Cosmic Rays In Multi-Orbit Images With The HST Wide Field Planetary Camera 2

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Session 9 -- HST Observing and Instruments
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

[9.09] Cosmic Rays In Multi-Orbit Images With The HST Wide Field Planetary Camera 2

Barbara E. Franklin and Rogier A. Windhorst (Arizona State University)

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.


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.

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