Thank you Greg, all taken on board and will give it a go next time out.
I tried Ha and OIII and combined to form a synthetic green, but I ended up with a red Running Man!! Did you mean a blend of Ha & OIII as a luminance channel?
So, if I capture some luminance, do I need to reprocess or can I simply lay the two frames over each other and select blending mode of colour or luminosity, depending on which way I layer the image?
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Ha and O111 often are used in a bicolour formula which is Ha red O111 blue and (Ha + O111)/2 for green. LRGB or RGB stars then layered in in lighten mode to replace the colourless bicolour stars.
M42 is not the prettiest in O111.
You could add in luminance as a luminosity layer after doing work on it first. Or you may need to do an LRGB first and then add in the Ha. depends how the Ha was layered in before. Do you save a file in PSD format so it retains the various layers so you can change it easily later on? If not its a good practice and makes a repro easier to do.
Greg.