M81 imaged using my C6 OTA (with 0.63x focal reducer) on my iOptron GEM45 mount. About six hours of three-min subs using my ASI533MC Pro camera with a UV/IR filter, plus calibration frames. Processed using Affinity Photo, AstroFlat Pro, and Denoise AI.

I’ve been thoroughly enjoying my time taking astrophotographs using my Sky-Watcher Esprit 100 f/5.5 apo refractor, ZWO ASI533MC Pro camera, and iOptron GEM45 mount over the past several months. The Sky-Watcher has been a joy to use, yielding sharp views in a field of view that works well for nebula and other larger targets. But with the onset of spring, most of the best targets in the night sky are galaxies–spectacular, but smaller. So I decided it was time to give my Celestron NexStar 6SE a try. Not the mount, mind you–it’s an alt-az mount with a lot of backlash in the motor drives–just the optical tube assembly (OTA). The C6 is a 150-mm f/10 with decent optics. A 1500-mm focal length seemed like quite a challenge for imaging, though, so I added the Celestron 0.63x focal reducer to the mix.

I didn’t really know how well this was going to work. I was pretty sure that the C6 could do a decent job imaging, but the 0.63x reducer was an unknown in terms of optical quality as well as the required back focus (the optimal distance from the back of the reducer to the camera sensor). I spent quite a bit of time searching for information about the best back focus for the reducer and came up with a modest consensus that 105 mm was a good number.

Another complication in using the C6 OTA was mounting it on the GEM45 mount. The OTA from the NexStar 6SE has a Vixen-style dovetail screwed to the side of the tube, and while the GEM45 mount will accept a Vixen dovetail, that particular configuration made it difficult to get the tube in the right position on the saddle for balancing purposes. Ultimately, I decided to replace that dovetail with a William Optics 245 mm long dovetail bar and a pair of tube rings. Now I can adjust the position of the tube in the tube rings, and I can adjust the position of the tube rings on the bar in order to make balancing easier (with or without the camera on the back end). It also made it easier to set the tube ring spacing so I could mount the William Optics 120 mm Handle Bar to the top, to hold my guide scope.

The C6 optical tube assembly after being fitted with a separate dovetail bar and tube rings. Note the William Optics Handle Bar on the top, holding the guide scope.
The C6 OTA mounted on the GEM45 mount. Not configured for imaging yet–I was working on focus and collimation in preparation for imaging.

The final piece to the puzzle was to add the Celestron focus motor. Stepper-motor-driven focusers make focusing for imaging so much easier, because most imaging software (I use N.I.N.A) can control the focuser and find optimal focus for you, greatly simplifying the imaging process and improving overall image focus. I personally have found the ability to autofocus to be a godsend.

Given all that, I was able to collect the data for the image of M81 you see at the top of this post. For this image I collected about 12 hours worth of 3-minute subs and then kept the top 50% of them (according to Deep Sky Stacker, anyway). The image was taken through a UV/IR filter. The final image was stacked using dark, flat, dark flat, and bias calibration frames, and then processed using Affinity Photo. I also used AstroFlat Pro and Denoise AI as part of the processing. I also used Affinity Photo macros from James Ritson, which greatly streamlined processing.

I’m hoping to add some more data to this image yet–perhaps some narrowband stuff shot through my L-Extreme filter. It appears, too, that the C6 OTA could stand a check of collimation, so I’m going to be exploring that as well. My attempt at collimating the C6 under the stars was a miserable failure, so I’m purchasing a collimation eyepiece and trying some techniques for collimating in daylight on the test bench. Wish me luck.

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