Cosmic-Ray Rejection by Laplacian Edge Detection

Conventional algorithms for rejecting cosmic-rays in single CCD exposures rely on the contrast between cosmic-rays and their surroundings, and may produce erroneous results if the Point Spread Function (PSF) is smaller than the largest cosmic-rays. This paper describes a robust algorithm for cosmic-ray rejection, based on a variation of Laplacian edge detection. The algorithm identifies cosmic-rays of arbitrary shapes and sizes by the sharpness of their edges, and reliably discriminates between poorly sampled point sources and cosmic-rays. Examples of its performance are given for spectroscopic and imaging data, including HST WFPC2 images.Comment: Accepted for publication in the PASP (November 2001 issue). The algorithm is implemented in the program L.A.Cosmic, which can be obtained from http://www.astro.caltech.edu/~pgd/lacosmic