Colour and Cave Nebula

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Colour and Cave Nebula

Postby Rkonrad » Fri Aug 10, 2018 1:56 pm

Hello,

I know there have been a number of discussions regarding the true scientific colour of ha emission nebulas in this forum. In this example, data which Eric Cole has graciously shared though the astroimaging channel, I consistently get a fair amount of blue in the nebula. My work-flow consists of masking the stars to get the correct white balance, then applying it to the whole. Noticing a fair amount of green flecks in the Max RGB screen I reduced the green by about 10.

Does this look right? Most ha images published, including Eric's have way less blue. Thanks!

https://www.flickr.com/gp/rkonrad/D80JL8

ps Data consists of Luminance of ha (75%) and RGB (25%) - latter combined to create luminance then RGB loaded to appropriate channels
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Re: Colour and Cave Nebula

Postby admin » Sun Aug 12, 2018 2:48 am

Hi,

First off, I think you're aware of this but I'll state it for the record for anyone else reading this - a large part of color representation in astrophotography is subjective.

Color perception itself is entirely subjective, compounded by the fact we can't see these colors in AP with our own eyes. Nor is there any prescribed white point for astrophotography. Muddying the waters even more is when we start including narrowband data in the signal path, as you have here.

The best we can do to convince ourselves (and perhaps others), is to reason our way through the color balancing and calibration.

I can only offer my reasoned personal opinion on your rendition;

From what I know about the region's chemistry and physics, I find it a convincing presentation of the region. HII areas like these tend to show a mixture of red and blue (therefore yielding blue, red and - when they're mixed - pink/purple). The red is caused by Ha emission. The blue by fainter Hb emission and reflection of starlight of the short-lived, bright O and B-class stars that are born in these gas-rich regions; these largely power the emissions in the first place.

The star field has a reasonable representation of star temperatures, ranging from red->orange->yellow->white->blue. This is no wonder; you used them to calibration the white point against.

The diffraction spikes show a decent continuum of colors, however lacking some green - not a biggie in my opinion.

Though StarTools does its best trying to retain color ratios as-recorded, independent of brightness, introducing a Ha bias into the luminance channel may make things look "different" (but by no means "wrong"!) versus a straight RGB rendering.
StarTools attempts keep colors constant psychovisually, independent of brightness, but there are limits to the color gamut that sRGB (as used by most screens and web standards) can encode. I find the coloring consistent with other color-ratio-preserved StarTools renditions of other regions of similar composition, with similar processes going on.

That's really all I can offer. All I can say is that I'd be happy with that. :)
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Re: Colour and Cave Nebula

Postby Rkonrad » Tue Aug 21, 2018 5:25 am

Thanks Ivo

Sorry for the late reply and thanks for your illuminating explanation. In fact the example I posted was was a luminance mixture of 75% rgb and only 25 HA. I mixed it back to front. I re-did it to 75ha and 25.rgb and got this https://www.flickr.com/gp/rkonrad/uEJ11F
I like this better but this only shows my artistic tastes rather than anything scientific which you have explained very well. I may have over saturated this as it is viewed on an iPad via flicker (edited the saturation ).
For this image, I processed a pure rgb and then blended it with the luminance 75ha 25rgb (colour of foreground) with a few tweaks.

A new question: when viewing the colours via the code (where rgb is shown in colour module,) how much green should i be removing? As green is not normally shown in DSO, should I reduce the green till is is gone or should there still be a remnant?

Cheers. Richard
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Re: Colour and Cave Nebula

Postby admin » Wed Aug 22, 2018 1:22 am

Rkonrad wrote:Sorry for the late reply and thanks for your illuminating explanation. In fact the example I posted was was a luminance mixture of 75% rgb and only 25 HA. I mixed it back to front. I re-did it to 75ha and 25.rgb and got this https://www.flickr.com/gp/rkonrad/uEJ11F
I like this better but this only shows my artistic tastes rather than anything scientific which you have explained very well. I may have over saturated this as it is viewed on an iPad via flicker (edited the saturation ).
For this image, I processed a pure rgb and then blended it with the luminance 75ha 25rgb (colour of foreground) with a few tweaks.


Again, a lovely image! If you've gone through the trouble of acquiring Ha, you might as well accentuate it, like you're doing here. It can both be a "scientific" and artistic choice; show otherwise "hidden" Ha emissions better because that's what you want the viewer to focus on, or just because it looks more interesting. As long as the data backs up your interpretation (e.g. you haven't painted on a moon that isn't there :P), then that's great! Personally, for me, the more you can articulate what we're looking at and what the reasoning behind your particular interpretation is, the more I am interested in an image.

A new question: when viewing the colours via the code (where rgb is shown in colour module,) how much green should i be removing? As green is not normally shown in DSO, should I reduce the green till is is gone or should there still be a remnant?


Green is usually only dominant in O III emission regions. Green also tends to exists in diffraction patterns/spikes. Natural green noise can also be reasonably expected, so as long as the green pixels seem to occur in no particular pattern (e.g. seem to be randomly distributed), then you should be good and can leave these in. I use MaxRGB mode a fair bit - I tend to be on the lookout for structures being visible in the green channel where they shouldn't. When they are, I know my green channel has too much emphasis.
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Re: Colour and Cave Nebula

Postby Rkonrad » Wed Aug 22, 2018 7:12 pm

Great - thanks.

Natural green noise can also be reasonably expected, so as long as the green pixels seem to occur in no particular pattern (e.g. seem to be randomly distributed), then you should be good and can leave these in


So I'm then assuming this green / noise will be dealt with in the next denoise step? I've also noticed quite often that large bright stars have a green core (viewed on maxRGB) and exist nowhere else in the image. Assuming that's ok...

Richard
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Re: Colour and Cave Nebula

Postby admin » Fri Aug 24, 2018 1:41 am

Rkonrad wrote:So I'm then assuming this green / noise will be dealt with in the next denoise step? I've also noticed quite often that large bright stars have a green core (viewed on maxRGB) and exist nowhere else in the image. Assuming that's ok...
Richard


Indeed, "normal"/natural green chromatic noise will be dealt with effectively by the denoise module.

You're right; over-exposed areas, such as star cores, can be safely ignored in the MaxRGB mode. Since they are over-exposed, their pixels carry data that has an 'indeterminate' value.
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Re: Colour and Cave Nebula

Postby Rkonrad » Fri Aug 24, 2018 4:20 am

Thank you so much Ivo for your detailed explanations!

Richard
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