COS FUV Time Dependent Sensitivity
The Far-Ultraviolet (FUV) Time Dependent Sensitivity (TDS) monitor is designed to obtain regular observations of flux calibration standards using the shortest and longest central wavelength standard settings of each grating. Additionally G130M/1291, and the “blue-modes”, G130M/1055 and G130M/1096, are also tracked. The time dependence is modeled as a piece-wise linear function, with several breakpoints (the most recent at decimal year 2015.5) and normalized to 1.0 at first light (May 1, 2009). The TDS slopes have remained steady at values between 0% and -3% per year for all modes. The trends for all the modes are shown in the figure, along with a plot of the solar flux at 10.7 cm, which is a measure of the solar activity directed towards the earth.
COS NUV Time Dependent Sensitivity
The COS NUV detector has a time-dependent sensitivity (TDS) that must be monitored and accounted for in flux calibration. The NUV TDS monitor shows that the G230L and G185M gratings, which are coated in MgF2, exhibit trends that are consistent with no change in sensitivity. The G225M and G285M gratings, however, which are bare aluminum, show significant sensitivity declines of about –3% per year and −10% per year, respectively.
Because the sensitivity of the G285M grating is now very low, it is no longer monitored starting in Cycle 26, and the COS team discourages its use for General Observer (GO) programs.
*G185M/2010, G285M/2850, G225M/2306, and G225M/2410 cenwave monitoring began in 2017, and may be misleading on the above figure. Please see COS ISR 2018-11 for more information.