M 101 with H-alpha blending - 2nd try
My processing skills (and tools) have improved in the last 3 months and it's cloudy outside so I reprocessed a stack of M101 subs from the spring that I posted here.
S50 in EQ mode, 17 hours of IRCUT 20s subs and another 5 hours with LP filter, stacked separately. I extracted the red channel from the LP stack as the H-alpha signal. Images were processed separately in Siril with RC Astro and Veralux tools and then combined manually with pixelmath using 40 percent Ha on the mid tones to avoid changing the core and background. No messing around with GIMP this time, it was all done in Siril.
The idea of the Ha blend is to make the areas of star formation pop out. The second image is a crop from my original post for comparison. It is the same underlying data.
After registering the 2 stacks, splitting the channels (r,g,b from IRCUT and Ha from LP) and a linear match on Ha channel, I used the same pixelmath as before:
red: min(r * (1-k) + iif(r > tl, iif(r < th, ha * as, r), r) * k, 1)
green: min(g * gs, 1)
blue: min(b * bs, 1)
the parameters are:
k: how much ha to blend into red 0-1. E.g. 0.4 is 40 % Ha and 60 % RGB
as: alpha scaling multiplier to enhance the ha signal e.g. 1.2= 120%
tl/th are the low and high thresholds (0-1) for the blending. This cuts off blending below and above a certain level to avoid noise in the ha image getting into the background and discolouration of the bright core.
bs/gs are just scaling for the blue, green to adjust the colours.
With these parameters it is easy to play around with the blending to get the best result.
For the image above I used k = 0.4 , as = 1.2 , tl = 0.15 , th = 0.71, bs = 1.03 , gs = 1.03
I know there are continuum subtraction scripts around, but I enjoyed doing it this way with maximum control.