FOS Instrument Science Report 099

"Serendipitous Background Monitoring of the Hubble Space Telesope's Faint 
Object Spectograph" by John E. Fitch and Glenn Schneider

Abstract excerpted from the report:
	The nature of the Hubble Space Telescope's (HST) low Earth orbit
	imposes scheduling restrictions and imterruptions in the data collection
	periods for it's compliment of scientific instruments.  During many of 
	these times the Faint Object Spectograph (FOS) is in full operational 
	configuration and is taking detector background measurements which are
	continually reported in HST's engineering telemetry stream.  These data
	are primarily used to monitor the instrument for changes in behavior
	resulting, principally, from intermittently noisy diodes in its digicon
	arrays.  These same data may be used to monitor teomporal changes in the
	charged particle environment of HST's near earth orbit.

	We present here the results of a study of two years of on-orbit FOS
	background data obtained serendipitously during periods while the FOS
	in an operational state, but not exposing on external, or calibration
	targets.  These in situ data, which represent more than 100,00 discrete
	samples (equivalent to more than 1100 orbits) have allowed us to 
	accurately measure variations in the background proton flux seen by the
	FOS.  An analysis of these variations have pernitted us to model the
	geomagnetic environment of the South Atlantic Anomaly (SAA) as a
	function of time as well as the change in detector background as a 
	function of geomagnetic latitude.


For more information, please contact the above authors.