Space Telescope Science Institute
COS Data Handbook 2.00
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COS Data Handbook > Appendix A: COS New Spectra Position > A.1 COS New Lifetime Position

A.1 COS New Lifetime Position
To overcome gain sag effects, the position of spectra on the COS FUV detector was moved on July 23, 2012 to a fresh part of the detector which has not yet experienced significant gain sag. All GO observations will now use this new lifetime position.  This new position is displaced in the cross-dispersion direction by 3.5" (no dispersion direction displacement), corresponding to ~41 pixels, and should, at least for the next two to three years, eliminate the deleterious effects of gain sag on detector throughput and flat fielding.
This change in lifetime position will result in a slight decrease in spectral resolution (~5-10%), but will avoid the low response areas that have resulted from the gain sag and which had been starting to compromise the quality of the data taken at the original position.  Small changes to the wings of the line spread function are predicted; new LSF models have been computed and will be made available to users once they are validated by calibration observations at the new lifetime position.
The flux and wavelength calibrations of the new lifetime position are expected to be very similar to those at the original position. Calibration observations will be executed during the Summer and Fall of 2012 and updates to calibration reference files, if needed, will be promptly made. These observations include measurements of the resolution, verification of the wavelength scales, verification of the FUV BOA operations, and flux and flat field calibration observations.
Changes have already been implemented in the calibration pipeline and associated calibration reference files so that data taken at the new lifetime position can be properly calibrated.These changes include new header keywords or modifications to existing ones.Table A.1 lists the keywords that are reelevant to the lifetime position, indicates the possible values and provides a brief description on the meaning of these keywords.
Table A.1: Selected Header Keywords relevant to the New Lifetime Position

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