I was recently looking for something similar. Finally I used PixelMath, based on what has been posted as Vicent's technic. But since I had different sub-exposures and binning for Ha and R I made my assumptions how the maths should be expanded to include this. Not sure if the assumptions are correct, but at least I'm getting pretty good results. I finished with the attached PixelMath, applied on either Ha or R - does not really matter which one, it will create a new HaR image anyway, which you use as a new R to produce a (HaR)GB image with LRGB tool. Not sure if the color weights have any meaning here, I used 1:1:1 and continued from there. Not something fancy, just a quick way to do the job with some try-and-error for the parameters. I also tried something similar with O3 + B and O3+G and seems that the trick works, but I had very bad O3 data and could not really test it. I was actually thinking to make a script with some sliders to allow playing with the parameters, but -as usual- time is not something easily found
And I'm pretty sure the blending tool will be a lot more superior than this.
Ha and R should be linear and aligned together
The math is combined to make it easier
har = (r * r_mult) + ( ha_mult * (ha - (bandw_Ha / bandw_R) * ( r - (tR / tHa) * ha ) ))
where
r the R image
ha the Ha image
bandw_Ha : Ha's bandwith (7 for Baader)
bandw_R: R's bandwith (100 for Baader)
tR: total time of R , calculated as subs*sub_time*bin
tHa: total time for Ha, same calculation
r_mult and ha_mult the mixing factors
Just my 2 cents
