Optimal Cosmic-Ray Detection for Nondestructive Read Ramps

Abstract

Cosmic rays are a known problem in astronomy, causing both loss of data and\ndata inaccuracy. The problem becomes even more extreme when considering data\nfrom a high-radiation environment, such as in orbit around Earth or outside the\nEarth's magnetic field altogether, unprotected, as will be the case for the\nJames Webb Space Telescope (JWST). For JWST, all the instruments employ\nnondestructive readout schemes. The most common of these will be "up the ramp"\nsampling, where the detector is read out regularly during the ramp. We study\nthree methods to correct for cosmic rays in these ramps: a two-point difference\nmethod, a deviation from the fit method, and a y-intercept method. We apply\nthese methods to simulated nondestructive read ramps with single-sample groups\nand varying combinations of flux, number of samples, number of cosmic rays,\ncosmic-ray location in the exposure, and cosmic-ray strength. We show that the\ny-intercept method is the optimal detection method in the read-noise-dominated\nregime, while both the y-intercept method and the two-point difference method\nare best in the photon-noise-dominated regime, with the latter requiring fewer\ncomputations.\n