Background Extraction

Screen

The sky background often has an unwanted gradient caused by light pollution, the moon, or simply the orientation of the camera relative to the ground. This function samples the background at many places of the image and looks for a trend in the variations and removes it following a smoothed function to avoid removing nebulae with it.

Tip

Samples can be automatically placed by providing a density (‘samples per line') and clicking on ‘Generate'. If areas of the image are brighter than the median by some factor ‘tolerance' times sigma, they no sample will be placed there. After generation, samples can also be added manually (left click) or removed manually (right click).

Note

Once the samples are properly placed, click on Apply to generate a background equivalent image with a polynomial representation of the configured degree (‘Degree order' at the top) that will be automatically subtracted from the original image.

The background gradient of pre-processed image can be complex because the gradient may have rotated with the acquisition session. It can be difficult to completely remove it, because it's difficult to represent it with a polynomial function. If this is the case, you may consider removing the gradient in the subexposures: in a single image, the background gradient is much simpler and generally follows a simple linear (degree 1) function.