bruce Waddington]

Hi Philip. It's a little hard to be sure what's going on because the performance changes quite a bit from one guiding sequence to the next.  But I think you are going down the wrong path by trying to blindly and quickly adjust the guiding parameters.  If you look at the guiding data, you'll see a lot of sudden excursions of 2-3 arc-sec, mostly in RA.  Those happen often enough to affect your overall performance, and they're not caused by guiding.  One obvious possibility is that you were dealing with bad seeing, which can wreak havoc on guiding.  This is especially a problem in the summer when heat convection from surrounding surfaces and tube currents can be hard to tame.  There are a few things you can do to gauge the seeing conditions beyond simply looking at the Clear Sky Clock for your location.  If you take a series of 10 second exposures with your main scope, you can measure an average FWHM of faint to medium stars in the image.  This size can change significantly from one night to the next as the seeing conditions change.  As a rough rule of thumb, your guiding RMS is not likely to be better than 1/4 the measured FWHM.  With experience, you may also be able to judge relative seeing based on the results from your automated focusing runs.  You can also use the PHD2 Guiding Assistant as an estimator of relative seeing assuming you have gotten decent baseline results when seeing is better than average.  Run the GA for several minutes, then look at the "high-frequency star motion" values for RA and Dec.  With degraded seeing, these numbers are likely to be significantly larger than what you'll see with good seeing.  If seeing is your problem, there's little you can do to get big improvements in guiding beyond raising the min-move values to avoid chasing the seeing.  But that just helps to avoid making things worse - bad conditions are just bad conditions if you're using longer focal length setups.

If you're convinced the seeing conditions were good, you'll have to look elsewhere to see what's causing the large RA excursions.  Without knowing anything about your setup, I can't offer much advice on that.  You might want to take a look at sections of the log analysis tutorial dealing with seeing conditions, flexure, and large guide star displacements:
http://openphdguiding.org/Analyzing_PHD2_Guide_Logs.pdf

If you don't have seeing problems, we can try to help you isolate the problems.  We'll need to have a GA run for at least 5 minutes followed by a 10-min guiding run that doesn't involve any tweak to parameters.  Before doing this, you should reset the guiding parameters to their default values.  We'll need both the guiding and debug logs along with a more detailed description of your set-up.

Good luck,
Bruce

[Philip Keyser]

Thanks for the reply Bruce. I don't think it was seeing otherwise I would expect the areas between the large RA spikes to be a bit more chaotic but maybe I am wrong. I am still trying to do some testing to narrow the problem down. I got some better results last night after I made some changes with the mount but those spikes are still there. I will try and gather some good data for you next time it is clear.

[Philip Keyser]

Last night things were looking good but I am still seeing the RA spikes and it almost looks like an overcorrection but the rest of the time the corrections are spot on so I don't think the aggression is too high. I am using a MX+ on a Losmandy tripod with a William Optics 102mm refractor with an OAG. I did not have Protrack enabled at all only PEC was applied. I just got back from a trip yesterday so I didn't mess with things too much. I just got it to a point that looked good and went to bed. This morning guiding looked real good except for these random spikes. I thought there was a correlation with the RA spikes and DEC corrections so I tried really increasing the DEC Min Mo. My PA is very good so I shouldn't need to correct in DEC very often at all. TSX reported my Az off by 11.6 arcseconds and Alt off by 59 arcseconds. Guiding and Debug logs are attached. I will try get the 5 minute GA run and 10min of Guiding you asked for tonight if the forecast holds and it is clear.

http://i.imgur.com/q8RjhaC.png

[Philip Keyser]

Also here is a 5 second frame through my Lum filter taken last night so that the seeing can be measured.

[Andy Galasso]

Philip,

The RA spikes appear to be correlated with dec pulse reversals. Notice when Dec pulses change from North to South or vice-versa, an RA spike follows.  This is not the first report we have had of this behavior with SB mounts.

legend:

   RA = blue, Dec = red

   red/blue lines = guide star offset from lock position in RA and Dec (each vertical division = 1 arc-sec)

   vertical bars = guide pulses sent to mount

   orange arrows label Dec pulse direction reversals

   

I think you'll need to contact SB support for help with this problem since the problem is coming from the mount itself.

Andy