Thanks David for the info
I have decided to try another target for four narrowband channels and attached is a screenshot of 4 single subs of M20 in Ha, NII, OIII and SII.
SNR in those subs is in the ratio of about 4:2:1:1 (Ha:NII:OIII:SII).
I nearly have enough Ha for this image, and pondering on which two I should collect to combine with Ha. Given enough clear skies, I may try collecting all four.
One advantage of getting NII is a possibility of integrating it with Ha for a synthetic Lum and increasing SNR in the LUM, and NII also nicely shows fainter nebulosities around M20, while SII has a weak signal and OIII does not show any gaseous structures surrounding M20, so I would probably not try adding them to the synthetic LUM.
Any advices and opinions will be highly appreciated.
Suavi