Previous
abstract 
Next
abstract
In the July 1994 PASP (Vol. 106, p. 798), we presented an optimized alogorithm that removes cosmic rays (CR's) from multi-orbit HST Wide Field/Planetary Camera (WF/PC-1) images. The algorithm was shown to work best at clipping levels of $\simeq 2.0-2.5\times\sigma_{Poisson}$ when 4--6 unshifted exposures are available, while for 6--12 exposures the optimal clipping level is $\simeq 1.8-2.0\sigma_{Poisson}$. For fewer than 4 exposures, the algorithm is unreliable. We now test this algorithm on a 12-orbit WFPC2 stack, again as a function of the number of available orbits and the formal Poissonian $\sigma$-clipping level for both the PC-1 and the WFC-2, 3, and 4 camera's.
\bigskip
We analyze the surface density of CR's detected in the WFPC2 images as a function of their apparent flux as following. First, we constructed a ``CR'' image made by subtracting the best CR-filtered image from the non-filtered 12-orbit image stack. We then use the ``FOCAS'' package to analyze the surface density of CR's as a function of the apparent V-magnitude (or DN-flux) they would have left in the images had they not been removed, and we compare this to the deepest available galaxy counts. For WF/PC-1, the power-law slope of the ``CR-counts'' ($\gamma\simeq$ 0.6 for $N(m_V)\propto m_V^{\gamma}$) is steeper than that of the galaxy counts down to the formal 6$\sigma$ CR-point source sensitivity of V$\simeq$28.5 mag. For WFPC2, the read-noise is $\sim 2.7\times$ lower, and the Loral chip is thicker. We thus find many more CR's per unit time in the WFPC2 images, especially at lower flux levels. The slope of the CR-counts remains the same for WF/PC and WFPC2, but the apparent magnitude limit of the CR-counts reaches much deeper, to V$\simeq$30.0 mag in a 12-orbit stack. We conclude that there will always be a non-negligible contribution to the sky-signal and noise-variance of the deepest WFPC2 images from faint CR's.
This work was supported by NASA/HST grants GO-2684-03-94A and GO-5308-01-94A from STScI, which is operated by AURA, Inc., under NASA contract NAS5-26555.
