M101 Pinwheel Galaxy in LRGB

I can’t believe it has been over 2 months since my last post. The weather here in the UK has been terrible; a combination of snow and heavy rain every day since January. I was worried that galaxy season was passing me by and so, when the weather finally broke last week, I was eager to try and capture something worthwhile. With that in mind, I decided to target M101 Pinwheel Galaxy as it is high in the sky but remains an elusive target for me due to being relatively dim (mag 7.8).

This would also give me a chance to finally use LRGB filters on my mono camera; something I have had a go at with little success in the past.

The weather last week was clear overnight three nights out of four so it made life a little easier. I set up on night 1, using drift align to get my polar alignment as close to perfect as possible. Now we are in British Summer Time, it isn’t dark until after 10pm and gets light again around 4am so, especially during the week when I need to get up at 5am for work, I didn’t want to be wasting time setting up each evening. I elected, instead, to leave my equipment in situe for the four days so that I could not only begin imaging as soon as it got dark, but that I could also quickly capture flats the following morning before putting the covers on.

As I mentioned earlier, I am not very experienced in LRGB imaging. I know the theory but have not had much practice. I wanted to use longer exposure times to go as deep as conditions would let me, not just in terms of overall integration, but also to try and capture fainter dust lanes. I ran a couple of test images on each filter to work out a similar exposure outcome in terms of ADU and ended up shooting:

Lum: 20×600″ bin 1×1
Red: 37×183″ bin 2×2
Green: 20×240″ bin 2×2
Blue: 23×440″ bin 2×2

My issues started from this point onwards! The subs all looked good quality and so I began calibrating in Pixinsight. That was when I realised my first schoolboy error: I had captured all calibration files in 1×1 binning which meant they were unusable to calibrate the RGB files. I had a dust bunny on the sensor (I really must get around to dealing with that) and so each sub had that on it too so I really needed flats to remove it. Then I had a brainwave – could I create master darks and flats, and then resample them to create synthetic 2×2 binned calibration files? Turns out you can! With those sorted, I tried calibrating the lights and then stacking them with limited success in Pixinsight. I have been having real issues recently with this element of the process using Pixinsight and so I bought a licence for Astro Pixel Processing a few weeks back.

For calibrating and stacking, I really like APP. It produces clean, well calibrated light stacks and has the added bonus of an effective light pollution removal tool in post processing that does, in my opinion, a much better job than ABE or DBE in Pininsight.

Once stacked, I moved back into Pixinsight for processing – it is still, for me, the most effective tool for processing light stacks into a final image.

I went through several iterations before settling on this as my final image. I am really pleased with the depth of the image in terms of its sharpness and the ability to bring out the fainter outer lanes in this distant galaxy. I would likt to reshoot this using a longer focal length to make the most use out of my small sensor and pixel size, but I also really like the wider field in this version.


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