[RCC] Do you mind the cyan cast in these dark nebulae?

Topic starter

Sep 30, 2023

Hi there!

This summer I took a deeply exposed shot of the North America nebula, revisiting one of my first deep sky targets when I began with this hobby.
The image was taken with a QHY600 mono camera, stacked in Pixinsight and color calibrated using SPCC:

https://www.astrobin.com/z6esnx/0/

However, an experienced astro photographer noted that my dark nebulae had a cyan cast.
I did not alter the colors resulting from the SPCC process and I actually liked the result.
To be honest, I did not notice that cyan cast at all and I have no idea why SPCC created the colors that way.

When I experiment correcting the colors by setting the black point in a way the dark nebulae are equally balanced, the result is a rather flat image with low contrast.
It seems the slight cyan cast creates a nice contrast to the pink emission nebulae (complimentary colors).

So, do you mind the cyan cast or do you notice it at all?
After all, the darker the luminance the lesser the hue appears.

Curious about your opinion!
Chris

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Guiem Kimi

Sep 30, 2023

For me, it is a very beautiful image. Congratulations!

I tend to judge the colors of an image taking the stars into account, and I think that star colors in this image feel very natural.

Even so, have you tried playing with the color curves instead of moving the black point? Removing the stars and playing with range masks may help you to target that parts of the image.

CS,

Guiem.

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psparkman

Sep 30, 2023

Hi Christian. You have a very nice image capture here, and I certainly feel the same feelings about this target as it was very exciting for me a few short years ago to image it for the first time.

As for the color cast. As the creator of the image, it is entirely up to you what you want it to look like. It does have a significant color cast in the background. If you look at the RGB colors in the background, you can see that the Red has been reduced to about 1/3 the level of the Green and Blue. That might be what you wanted as you moved along processing the image to try and get more blue to show in the nebula area. But an accurate RGB color balanced and background neutralized view of this area will pretty much be magenta for all of the Ha and a little shift in the Oiii and Hb regions. My personal approach is to get the background neutral and remove any artificial gradients, but you are in control and should make the image the way you like it.

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Brian Puhl

IOTD Staff

Sep 30, 2023 · Edited: Sep 30, 2023

Using Histogram transformation, and also cycling through each channel, you'll see that you agressively stretched the red channel, but the green and blue remain very conservative. This created quite a bit of misalignment in the color channels, leaving you with what you've published. Adjusting the black point wasn't appealing because then the white points didn't line up either. I'd go back to the drawing board and try this again from the beginning. Just my 2 cents.

Keep in mind, if you perform color correction, then adjust the curves individually, or stretch them independently, then your color correction is no longer accurate.

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Joon Ren

Sep 30, 2023

I don’t mind the colour cast as it goes well with the nebula in this case. If you want to reduce the cast, try desaturating or colour curves adjusting the darker areas with a mask.

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Oscar

Sep 30, 2023 · Edited: Sep 30, 2023

You need to align red, green, and blue in the histogram; if you did that correctly, things like this color cast should never happen. Sometimes, you still might not get a good image; in that case, you need to go by eye.

Francois Theriault

Sep 30, 2023

Chrisitan,

Nice image. I have always thought at the NA nebula as a narrowband target - you proved me wrong !

Instead of being concerned about the cyan cast, why don't you emphasize it ? As Messierman suggested, align your color channels to make sure that the R G and B all contribute equally to the image.

I see you are using Photoshop. I have an older version, but there is a couple of functions that are my go-to:
On you combined RGB image
Image –> adjustments –> Brightness and contrast - imcrease the contrast.
Image –> adjustments –> Vibrance - you can increase both the vibrance and the saturation.
You can then play with the Image –> adjustments –> color balance and Image–> adjustments –> channel mixer to adjust and control each color separately.

Experiment with it and have fun !

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Christian Koll

Topic starter

Oct 2, 2023

Hello,

thank you very much for your comments and suggestions!

As for the impropper color balance in the background, I have no idea why SPCC did that.
SPCC is supposed to do a background neutralisation - why this did not work, I don't know.

Maybe I'll try SPCC again and select some different background area for background neutralisation.
To be honest, I'm not very fond of Pixinsight's SPCC - the results, especially for galaxy pictures, are just unuseable.

As for my NA-nebula image, I did not stretch the color channels individually, but as a combined RGB image (all color channels stretched the same).2
So it definitely has something to do with SPCC.

Maybe it's back to the old times, when you adjusted the histogram peaks on top of each other and adjusted black- and white points in levels….

Chris

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Brian Puhl

IOTD Staff

Oct 2, 2023

Christian Koll:
Hello,

thank you very much for your comments and suggestions!

As for the impropper color balance in the background, I have no idea why SPCC did that.
SPCC is supposed to do a background neutralisation - why this did not work, I don't know.

Maybe I'll try SPCC again and select some different background area for background neutralisation.
To be honest, I'm not very fond of Pixinsight's SPCC - the results, especially for galaxy pictures, are just unuseable.

As for my NA-nebula image, I did not stretch the color channels individually, but as a combined RGB image (all color channels stretched the same).2
So it definitely has something to do with SPCC.

Maybe it's back to the old times, when you adjusted the histogram peaks on top of each other and adjusted black- and white points in levels....

Chris

Dumb question here but you are using the correct settings right? SPCC has presets for your camera and specific filters. I've literally never had any issue with SPCC, so I find it strange.

Christian Koll

Topic starter

Oct 2, 2023

Brian,

yes, of course I am using the correct settings for my sensor and filters.

CS
Chris

Christian Koll

Topic starter

Oct 7, 2023

Hello,

I processed this image again from scratch - even with the new GraXpert AI background extraction tool SPCC still does not align the histogram peaks on top of each other (apparent in the Astrobin histogram).

Howerver, I left the histogram as it was and de-saturated the background using a range mask, as sugested by Joon.

https://www.astrobin.com/z6esnx/E/

Best regards!
Chris

Do you mind the cyan cast in the dark nabulae?