Cosmic Origins Spectrograph: Flat Fields And Signal-to-noise Characteristics

Details Cosmic Origins Spectrograph: Flat Fields And Signal-to-noise Characteristics Cosmic Origins Spectrograph: Flat Fields And Signal-to-noise Characteristics

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21546425s&link_type=abstract

American Astronomical Society, AAS Meeting #215, #464.25; Bulletin of the American Astronomical Society, Vol. 42, p.503

Astronomy and Astrophysics

Astronomy

Scientific paper

The Cosmic Origins Spectrograph (COS) employs different microchannel plate detectors for its two channels: a cross delay line (XDL) for the FUV, and a multi-anode microchannel array (MAMA) for the NUV. These detectors show non-uniformities due to the intrinsic `chicken wire’ and moiré patterns of the microchannel plates, dead spots, hot regions, and for the XDL, shadowing by QE grid wires. Signal-to-noise (S/N) improvements can be achieved by applying a high-quality flat field during data reduction. For the highest S/N, multiple exposures can be taken using the FP-POS technique, where spectra are stepped to different locations on the detector. During the COS Servicing Mission Observatory Verification (SMOV) program, observations of bright astronomical targets and an internal deuterium lamp were made in both channels to investigate methodologies to improve the S/N of on-orbit observations.
For the NUV channel, flat field exposures were obtained with the onboard lamp. Comparisons of the data with a flat field constructed from prelaunch data indicate that there have been no changes, so a high S/N flat has been built by combining ground and flight data. Analysis indicates that S/N = 100 per pixel is achievable using flat fielding alone.
For the FUV channel, which does not have a ground flat of such high quality, exposures were obtained of white dwarfs at various cross-dispersion locations on the detector. Comparisons of different reduction techniques for this data set will be presented. Until high-quality flat fields are implemented in standard pipeline processing, high S/N spectra are best achieved by the FP-POS technique, which has demonstrated S/N of > 50 per resolution element.

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