2.6 Time-Dependent Sensitivity Changes

During Cycle 17 the throughputs of some COS modes—the bare-aluminum NUV G225M and G285M gratings and the FUV channels—were found to be declining with time significantly more rapidly than had been expected. Since that time the decline of the G225M and G285M grating throughputs has continued at approximately the same rate. However, the rate of decline of the throughput of the FUV channels has fluctuated, first becoming more modest and later showing variations that appear to be correlated with Solar activity (Section 5.1.5). It is believed that these latter changes are due to degradation of the photocathode caused by reactions with residual atmospheric atomic oxygen. As Solar activity increases, the Earth's atmosphere becomes slightly inflated, exposing the open-faced COS FUV XDL detector to increased levels of atomic oxygen.

Despite the decline with time, the sensitivity of the COS FUV channel remains outstanding, and it remains the instrument of choice for observing faint FUV targets.

It is strongly recommended that observers use the COS Exposure Time Calculator (ETC; see Chapter 7) to estimate the required exposure times for their proposals. The ETC calculations account for the time-dependent sensitivities by using the same model as the calibration pipeline. The sensitivities used by the ETC for each cycle correspond to various values extrapolated to the mid-cycle date and will provide sufficient accuracy over the whole cycle for the purposes of an observing proposal.