Spiral Galaxy Messier 106

Springtime weather in the Colorado Rockies can be a real adventure. Shorter nights, frequent clouds, and weather that changes quickly can make it a challenging season for astrophotography. The last few weeks have not seen decent conditions for imaging, either because of the weather or the bright moon. So when I saw what looked like a window of opportunity a few days ago, I quickly planned an imaging session and set up my equipment in hopes that I’d be able to get at least a few hours in. This would be my second outing with my Celestron C8 OTA and Starizona SCT Corrector IV, and I was looking forward to the same success I had with this setup when I shot M51 a few weeks ago.

Once again I shot using a mono camera (the ASI533MM Pro) and Optolong LRGB filters mounted in a ZWO electronic filter wheel. For guiding I used the ZWO off-axis guider and a ZWO ASI290MM mini guide camera. This time I also had an AstroZap dew shield/heater mounted on the C8. The whole setup rides on my iOptron GEM45 mount on a concrete pier:

My imaging setup.

This evening I was planning to shoot using 2×2 binning in my main camera–using 1×1 binning with this combination of focal length (1470 mm) and camera pixel size results in significant oversampling. Oversampling isn’t the end of the world, but changing the binning to 2×2 resulted in a good match between camera and optics. I also planned to use 2×2 binning in my guide camera, for the same reason. I took care to alter my N.I.N.A. settings to use 2×2 binning and did the same with PHD2, even building a new dark library for 2×2 binning in the guide camera.

Well, it was a good plan, anyway. In the heat of battle–er, imaging–plans often change, and sometimes for the worse. Once darkness arrived and was able to start the imaging process, I encountered problems finding and holding on to guide stars in PHD2. The lack of guide stars can sometimes be a problem with off-axis guiding, but I could see at least one or two in the PHD2 display. Despite that, PHD2 was frequently losing the guide star, resulting in subs that were worthless. Until I could get this issue resolved, my subs would be worthless.

Truly, I had little idea what was causing the problems with guiding. Initially I blamed the poor guiding on the fact that I was using 2×2 binning, but that was probably a faulty supposition. Regardless, I changed the setttings in PHD2 to use 1×1 binning, and PHD2 immediately warned me that there was no matching dark library for the camera for that binning. I pressed on anyway, with no better luck. After continuing to struggle, I finally decided to go out to the telescope and put the dust cover on (removing the dew shield in the process) so I could shoot some new darks for the guide camera. While I was out there, I adjusted the off-axis guider so that it stuck a little farther into the optical path in hopes that it would find more guide stars. That required refocusing the guide camera, which in retrospect I probably needed to do anyway. Finally, those adjustments cleaned up the guiding and all was well, or at least better. I went back inside (failing to reinstall the dew shield, of course).

At this point it was about 11 PM, and I was basically restarting the imaging run. For no good reason, I decided to revert to 1×1 binning in the main camera since I’d already done so for the guide camera. Also, because I had adjusted the off-axis guider, I noticed that it was now casting a shadow on my main camera sensor for each sub. Fine, I thought–I’ll just crop that out in post processing. At least I was getting images. Things were humming along was well as they could at that point, and I went to bed about 12:30 AM.

When I got up the next morning, I checked the results from the night’s imaging run and was discouraged to see that the clouds had moved in about 15 minutes after I went to bed. I ended up with only 80 minutes of data split across the four filters (I was shooting 5-minute subs in an L-L-R-G-B rotation). I was even more discouraged when I went outside to bring my telescope back into the house and discovered dew on the corrector plate. I was paying now for my failure to reinstall the dew shield the previous evening. The forecast for the evening predicted the temperature would stay well above the dew point, so I thought I was safe. So much for the forecast. Lesson learned.

The whole episode left me feeling a little discouraged and frustrated, and I almost didn’t bother processing the data. Finally, I set the scope up on the workbench, took some flats and bias frames, and then fed the whole works into PixInsight. With a bit of work, I was able to coax an acceptable image (see above) out of the data I had collected. More importantly, I was able to mentally review the imaging process I had just used and identify where I most likely went wrong.

First, I never should have switched out of 2×2 binning, which was an appropriate choice for that focal length. Second, I should have ensured in advance that the off-axis guider was positioned properly–far enough in so that it sees guide stars, but not so far that it casts a shadow on the sensor. Third, I should have taken the time to check that the off-axis guiding camera was focused properly. Fourth, I should have reinstalled the dew shield/heater before continuing the imaging run (both for the sake of dew protection and because the equipment had been balanced with the dew shield in place and would be out of balance without it). Fifth (and I didn’t think of this until later), I should have reoriented my guide camera so that the long dimension of its sensor would stick farther into the imaging train. I had previously oriented it in the worst possible way.

It seems like every time I switch to a new and different imaging configuration, I go through a painful process like this. Hopefully I’ve learned most of the important lessons and will have better luck next time.

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