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Is It Necessary To Remove Color
Bias Before Applying Color Ratios?

Color bias and gradient removal are linear processes using only addition or subtraction. Color balancing uses multiplication or division. It is generally accepted that color bias correction and gradient removal should be performed before color balancing. For this exercise, the gradients were removed from the individual red, green and blue images.

This test shows that removing the color bias before or after applying color ratios makes little or no difference in the final color. However, this is no excuse for not doing the processes in the proper order. It may make a difference.

Aperture photometry subtracts the surrounding background level from a star's brightness. So addition or subtraction pixel math has little or no effect on color balance calculations. When using several stars for determining color balance, gradient removal may have a very small effect. The eXcalibrator results, in this example, were essentially identical before and after color bias removal.

The eXcalibrator RGB color balance factors are 1.00, 1.26 & 1.63    


Software Used:

PixInsight (PI) for gradient removal.

eXcalibrator for color balance calculations.
CCDStack for pixel math and RGB image creation.




The first pair of images show the initial RGB color combines. Pixel math was used to remove the color bias. The blue data were increased by 484 and the green reduced by 764. The pixel math was applied to unstretched images. The results were equally stretched to make the difference more visible.


RGB Ratios 1,1,1 Before Color Bias Removal


RGB Ratios 1,1,1 After Color Bias Removal




The second two images are with the eXcalibrator color ratios applied to the above images.  These two images are not streched. It is not surprising that a strong color bias is introduced.


RGB Ratios 1.00, 1.26, 1.63 applied w/o Color Bias Removal


RGB Ratios 1.00, 1.26, 1.63 Applied After Color Bias Removal




The final image pair is the above two images with the color bias removed using pixel math. Non-linear stretching and color saturation were applied equally. These images are the answer to the original question.

The images are essentially identical.


Color Balance Set Before Removing Color Bias


Color Balance Set After Removing Color Bias