SHO pallete ... advice needed - Forums

Topic starter

Mar 23, 2022

I'm wondering how people combine their SII, Ha and OIII images to get a natural look. I have just taken my first SII narrowband images of the crab nebula and did a straight SHO channel combination and ended up with:

The colours to me look artificial. What formulas do people use in Pixelmath to get a more natural combination?

Thanks in advance.

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Tim Hutchison

Mar 24, 2022

Andy.

I think this is pretty nice.

What I try to do is put s, h, and o in r, g, and b. Then balance the histogram between the three channels. In my opinion, don't make the mistake of using scnr to remove all of the green. There should be some green in a Hubble palette image… Same for magenta. Also, I typically remove all of the stars, process the tonemap, then add the stars back. They should basically be white.

Just my opinion… The cool thing about narrowband is having some freedom to play with color.

Tim

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Sean van Drogen

Mar 24, 2022

Depending on what you are using for processing, but after straight SHO combine most people apply a SCNR on green to get the more gold blue outcomes

There are several pixelmath blend formulas

Think this is the Forax one I use the most
R=(OIII^~OIII)*SII+~(OIII^~OIII)*HAG=((OIII*HA)^~(OIII*HA))*HA+~((OIII*HA)^~(OIII*HA))*OIII
B=OIII
CS,
Sean

andrea tasselli

Frequent contributor

Mar 24, 2022

Andy Wray:
The colours to me look artificial. What formulas do people use in Pixelmath to get a more natural combination?

The colours will always look artificial, that's SHO palette for you. The very rare times I do SHO I use hue curves to get the right tonality.

AstroDarkSky

Mar 24, 2022

Like everything in this hobby, there are multiple ways to do things and I can't say this is the "right" way to work with SHO. But I like the results at least. Since you mentioned PixelMath, that gave it away that you use PixInsight.

With Pixinsight, SCNR can be a powerful tool, but the main idea is not to use it at 100%.

Try between .5 and .25. Then use Curves to adjust the b, c, (maybe H) channels to your liking. Possibly run multiple iterations of partial amount SCNR. Multiple iterations of smaller amount settings of SCNR really help narrow it down versus a one shot reduce all-green in one click approach. I try to go for golds and teals vs. green and neon blue but my color vision isn't like everyone else's so I'm kind of guessing at that point.

With multiple runs of SCNR, you will take care of the green, but probably start losing saturation in other colors and in that case, you can use masks for Oiii and/or Sii channels to boost those specific colors back up again. The Sii/Oiii masks will need to be HistogramTransform(ed) to boost the bright areas and blacken out the background on the mask. Apply a small amount of convolution to that mask. Then apply the mask and boost the colors you want to bring up in value for that particular channel.

I try this approach for all SHO now and I intend to go back and redo past images since I prefer the results using this approach. Most people can probably adjust the curves completely by eye on the G channel, but I can't do that since I'm red/green color blind and I find this way works for applying changes equally across the palette without throwing other colors too far into an "unnatural" level.

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Kevin Morefield

IOTD Staff

Mar 24, 2022

Tim Hutchison:
Andy.

I think this is pretty nice.

What I try to do is put s, h, and o in r, g, and b. Then balance the histogram between the three channels. In my opinion, don't make the mistake of using scnr to remove all of the green. There should be some green in a Hubble palette image... Same for magenta. Also, I typically remove all of the stars, process the tonemap, then add the stars back. They should basically be white.

Just my opinion... The cool thing about narrowband is having some freedom to play with color.

Tim

I second what Tim has said about not removing the green with SCNR. Better to balance the colors than remove data. Not every image will respond to the same process to so try lots of things till you get what you want. After rebalancing the colors I often do a pseudoluminance with just Ha and OIII to keep SNR high. Usually the SII structures are all contained in the Ha.

Kevin

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Tim Hutchison

Mar 25, 2022

Kevin Morefield:
After rebalancing the colors I often do a pseudoluminance with just Ha and OIII to keep SNR high.

That is what I do as well. And that's how I get the stars back in the image after removing them to process the tone map.

JDJ

Mar 25, 2022

I use the following approach in PixInsight:

Starting with individually processed Ha, OIII, and SII images generated the SHO and HOO pallette images using the following steps in PI:
(1) Used the SHO-AIP script to generate the image: Ha for luminance, R=SII, G=Ha, B=OIII)
(2) Reduced the magenta tone of the stars using the following steps: Inverted image, applied SCRN process (set to Green), inverted image again.
(3) Tweaked the color palette from green dominated to more orange dominated by first using Curves Transformation in Hue mode followed by SCNR to remove green.
(4) Color saturation boost

Recently noticed that there is a "reduce pink halos" process in SHO-AIP that does the job of step 2 nicely.

Here is a recent example for the California Nebula: https://astrob.in/mqt5rd/0/

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