2.5 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 these challenges, the COS FUV sensitivity is still outstanding, and COS remains the instrument of choice for most spectroscopic observations of faint FUV targets.

Observers are reminded to use the COS Exposure Time Calculator (ETC; see Chapter 7) to design their proposals. It will be updated as the instrument sensitivity and background rates evolve.