More color questions

[astrodoc71]

What is the relationship between color calibration and the color histogram alignment? In other words, let's say you do the color calibration process and you notice in your color histogram that one channel is displaced relative to the others. Does this mean the color calibration is wrong? Or is this something totally different? Does the color histogram have to be aligned for accurate color representation?

[Philip de Louraille]

I think this depends on many factor, the first one being... is this an RGB picture or a Ha-OII-SI one.
If the latter, you should not expect a color histogram alignment.
If the former, and the field is mostly stars, one could expect to have color balance. But if the picture is of a gas nebula... then not really.
Color calibration is the step I use to "normalize" the color channels according to exposure time (in case R, G, and B did not get equal time perhaps), and where I can eliminate a color tinge I know should not be there (is the starry background neutral?)
Personally, I use the HistogramTransformation (color channels) to maybe boost a color (without displacing its peak relative to the others) and the color saturation to intensify faint colors where appropriate (gas nebulae in a galaxy for example.)
But I can't say that this is always what happens if I am looking for a good, pleasing visually stimulating picture...

[astrodoc71]

Thanks for the reply. So what happens if after the color calibration step for an RGB image you look at the histogram and the channels don't line up? Should you adjust the histogram until they do or leave it alone? Or do you need to recalibrate the RGB again?

[Philip de Louraille]

I guess it depends on how good was your color calibration done.

http://www.harrysastroshed.com/pixuser/colcal.html

http://www.youtube.com/watch?v=P85H7Ra91_s&feature=youtu.be

http://pixinsight.com/forum/index.php?topic=3360.0

[Ignacio]

I understand that, when you pick an area of the image as unstructured "white reference", it should have on average and after color calibration,  aligned RGB channels. But this does not apply to the image as a whole. In fact, you should know this apriori in order to make the proper selection, similar to what happens with the background reference.

Ignacio