Sensitivity

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 standard central wavelength settings of each grating. Additionally, G130M/1222, the blue modes G130M/1055 and G130M/1096, and cenwaves G140L/800 and G160M/1533, introduced in Cycle 26, are tracked. The time dependence is modeled as a piece-wise linear function, with several breakpoints (the most recent at decimal year 2022.0) and normalized to 1.0 at first light (May 1, 2009). Due to a recent increase in solar activity, the current TDS slopes are between –2% and –5% per year. 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, and is correlated with the TDS.

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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.

Last Modification Date: Thu. Mar. 14, 2024 1:32PM

*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.