Wednesday, April 27, 2016

Auto Focus Issues Resolved and New Pointing Model

Got out to Kumeu Observatory a bit late last night but ended up working until 5:30 am so it was a pretty productive night, first thing I did after setting up was to loosen the shipping bolts in the C14 so I could move the primary mirror, once that was done I attempted to do an auto focus run in MaxIum DL but for some reason it would focus on a hot pixel and not the stars on the frame, so guessing that I needed to get it closer to focus manually I proceeded to focus the telescope as best I could, after getting the stars as focused as best I could I then selected a star and attempted another auto focus run but again it would just take loads of images and then fail due to too many exposures even though I had entered the parameters that Grant Christie had suggested, feeling a bit frustrated and thinking that the focus was probably good enough for a pointing model I switched the camera control over to the Sky X and attempted to do an automated calibration run, the newly slaved dome couldn't keep up with the randomized pointing model so I ended up manually moving the dome which worked fine, the problem I run into was that after 20 or so pointing samples it would stop astrometricaly solving the images, after a couple of attempts I noticed that the temperature compensating focuser was not tracking the changes in temperature and I knew this was a problem, so I figured I better try resolve the focusing issue before attempting another pointing model, after attempting a few more auto-focus runs I noticed that the focal ratio in the focus setup was in error so after entering the correct focal ratio I was overjoyed to see a lovely V curve and got my 1st auto focus successfully completed, I measured the full-width half-maximum of the focus star at 3.548 which isn't that bad considering we haven't collimated the telescope yet.

With the focus looking better than ever I proceeded to do another pointing model, this time I got the entire western side of the sky successfully with 53 sample points I then got through 3 sample points on the eastern side before running into trouble, I'm not sure why but it could be due to mirror flop seeing as the focuser was tracking the changes in temperature at this point, what would happen is that the mount would become confused and want to point to the ground and the only way to fix that problem was to re-home the mount, knowing that you really can't continue a pointing model after homing the mount I just finished and saved the pointing model at 56 points, hopefully if the sky is clear tonight I'll try another pointing model but start on the eastern side this time and see if I run into the same problem, it was getting late by the time I finished the 56 point model but I couldn't resist testing the pointing model, at 1st the model was saying we needed to make an adjustment in Altitude but after converting the model into a super model it was saying not to make any adjustments to the polar alignment at all, so this is looking hopeful and seeing as my test images at 200 seconds were showing only the slightest of drift I figure our polar alignment must be pretty good, so I slewed to a few objects and was happy to see each object on the frame although each object was a bit off to the right of center, probably due to the pointing model not being completed perfectly and without hardly any pointing samples on the eastern side of the sky, below are two images from my pointing tests, vignetting is worse than normal due to the gibbous Moon and the calibration still needs to be re-done, I was really happy to see the SBIG CCD get down to -19.5 though, so I think as we move into Winter we should have no problems getting down to -20.

M27 The Dumbbell Nebula was a pointing test low to the horizon ^

Lenticular galaxy IC 5122 was a pointing test high in the sky ^

Posted by Jonathan Green

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