Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21546206g&link_type=abstract
American Astronomical Society, AAS Meeting #215, #462.06; Bulletin of the American Astronomical Society, Vol. 42, p.493
Astronomy and Astrophysics
Astronomy
Scientific paper
All images taken with the restored ACS Wide Field Camera (WFC) exhibit a modest additional noise component in the bias level that is constant along rows of the two WFC CCDs (spanning both amplifier quadrants on each CCD). We believe this to be caused by low-frequency (1 mHz to 1 Hz) 1/f-noise on the reference voltage from the new Application-Specific Integrated Circuit (ASIC) that governs the offset of the pixel signal subsequent to its correlated double-sampling. The striping amplitude is roughly Gaussian with FWHM = 2e-, as compared with the underlying WFC read noise of 4.2e- (gain=2). The OPUS pipeline superbias and superdark images, averaged from dozens of individual biases and darks, show very little residual striping and thus do not exacerbate the striping in the pipeline-processed science images.
The impact to most science programs will be insignificant; however we recognize that the highly correlated nature of this striping may complicate analyses at very low surface brightness, particularly for images where the background counts are also low. In addition to our detailed characterizations of the striping, we present results from our testing of stripe-removal codes to correct a broad variety of science images. We have determined that the WFC overscan is a poor estimator of the stripe amplitude, requiring the stripes to be fit across the imaging area where care is needed to avoid removing source flux as well. STScI is making these algorithms available to the community as stand-alone software to be run in conjunction with by-hand CALACS processing.
Grogin Norman Andrew
Hook Richard N.
Lim Paul
Loose Marcus
Maybhate Anil
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