REMOVING COSMIC-RAY HITS FROM MULTIORBIT HST(1) WIDE-FIELD CAMERA IMAGES

We present an optimized algorithm that removes cosmic rays (''CRs'') from multiorbit HST Wide Field/Planetary Camera (''WF/PC'') images. It computes the image noise in every iteration from the WF/PC CCD equation. This includes all known sources of random and systematic calibration errors. We test this algorithm on WF/PC stacks of 2-12 orbits as a function of the number of available orbits and the formal Poissonian sigma-clipping level. We find that die algorithm needs greater-than-or-equal-to 4 WF/PC exposures to locate the minimal sky signal (which is noticeably affected by CRs), with an optimal clipping level at 2-2.5 X sigma(Poisson). We analyze the CR flux detected on multiorbit ''CR stacks,'' which are constructed by subtracting the best CR filtered images from the unfiltered 8-12 orbit average. We use an automated object finder to determine the surface density of CRs as a function of the apparent magnitude (or ADU flux) they would have generated in the images had they not been removed. The power law slope of the CR ''counts'' [gamma congruent-to 0.6 for N(m) is-proportional-to m(gamma)] is steeper than that of the faint galaxy counts down to V congruent-to 28 mag. The CR counts show a drop off between 28 less than or similar to V less than or similar to 30 mag (the latter is our formal 2sigma point source sensitivity without spherical aberration). This prevents the CR sky integral from diverging, and is likely due to a real cutoff in the CR energy distribution below approximately 11 ADU per orbit. The integral CR surface density is less than or similar to 10(8) deg-2, and their sky signal is V congruent-to 25.5-27.0 mag arcsec-2, or 3%-13% of our NEP sky background (V=23.3 mag arcsec-2), and well above the EBL integral of the deepest galaxy counts (B(J) congruent-to 28.0 mag arcsec-2). We conclude that faint CRs will always contribute to the sky signal in die deepest WF/PC images. Since WFPC2 has approximately 2.7 X lower read noise and a thicker CCD, this will result in more CR detections than in WF/PC, potentially affecting approximately 10%-20% of the pixels in multiorbit WFPC2 data cubes.