The Pinwheel Galaxy in HaGB (no red filter)

May 14, 2020 - Reading time: 2 minutes

The Pinwheel Galaxy (M101) could be our galaxy's giant cousin--very distant cousin, since it's 21 million lightyears away (that's like 6 megaparsecss to you serious astro nerds). It's almost twice the diameter of the Milky Way galaxy, with a trillion stars. But what also makes the Pinwheel so wonderful is the way it's angled, so that we see it from the top (or face?). 

I shot this in three sets of images, first with hydrogen-alpha, then with broadband green and blue filters, so instead of a normal red filter, I'm using narrowband Ha, which only allows a fraction of the hydrogen bandpass through--on the red end of the spectrum. I know HaRGB is popular, using the stacked Ha frames for a luminosity layer, but I only have five slots in the filter wheel, and thought I'd give this a try. (My filter order: Ha, OIII, SII, Green, Blue). 

Here's M101 in HaGB mapped to RGB color, so that the galaxy's massive H II regions (pink and red masses swirling along the Pinwheel's spiral arms) really stand out. If I shot this in normal broadband red, these would just blend into the overall color of the galaxy. Astro Notes: William Optics SpaceCat 51 250mm fl Apo Refractor, ZWO ASI1600MM-Pro cooled monochrome camera, iOptron CEM25P EQ mount, Astronomik 6nm Ha filter, Baader Green and Blue filters.

I shot the 60 sub-exposures that make up this image (20 subs for each filter) with everything you see here: