How background matching works

When combining images into a mosaic, each input image may have an arbitrary additive offset in its background level, due for example to different observing conditions for different tiles. The reproject_and_coadd() function can correct for this if the match_background option is set.

The background matching works by finding all overlapping pairs of images and determining the median difference for each pair, then using a stochastic gradient descent method to find the optimal additive corrections (a positive or negative constant for each image) to minimize differences. We additionally place the constraint that the average correction should be zero, but since there’s no reason that the average correction should be exactly zero, you should be aware that the final mosaic may be offset from the absolute surface brightness/flux by a constant additive factor. The algorithm should be robust for many situations and does not currently have any exposed options for fine tuning.