Adjusting Color Correction in Nebula

[sreilly]

Seems I've been a bit hard headed on my color correction in always trying to use the Color Correction tool on nebula. I'm working on an image now that the color correction tool just won't work and seems I had this issue back some time ago with M42. For some reason after several failed attempts using the CC tool I decided to use the Histogram Stretch tool and adjust each channel while keeping an eye on the top graph. By making several RGB adjustments to get a good scale on the top graph, I then adjusted each channel by moving the black point (left slide adjuster) starting with the red channel and then matching the peaks in the blue and green channels. By having the real time previewer open I could see the affect I was having on the whole image.

I'm not sure why this eluded me this long but it really is quite simple and effective. Maybe this helps someone or there is a simpler or more accurate way.

Thanks,

[papaf]

How do you use CC normally?
CC must be used on linear images. Also, it is a good practice to use it after backgroundneutralization, which automatically does what you did with the histogram tool.

[sreilly]

Normally I calibrate the image, align, stack, crop for clean edges (I dither), use DBE if necessary, background neutralization, and then color calibration. But getting a clear area of background when the entire image is nebula is almost impossible. Then I would do multiple previews, combine using preview aggragator and color calibrate. But this doesn't work for all images, at least not for me when the image is overwhelmed with nebulousity. Then I do as posted above.

-Thanks,

[Juan Conejero]

But getting a clear area of background when the entire image is nebula is almost impossible.

Actually, clear sky background areas almost don't exist if the image is deep enough. When we refer to free sky regions, we are actually referring to the local background of the image. For example, take a look at this image of NGC 7331. The image is so deep that there are no free sky regions; everything is filled with light from our own galaxy. That's why the whole image has a purple/bluish background. However, we managed to neutralize the background by defining a small preview on the bottom left quadrant, after a careful inspection of the image. The preview in question still has no free sky, but it is valid as a local neutralization reference.

Take a look at this example with an M42 image. Obviously, the background reference preview is not a free sky region---we all know that this region of the sky is completely filled with nebulosity. However, the image isn't very deep so the selected region can be used as an approximate local reference for neutralization with quite good results. With a deeper image the situation would be more complex, but still manageable if the data had been accurately calibrated (which includes gradient correction). Another important item in this example is the fact that the whole image can be used as a white reference when it has a rich representation of objects.