The Great Hercules Cluster (M13) and Meade Guiding on the G11

The objective of this session was to attempt guiding with the Meade LX850 (12″ f/8) on the G11 mount. The session did not go too well.

I had a better result with the Pronto last night than I did with the OAG the night before. Even so, there are plenty of problems to work through before I can claim success, and I hold open the possibility that I might not ever be able to do guided DSO imaging with the G11/Meade. It also occurs to me that I can usually break through what may at first appear to be an insurmountable problem if I keep chipping away at it. This will be a tough one!

The Meade with the ASI6200 were already on the G11 from the night before. I added the Pronto as a guide scope and moved the guide camera from the OAG to the Pronto. I had planned to move it back to the OAG and re-test that configuration. My present thinking is that I will make some adjustments and try the Pronto again for at least one more session, and that I will try the OAG again to see if I get a better result with better sky conditions than I had when I used it on 2022-06-04.

I went out the to scope to begin powering up and connecting software to equipment at 2105. The temperature and dew point were 66F and 53F, respectively. The air was still, and the sky was clear. A 33% illuminated moon was in the west. Astrospheric forecast was for Below Average transparency and Average seeing.

Focusing both scopes took 45 minutes, which was way too long. Focusing the main scope with the 6200 and the Bahtinov mask took longer because I had to work out the technique for this setup. All of the stars in the Meade FoV were rather dim, so it was hard to read the mask spikes due to the Meade’s f/8 slower speed. I overcame that by upping the exposure time on the camera, which added more time to the process. Next time I will try nudging the scope around to find a brighter star. On the guide camera, I had trouble finding focus, and ended up needing a longer extension. Also, there were halos surrounding the stars when focused to the lowest FWHM number and I could go a little higher than that FWHM and see pin-point stars. Not sure which position is the best to use. There was no moisture on the objective, but maybe I need to clean it. Even starting from an unknown focus position next time, I can complete this activity much more quickly.

After focusing I ran though a guiding assessment. My approach was to go through the process without making adjustments to get a baseline result, to include image results, afterward.

I slewed to meridian-equator for calibration. PHD reported the result as suspect. I examined the orthogonality graph and found that it was close, and there were maybe three plots that were out of line. Not having any idea what the issue was, I decided to proceed.

Initial guiding RMS was Ra 1.19”, Dec .62” and Tot 1.28. This was not a bad starting point compared to what I got with the TV on the mount, but it was at times much worse. With the Meade/6200 image scale, I’d like to see a total RMS of about .5” with Ra and Dec values being about equal. That’s going to be tough!

Next I ran the Guiding Assistant tool. After a 3-minute run, I was seeing a polar alignment error of 36.4’ (yes, minutes). I don’t think that it was actually that far off, but I did not recheck with PoleMaster. The backlash measurement failed, because the mount did not consistently respond to south guiding commands. After accepting the GA tool recommendations, the guiding RMS was Ra 2.76”, Dec 1.31” and Tot 3.02”.

I noticed a curious pattern in the PHD2 scatter graph. There were two groups of plots. A small number of plots was located in a rather linear formation just below the Y axis between the X axis and the outer circle. The majority of the plots were in a decently tight circular formation located completely outside of the circle at about the 7 o’clock position. I am not sure what to make of the two groupings or of the main one being so far from the center.

For imaging, I let NINA handle the slew to and center on M13.

I believe that the initial slew is based on Ra/Dec coordinates that are passed to the mount. Then plate solving engages to center the target, and in this case it failed. The target ended up slightly off center, but it was good enough for my purposes. I suspect that ASTAP (plate solver) did not have correct parameters for the Meade/6200.

I set up and ran a sequence for 60s on the L filter, and 120S each on the RGB filters. The first images were completely horrible from a guiding perspective, and later they cleaned up a good bit. I noticed at the end that 60s on L and 120s on the colors did not move the histogram completely out of the noise. While these exposure values worked well with the TV at f/5.4, they were completely inadequate for the Meade at f/8. Experimentation is needed, but I suspect that 3x longer exposures.

When I ended the session, the temperature was 60F and the dew point was 52F. There was dew on the grass but not on the metal surfaces. The sky was clear and there was no breeze, and by my estimation met the Astrospheric seeing forecast. I wore a light jacket but was a little chilly by the end.

After thoughts:

One possible (probable?) guiding problem is the weight on the mount. I see two prevalent points of view in Cloudy Nights. First is that the 12” LX850 is near the G11’s max capacity, and that imaging is best done at about half of a mount’s max capacity. In my case, it gets worse as I add the electronic focuser, five pounds of imaging package, and supporting accessories. Another point of view is that the G11 can handle it. For the time being, I’m going to assume that the G11 can handle it, and that its up to me to figure out how to help the G11 guide with some extra weight.

I also see to CN points of view on guide scope vs. OAG, and most come down on the side of OAG as the best guiding performer. It is too early to rule out OAG!

This morning I have thought of three more things to try. First is to make sure that I am balance with an east bias. That went through my mind last night, but I was so freaked out by the amount of counterweight on the bar (59 lbs) that I dare not touch it. I really need to touch it. Perhaps pre-setting it to east bias in the day light will help me get past that. Next, I will re-test the OAG with better sky conditions, probably in a separate session after I am through working with the Pronto. And finally, if it turns out that using a guide scope is going to be the best solution, then I will compare commonly available guide camera weights with the Pronto’s weight. If the difference is significant and if I think that the prospects are good for improving guiding with a lighter scope, then perhaps I’ll purchase one.

Post Session to-do list:

  • Clean objectives on main and guide scopes
  • Balance Ra for east bias when the scope is on the west side of the mount.
  • Troubleshoot ASTAP plate solving issue
  • Research guide scope and Pronto weights
  • Post some test images

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