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Hubble Space Telescope
Vignetting Removed from COS NUV Flat Field

During SMOV the COS NUV detector showed vignetting in observations of external targets, characterized by a 20% depression that increases linearly along the first ~ 150 pixels at the shortest wavelengths of each stripe for any exposure. Ray-trace modeling found the observed vignetting to be consistent with the optical beam partially missing the NCM3 mirrors. The vignetting was not observed in the flat field as the path of the D2 lamp is sufficiently different from the path of light from an external target. As the vignetting appeared to be stable, a correction was added into the COS NUV Flat field. Analysis of high signal-to-noise data obtained during Cycles 17 and 18 revealed that the location and depth of the NUV vignetting varies with time and stripe. This variation is strongly correlated with the spectrum's location on the detector in the dispersion direction and is thought to be due to the OSM non-repeatability. While an effort is underway to provide an accurate correction for the NUV vignetting in CalCOS, the static vignetting correction has been removed from the flat field. A new NUV flat field, without the vignetting correction, was delivered to CDBS on February 14, 2011.