Hi Craig,
I am still relatively new to NB filter imaging, only got my filter-wheel working at end of February.
The M24 image is apparently a predominantly Hydrogen emission region, what I think I am seeing is that for more conventional nebula targets e.g. Orion Nebula, the normal practise seems to be to back off the Ha as it is a lot brighter than the other two SII and OIII channels to get a pleasing 3 colour RGB image, but this technique applied here would understate the Ha line.
In the M24 area the three filters are narrow-band filtering the normal range of "white" light plus any emission lines.
This image is the Ha 656nm as red with the SII 672nm (green) subtracted. These 7nm filters are very close together so it is reasonable to assume that the camera sensitivity and filter characteristics are similar enough to treat them as equal as far as "white" light is concerned, and therefore taking away the SII filter channel will leave the Ha. If there is some SII then that would be subtracted too, so the assumptions as to the Ha remain sufficiently valid since it is a lesser crime to underreport in this instance
The background mottling is from the much fewer SII frames (40) compared to the Ha (178) frames.
The stars down into magnitude 12 have been made blue to contrast with the red.
There seems to be a fair amount of Ha emission going on, above and beyond the background "white" light.
Perhaps someone more knowledgable would care to comment?
For other targets, a SII filter would be directly contributing to the final RGB image.
For fun, look at the human eye response, the Red and Green cones are shockingly close together compared to the blue:
E.g.
https://upload.wikimedia.org/wikipedia/commons/3/32/Opsin_Absorption_Spectra.png... then compare to a colour camera's coloured matrix filter which is much more evenly spaced.
Best,
Susan.
BTW
Thanks Vince
