The Pinwheel Galaxy (M101)

This past week I finally received the brand-new ASI533MM Pro cooled monochrome camera from ZWO. After having used its color brother (the ASI533MC Pro) for over a year now, I was excited when ZWO announced the monochrome version a few months ago. Mine finally arrived, and all I needed was a decent night with clear skies to give it a try. Monochrome imaging is something I hadn’t attempted before, and I was eager to give it a shot.

Why monochrome? In my opinion, the best astrophotographs I’ve seen have been shot using monochrome cameras, often with narrowband filters. Imaging with a monochrome sensor and color filters yields a higher-resolution image that doesn’t require pixel-to-pixel interpolation like a color camera with a Bayer matrix. I’m not going to bore you with the details, but Sky & Telescope magazine published a good article describing the differences.

Ultimately, I want to image nebulae using H-alpha, O-III, and S-II filters so that I can produce images that use a color palette similar to that of the Hubble Space Telescope. However, good nebula targets are few and far between in the night sky during the warmer months here in Colorado, so I settled for a fairly prominent galaxy, The Pinwheel Galaxy (M101), as my first monochrome target.

Along with my camera, I acquired a set of LRGB (luminance, red, green, and blue) filters and a set of SHO (Sulfur II, Hydrogen alpha, and Oxygen III) filters (6-7 nm bandwidth) from Optolong, and a filter wheel from ZWO. These would give me a good start with monochrome imaging. For M101, I chose to image with LRGB since it’s a broadband target, but also captured H-alpha data to jazz it up a bit. One other recent addition is an off-axis guider (OAG) from ZWO in hopes of improving my guiding.

This time of the year, I’m lucky if I can grab 5 hours of data in an evening. In this case, I only got about half that thanks to clouds that rolled in during the night. I ended up getting about 40 subs, with 16 of them being luminance and the rest were R, G, B, and Ha. I wasn’t sure if that would be enough to produce a decent image, but working with PixInsight (and the Mastering PixInsight book by Rogelio Bernal Andreo) I was able to produce the image above, which I’m reasonably happy with for a first attempt at monochrome imaging.

It’s also worth mentioning that the OAG I mentioned above did indeed improve my guiding. For the first time ever, I was able to consistently guide with sub-0.5-arc-second accuracy. I was very pleased with that.

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