Color Callibration producing undesired effects in bright yellowish stars

[xatamec]

I have a problem with this tool when callibrating the colors of my DSLR images. Usually, in bright yellowish stars the core of the star is saturated on the red channel, but not on the blue and green.
The red channel is always the most prominent in my images, usually its median is more than twice the median of the blue and green channels.
As far as I understand it, Color callibration has been designed to apply coefficients between 0 and 1. Usually, after computing callibration (always after a previous background neutralisation) the blue channel is applied a coefficient 1, green channel near 1 and red channel a much lower coefficient, to match somehow the weight of the channels.
The outcome of this is the cores of the yellowish stars have a red channel between 0.2 and 0.4 while the blue and green channels weight much more. So, the halo is still yellowish but the core has become bluish-greenish. This is very unpleasant and hard to fix in post processing. So I'm not using this module, instead, I'm applying coefficients bigger than 1 to the green and blue channels, usually values near to 2.2 and 3 respectively, and leave the red channel as it is. Of course, this produces the saturation of the brighest features in this channels, in particular, the core of the stars, but I think it is more affordable to have all brightest stars with white cores, than cyan cores in yellowish stars.
This problem is not occurring with bright bluish stars, because in this stars ususlly the cores have similar weitghts in the callibrated images.

I don't know if more users have noticed this problem. Maybe it is not so important in images from higher dynamic range CCD cameras, but definitely it is a big problem in 12-bit DSLRs at least.

So, my suggestion is that this tool had 2 working modes, one mode applying coefficients as now between 0 and 1, and another mode applying coefficients bigger than one to the weaker channels.

Cheers,
Sergi

[vicent_peris]

Hi,

this is very easy to fix. Multiply (without rescaling) the image by 1/R_channel_scaling_factor. You will saturate some parts of the image, but they are unusable because one or more color channel is already saturated.


Regards,
V

[Simon Hicks]

Sergi,

I have exactly the same problem. I have a modded DSLR and the red channel often saturates the cores of stars well before the blue and green.

The only way I have found to cure this is to take two sets of images, long exposure and short exposure. The long exposure images get all the deep nebulosity, but the stars are saturated, especially in the red.

The short exposures are short enough so that none of the stars are saturated, even in the red.

You then need to use HDRComposition (or a manual version of the same thing) to create a high dynamic range image out of the two sets. In this way even your brightest stars will not be overexposed in the red, and will have the correct colour balance even in the cores.

Good luck.

Cheers
         Simon