One way to find the remaining cosmic rays and other defects is to compare the
four images taken with the four different filters. Another way is to compare
the pictures with the image profile of a point source such as a star. If we see
a sharp bit of light that doesn't have the right profile to be a star, it's
probably a cosmic ray residual or other defect that we need to flag and remove.
This image shows the remaining undesirable bits flagged in white against the
desired image, shown for the moment in red to ease visibility.
Now let's delete the remaining "flurries" and see what we've got.