The Detection and Removal of Large-scale Detector Background Structures in NICMOS Observations

After applying the standard corrections for well-studied NICMOS anomalies, significant large-scale spatial background variation remains. We report on the detections of a sky-dependent fringe pattern in the F110W filter, and a time-dependent residual corner glow in the calibrated NICMOS deep science images. We also describe methods developed to further correct these anomalies. A model of the background structure is derived from the algebraic manipulation of stacked science images and consists of the following two components. The first component is constant, dominated by a residual glow as high as 20 DN at the corners and by residual flat and persistence structures at the center. The second component, which scales with sky level, displays a clear fringe pattern with 10% variation for F110W images. However this pattern is not detected for F160W images. Using these model components to correct for the anomalies significantly improves the cosmetic appearance of NICMOS images and reduces the magnitude scatter in the photometry of distant galaxies by 20%.