B.1 COS High Voltage History

The high voltage on the two FUV detector segments has been adjusted numerous times since launch in order to optimize the performance. Figure B.1 lists the nominal high voltage values used since COS installation in 2009.

The initial values used were identical to those used during ground testing. After early on-orbit tests showed that the gain of the MCPs was higher than expected on both segments, the voltages were lowered to return the gain to the pre-launch values. As described in Section 1.2.1, exposure to photon events lowered the gain in the spectral region at the original lifetime position, so the voltage was raised on several subsequent occasions as shown in Figure B.1 to keep the gain high enough to minimize throughput loss. The voltage has been adjusted as needed since then to keep the gain in the spectral region at acceptable levels.

In addition to the nominal voltages listed in the table, other values have been used in a number of calibration activities, so some data files in the archive have nonstandard voltage levels. For a complete listing of the FUV high voltage, users should refer to:  FUV High Voltage History.  The HV used during an exposure can be found in the first extension header keywords HVLEVELA and HVLEVELB for segments A and B, respectively.