It is my hope that using stellar photometry during preprocessing will be copied widely, because when done correctly, it can produce extremely accurate results. This is vital in order to produce the optimum signal to noise ratio in the final stacked image. The accurate scale factor also makes it possible to accurately determine the correct weights to apply to the images when stacking them. Once this is accurately known, the relative gradient can be accurately removed. My innovation was to use stellar photometry to determine the brightness scale difference. Any errors here might not be immediately apparent, but will cause a contrast difference between the images. The problem is that it can be ambiguous how much of the correction should be applied as scale or offset. When correcting images to match a reference, the gradient must be removed by subtraction, and the brightness scale factor must be corrected with multiplication.
It is currently available as a PixInsight script. My aim has always been to create something that would help the amateur astronomy community. I am very happy for the code to be used within any image processing software, free or commercial, and I am willing to provide any help and advice that might be needed. This script is free, and it is also open source (GNU General Public License). This is a crucial preprocessing step in image processing that makes a significant difference to the quality of the final stacked image. NormalizeScaleGradient is designed to normalize the scale and gradient of registered images to that of a reference image.
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