It could also be an artifact of the image reconstruction technique. It may be able to suppress high-spatial-frequency noise (salt-and-pepper) but not low-spati

It could also be an artifact of the image reconstruction technique. It may be able to suppress high-spatial-frequency noise (salt-and-pepper) but not low-spatial-frequency noise (for example). Or, the algorithm may introduce artifacts due to modeling errors (say, imperfectly suppressing scattered light from the main star). Like the structures that you see in sonograms or some CT images.