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 on several subsequent occasions as shown in Table 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 updates on the FUV high voltage, users should refer to:
http://www.stsci.edu/hst/instrumentation/cos/performance.


Table B.1: Lifetime Position and Nominal High Voltage Values (Segment A/B) for the COS FUV Detector.

Grating:G130MG150MG140L

Central
Wavelengths

1055, 1096

1222

1291

1291, 1300, 1309, 1318, 1327

1577, 1589, 1600, 1611, 1623

11051

12802

5/11/2009

Not used

Not used

LP1

178/175

LP1

178/175

LP1

178/175

LP1

178/100

LP1

178/175

8/12/2009

LP1

169/1673

LP1

169/167

LP1

169/167

LP1

169/167

LP1

169/167

LP1

169/100

LP1

169/167

3/8/2011

LP1

169/175

LP1

169/175

LP1

169/175

LP1

169/175

LP1

169/175

LP1

169/100

LP1

169/175

3/26/2012

LP1

178/175

LP1

178/175

LP1

178/175

LP1

178/175

LP1

178/175

LP1

178/100

LP1

178/175

7/23/2012

LP2

167/163

LP2

167/163

LP2

167/163

LP2

167/163

LP2

167/163

LP2

167/100

LP2

167/163

6/24/2013

LP2

167/169

LP2

167/169

LP2

167/169

LP2

167/169

LP2

167/169

LP2

167/100

LP2

167/169

7/21/2014

LP2

167/175

LP2

167/175

LP2

167/175

LP2

167/175

LP2

167/175

LP2

167/100

LP2

167/175

11/3/2014

LP2

173/175

LP2

173/175

LP2

173/175

LP2

173/175

LP2

173/175

LP2

173/100

LP2

173/175

2/9/2015

LP2

173/175

LP3

171/167

LP3

167/163

LP3

167/163

LP3

167/163

LP3

167/100

LP3

167/163

1/18/2016

LP2

173/175

LP3

171/169

LP3

167/169

LP3

167/169

LP3

167/169

LP3

167/100

LP3

167/169

10/17/2016

LP2

173/175

LP3

171/175

LP3

167/175

LP3

167/175

LP3

167/175

LP3

167/100

LP3

167/175

10/2/2017

LP2

173/175

LP4

163/167

LP3

167/175

LP3

167/175












LP4

163/163

LP4

163/100







1 A commanded high voltage of 100 means that no data is collected on that segment.

2 In Cycle 17, the 1230 Central Wavelength setting was replaced by 1280.

3 Segment High Voltage in Volts = –(Commanded voltage*15.69 + 2500).