AN ANALYSIS OF FOS BACKGROUND DARK NOISE

E.t. R.osenblatt, W.A. Baity, E.A. Beaver, R.D. Cohen,
V.T. Jurtkkarinen, J.B. Linsky, and R.W. Lyons
University of California, San Diego, CASS

Instrument Science Report CAL/FOS-07j.
April 1992


Abstract

We present results of an analysis of dark data (proposals OV1.533 and
0V2748) obtained with the Faint Qbject Spectrograph (FOS) during
Orbital Verification 1 and 2 (OV1,2). All observations were niade with
high voltage an. Outside the Soutli Atlantic. Anomaly (SAA), the
average count rate was roughly 0.012 cts s d for tlie red detector and
0.0065 cts s~ d for the blue detector. However, we have fouud that the
background count rate varies with geomagnetic latitude such that
higher rates are observed at higher latitudes.


We have examined tnany different possible sources of background noise
outside the SAA. Tue dominant source with high voltage an was found to
be Cerenkov radiation from cosnlic rays ancl trapped protons. Cerenkov
light, emitted whenever a high energy (>300Mev) particle traverses tue
detector window, can result in large portions of tue diode array being
flashed simultaneously. Ta mødel the Cerenkov effect in the FOS
Digicons, we have developed a Monte Carlo Computer code which
accurately simulates in detail the FOS backgroui-td
characteristjcs. Using this model we have estimated the extent to
which this burst noise can be rejected as a function of source flux,
live time,and threshold setting. lt was found that the background can
be significantly reduced using the proper software rejection settings.


Data were also collected inside the SAA witb high voltage an
(0V2748). The average peak SAA background count rate was roughly 1.20
cts s~ d1 for the red side and 1.05 cts s~ d for the blue
side. Lastly, outer contours of the SAA were determined empirically
from FO8 dark data which describe where astrononiical observations may
start artd/or end, so as to avoid taking data within this region of
high background.