October 2019 STAN
Planned Updates to the COS NUV Flux Calibration
COS gratings lose sensitivity over time as shown on this monitoring webpage. This time-dependent sensitivity (TDS) is characterized in two TDSTAB reference files read by CalCOS, one for the FUV detector and one for the NUV detector. After the initial conversion of counts to fluxes, the fluxes are further modified by a factor that increases with time and depends on wavelength and grating.
The NUV gratings have lost sensitivity at slower rates than estimated in the current NUV TDSTAB, delivered in Cycle 17. As a result, NUV fluxes are slightly overcorrected by an amount that grows with time. The discrepancies are calculated by comparing pipeline-calibrated spectra of white dwarfs to model spectra. While the discrepancies were minor for most of the period since the last TDSTAB delivery, they are now outside of the 5% specification for many combinations of grating, cenwave, and stripe. Figure 1 shows how the fractional error has increased for each monitored cenwave and stripe in each of the three monitored gratings: G185M, G225M, and G230L. (Monitoring of G285M ceased after Cycle 25 due to its high TDS and lack of use in GO programs.)
Early in 2020, we expect to deliver a new TDSTAB, and all NUV spectra will be recalibrated. Until then, users should exercise caution in interpreting NUV fluxes measured with COS.