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. Table 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 prelaunch 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 in March 2011 (Segment B) and March 2012 (Segment A) to keep the gain high enough to minimize throughput loss.

The voltage was adjusted once more when the spectrum was moved to the second lifetime position in July 2012, since that location has not yet been affected by gain sag. The new voltages are lower than those used in 2009 in order to keep the rate of gain sag as low as possible while ensuring optimal detector performance. The voltage will be adjusted as needed in the future in order 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 updates on the FUV high voltage, users should refer to:

http://www.stsci.edu/hst/cos/performance/high_voltage/.

Table B.1: COS FUV Detector Nominal High Voltage History

DATE	HV LEVEL COUNTS (SegA / SegB)	HV LEVEL VOLTS (SegA / SegB)	DESCRIPTION
05/11/2009	178 175	-5293 -5246	Initial on-orbit values
08/12/2009	169 167	-5152 -5120	Lowered both segments to decrease modal gain
03/08/2011	169 175	-5152 -5246	Increased Segment B to compensate for gain sag
03/26/2012	178 175	-5293 -5246	Increased Segment A to compensate for gain sag
07/23/2012	167 163	-5120 -5058	Initial values at Second Lifetime Position