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Old 23-09-2020, 06:46 AM
Placidus (Mike and Trish)
Narrowing the band

Placidus is online now
Join Date: Mar 2011
Location: Euchareena, NSW
Posts: 3,573
Being in Queensland helps with this one. Depth and sharpness are good.

I'm understanding you've mapped Ha to red, SII to green, and some mixture of Ha and OIII to blue.

The point of narrowband imaging is to understand something about the astrophysics of what is happening up there. Very briefly:

H-alpha tells us where the bulk of stuff is, because it is easily ionized by UV light from hot young stars.

OIII tells us where the harder UV light is. (More particularly in tenuous regions, because in dense regions the excited state is destroyed by thermal collisions before there is time for the forbidden transition to occur)

SII tells us where the deep star-guts, heavily processed material, from supernova explosions, WR stars, and white dwarfs is to be found. The same requirements for hard UV and tenuous material apply.

Mixing any two channels together for aesthetic reasons completely destroys this information, and one would do better to have just taken a broadband image.

(Mike now seriously gets onto his hobby-horse)

There might be some argument for mapping Ha to red, because that is its "natural" colour, and some argument for mapping OIII to blue-green because again that is its "natural" colour, and perhaps even some argument for mapping some of the Ha to blue because where there is Ha there is H-beta, but again this is misguided, because you are always losing information if you do this. There is no conceivable argument for mapping SII to green. It is mixing metaphors, like "The long arm of the law finally put its foot down".

There is some reasonable argument for taking a wideband (straight RGB) image, and then using H-alpha as a luminance channel, to find where the bulk nebulous stuff is, and reduce the impact of background Milky Way stars.

In summary, if you're going to do three channels of NB, the only sensible motivation should be to convey astrophysical information, and the best way to do that is to use NASA's standard Hubble mapping of SHO.

Since you've taken exceptionally fine data, I'd strongly recommend that you reprocess it using the SHO mapping.

Because H-alpha is usually exceedingly abundant, OIII less so, and SII is very scarce, the usual approach is to increase the OIII a bit, and the SII brightness a lot, relative to H-alpha, so that you are not swamped by the H-alpha. (Note: the purpose is NOT to stop the image being green, because 'there ain't no green in space'. That is a misinformed and ignorant myth. In narrowband imaging, the colours are arbitrary, like the colours on a map of the world, or the colours on a roadmap. The M2 isn't really blue. The purpose is just to see where stuff is relative to other stuff, in order to understand the astrophysics).

Stretching the SII a lot and the OIII a bit relative to Ha so that you can see them means that your stars will end up with magenta haloes. If you're just beginning, magenta haloes are fine. NASA doesn't mind magenta haloes. If you are advanced, there are complex techniques such as separating the image into stars and starless, mapping the stars to white and recombining, or dropping in RGB stars, but these "aesthetic" tricks are to a degree missing the point).

(Mike dismounts from hobby-horse).

So, safely back on the ground, it is a lovely crisp deep image of something that is difficult to capture from down south.

Well done.
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