New COS/FUV TDSTAB Delivered
The sensitivity of the COS FUV detectors declines with time and is monitored with regular observations of spectrophotometric standard stars (the white dwarfs WD 0308-565 and GD71). A decrease in the slope of the Time Dependent Sensitivity (TDS) has been observed since July 2015. The slope of the Time Dependent Sensitivity (TDS) depends on wavelength, grating, and detector segment, and currently varies between ~0% per year and ~4% per year, with steeper declines at longer wavelengths and with the Segment B channel showing overall shallower slopes than Segment A (see here). A new COS/FUV TDS reference file, 23n1744nl_tds.fits, that takes into account the shallower TDS slopes was delivered on March 23 2018. This new reference file applies to all the COS FUV settings with the exception of G130M/1055 and 1096, for which the TDS reference file is left unchanged. The absolute and relative flux accuracy is within 5% and 2%, respectively, for all modes except for the G130M 1055 and 1096 CENWAVE settings, and for G140L data on Segment B. The absolute flux accuracy for the 1055/1096 settings is 20% while it is 40% for G140L/1280 FUVB (see the December 2013 STAN and the COS Data Handbook section 4.3.1)
Users concerned with the absolute or relative flux calibration of COS FUV spectra taken after July 2015 should re-retrieve their data from the HST Archive.