FOS Instrument Science Report CAL/FOS 146

The FOS Dark Count-rate: Cycles 1 to 4

Jeffrey J.E. Hayes and Don J. Lindler

ABSTRACT:

This document gives an analysis of all FOS dark observations
taken on-orbit from Cycle 1 through Cycle 4.  We find that overall,
the FOS/RED dark count-rate is 0.0109 +/- 0.0022 counts/sec/diode, while
for the FOS/BLUE, the count-rate is 0.0064 +/- 0.00083 counts/sec/diode.  
We confirm the finding of Rosenblatt etal (1992) of the correlation between
dark count-rate and geo-magnetic latitude, but see no evidence for any
correlation between geo-magnetic longitude, solar position angle, or solar
cycle.  STScI is in the process of making a dark count-rate model suitable
for both the GHRS and the FOS, which will be used in the pipeline.