|Space Telescope Science Institute|
|COS Data Handbook v.3|
To reduce the harmful effects of gain sag on detector throughput and flat fielding, the location of the COS FUV spectrum is periodically moved to a fresh (unsagged) part of the detector. The move to Lifetime Position 2 (LP2) occurred on July 23, 2012, and the move to Lifetime Position 3 (LP3) occurred on February 9, 2015. Since that date, all COS FUV GO observations have used LP3, except for the blue modes G130M/1055 and 1096, which remain at LP2. LP3 is displaced in the cross-dispersion direction by -2.523" relative to LP1, with no dispersion direction displacement. LP2 was located at +3.5” in the cross-dispersion direction and at -0.05” in the dispersion direction relative to LP1. It is anticipated that LP3 will be used as the primary FUV location for two to three years.The change in lifetime position from LP2 to LP3 has resulted in a slight decrease in spectral resolution (~5-10%), but will place FUV spectra outside the gain-sagged low-response areas that have been starting to compromise data quality. In addition to changes in the core of the line profile, 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 LP3.The flux and wavelength calibrations at LP3 are expected to be very similar to those at LP1 and LP2. Calibration observations are executed following lifetime moves and updates to calibration reference files, if needed, are made. These observations include measurements of the spatial and spectral 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 LP3 can be properly calibrated. These changes include new header keywords or modifications to existing ones. Appendix A:COS New Lifetime Position lists the keywords that are relevant to LP3, indicates their possible values, and provides a brief description on their meaning.
-11: this value, only used in reference files, indicates the file should be applied to all data with LIFE_ADJ = 1 and -1.