COS FUV Flat Fields and Signal-to-Noise Characteristics

Details COS FUV Flat Fields and Signal-to-Noise Characteristics COS FUV Flat Fields and Signal-to-Noise Characteristics

Jul 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010hstc.worke...4a&link_type=abstract

"2010 Space Telescope Science Institute Calibration Workshop - Hubble after SM4. Preparing JWST, held 21-23 July 2010 at Space T

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The COS FUV channel employs a detector comprised of two microchannel plate (MCP) segments with cross delay line anodes. The detector shows several types of non-uniformities due to the hexagonal and moire patterns in the MCPs, dead spots, gain variations, and shadows from the wire grid installed in front of the MCPs to increase quantum efficiency. These features induce fixed-pattern noise in FUV spectra. The effects of these artifacts can be reduced by dividing the data by a flat field and combining exposures taken at different grating settings. A spectral iterative technique, similar to that used for GHRS and FOS, shows that S/N > 100 can be achieved in extracted spectra. Although flat field observations were obtained during SMOV using white dwarfs, a two dimensional flat field of sufficient quality for standard CALCOS processing was not achieved. Other methodologies are being explored for flat field correction and are expected to be installed in CALCOS to improve the S/N of data incrementally. As an initial step, CALCOS currently ignores grid wire regions when creating a summed spectrum from exposures taken at different FP-POS positions. Average one-dimensional flats generated through spectral iteration have been investigated to correct individual exposures and show promise as an alternate flat fielding methodology. These may require separate flat fields for different cross-dispersion locations. An important result is that the flat fields and flux calibrations used by CALCOS are dependent on each other and should be derived together.

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