The Target-

     Messier 101, or the Pinwheel Galaxy, is a face on, unbarred spiral galaxy located in the constellation Ursa Major. It is a very irregular galaxy with a morphological type of SAB(rs)cd.   It was discovered in 1781 and included in the Messier Catalog as one of the last entries in the same year. M101 was one of the first targets I shot with a canon dslr and a 500mm telephoto lens on a star tracker when I first started my astrophotography journey. The result was... not great. I knew then that I would need to wait and retry the target with a better setup. I have since moved to a 6" Celestron C6-N Newtonian that I've upgraded for imaging. With the gear upgrade, I knew I wanted to try galaxy imaging again. Unfortunately, the area I live in isn't great for weather. We usually have nice imaging weather around the full moon (go figure). So for this target I just started with Ha imaging and decided to "see where it went", weather depending. In total I got ~85hrs of data with ~50 of those hours being in Ha. The rest was split in the LRGB filters. I was also testing exposure length and gain settings during this target so i ended up with 120"/180" exposures in the LRGB filters and 300"/600" exposures in the Ha filter. 

The Processing-

    I utilized DrizzleIntegration set to a scale of 1 and a drop shrink of .35 for this data because I wanted to get the most detail I could while maintaining a natural result. I found if I didnt use it BlurX was giving a bit of mushy or "stringy" result.  I used BlurXterminator with Non-stellar sharpening set to .5 as that gave the most natural sharpening.  I then went on a deep dive with Color Calibration. My attitude toward color is that I want to represent the data with the least amount of subjectivity possible. Of course there will always be some subjectivity involved. Even in this result the Ha addition is completely subjective. The effort is to reduce it, that's all. Spiral Galaxies in general have younger hotter stars than say elliptical galaxies so we should expect spiral galaxies to be shifted more toward the blue side of spectrum overall.  Recently I came across some information that indicates we should be applying a Color Correction Matrix (CCM) to our astronomical images. There is one major caveat to implementing this method and that is that for it to "work" the data needs to be calibrated to a G2V Star (Our Sun) as a white reference. This is because the CCM is a color weighting scheme that is applied to the out-of-camera jpeg images in DSLR and Mirrorless cameras today. Personally, I like to test things though. I used a CCM published for the Sony a6400 camera online as this was the closest camera I could find to the IMX533 sensor. I used SPCC set to a G2V Star as the white reference and then applied the CCM. This resulted in an overly warm white balance akin to an elliptical galaxy. So I landed on an Sd galaxy (close the type of galaxy that M101 is) as a good representation of the younger star population expected from spiral galaxies. Below is the comparison I made between using the CCM and using only SPCC/Saturation Curve and its pretty telling: 



Immediately I noticed an overall green tint (especially in the spiral arms) to the image with the CCM applied (left). This makes sense when considering that these CCM's are implemented to adjust the color and saturation of daylight balanced terrestrial jpegs. The result is less than stellar in my opinion as this green is overwhelming and would likely be adjusted out later with either SCNR or CurvesT. The next thing I noticed was that I needed to saturate the non-CCM result (right) pretty heavily to get a comparable saturation between the two. So the CCM  is not only adjusting the colors towards terrestrial balance but also saturating them pretty heavily. Again, this makes complete sense when your aim is to produce nice jpeg images from a daylight DSLR/DSLM camera. 

I decided to go with the non-CCM method.  I stretched both the Luminance and Color files the same way and used LRGBCombination for the Luminance information integration. I then used @Charles Hagen 's new PhotometricContinuumSubtraction script to prepare the Ha for addition into the LRGB file. Augmenting the Ha features is not a "natural" look, but I like to highlight the star forming regions in galaxies. I added the Ha in with this method  from Charlie's tutorial. I used these color balancing weights to try and achieve a realistic color balance for the Ha in the addition: 

R = .5,
G = 0.09,
B = 0.15,

   Once the Ha was added it was final black point setting, saturation tweaks for the star field and slight tweak for the galaxy itself and cropping. 

This was a fun dataset to test some color theory on and overall I am very happy with the result. There needs to be much more color data though. I now only wish to have much more aperture to resolve individual stars in M101. Haha. 

Thanks for looking. I hope you like it!
Joe