Other
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.6910f&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #69.10; Bulletin of the American Astronomical Society, Vol. 24, p.1231
Other
2
Scientific paper
We present the careful removal of bad pixels and cosmic-ray (``CR'') events from HST/WFC images. The WFC bad-pixel map is known to vary with time, because the intense on-orbit CR-flux (see below) increases the dark-current of pixels that have been hit by CR's many times. This is not always properly reflected in the standard WFC dark-frames. We have thus begun a program to monitor and update ``new'' WFC bad pixels over time. Similarly, we present the construction of a library of PSF-stars from available Cycle 1 images across the WFC field-of-view in various filters. This will allow us to monitor the PSF's time- and focus-dependence, and to properly deconvolve objects seen in sub-sections of WFC images with no available nearby PSF-star (see our other HST AAS-posters this Volume). To assess the on-orbit cosmic-ray flux quantitatively, we constructed CR-clipped images from our 8---12 orbit WFC data-cube through an iterative local 2sigma -clip that uses and preserves the local S/N per pixel, and subtracted these from the straight image average. The resulting ``CR-images'' were processed with FOCAS using various detection thresholds. Differential CR ``magnitude-counts'' follow from their summed DN/unit time (as if all CR's had been misclassified as objects). The CR-``counts'' have a power-law slope gamma =~ 0.60 [N(m)~ m(gamma ) ]. At Gunn g=25 mag, the CR-``counts'' are ~ 1.0 dex higher in amplitude than the galaxy counts. They continue with a nearly ``Euclidean'' slope down to g=27.5 mag (our 1-sigma point source sensitivity), below which they turnover due to ``incompleteness'' and FOCAS splitting problems. Their formal sky-integral thus ``diverges'' into the noise, and their integral surface density approaches ~ 10(8) \ deg(-2) down to g=28 mag, which corresponds to 1 out of every 12 pixels in the WFC data-cube, or an integrated sky-signal of at least V =~ 25.0--25.75 mag/arcsec(-2) . This work was supported by NASA/HST grants GO-2405-01-87A from STScI, which is operated by AURA, Inc., under NASA contract NAS5-26555, and by the Alfred P. Sloan Foundation.
Duchene N. S.
Franklin Barbara E.
Gordon Jeffrey M.
Neuschaefer Lyman W.
Schroeder L. L.
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