SPACE	INSTRUMENT SCIENCE REPORT
	TELESCOPE	         CAL/EOS-025
	SCIENCE
INST[FUTE
TITLE:	FOS Linearity Correctipns

ALTHOR: D. Lindler and R. Bohlin	DATE:___January 1986 ABSTRACT

Data taken during June and August 1984 show that the FOS diode
non—linearity (pai.red pulse correction) can be modelled by:

		x

(1  ty)

where y i.s the observed count rate (counts/sec) and x is the true
count rate. The time coefficient t is constant (9.43 microsec) up to
51,000 observed counts per sec, which corresponds to a true input rate
of about 100,000. Above 51,000 counts t can be modelled
by:


10
		t= 9.43 x 10 6 + 2.568 x 10	 (y  51000)


This variation in t may be caused by photocathode fatigue.

No significant diode to diode variation in the nonhinearity is seen
below 100,000 true (input) counts/sec. Above this count rate the
observed count rate varies from diode to diode by as much as ± 10%.


The a~curacy of this model in predicting true corrected count rates
should be better than 1% below 20,000 and degrading to about 5%föT an
average of many diodes at 100,000 true counts/sec.


The results presented here sbould be used only as a guide to the
reduction of flight data, because the red tube has been replaced and
the blue tube has not been studied at the highest counting rates. The
~atigue efrects may be quantitatively different for flight conditions.