LAB TEST RESULTS FOR THE FOS DETECTOR PERFORMANcE IN A VARIABLE
EXTERNAL MAGNETIC FIELD


E. A. Beaver and P. Foster
University of California, San Diego

Instrument Science Report CAL/FOS-082
August 1992

Abstract

Using lab test results from spare FO8 Digicon detectors, we have
extended the rnodel for the expected behavior of the FQS detectors in
an external magnetic field to account for anomalies seen in the 8V FO8
detector test proposal ET3138. Our lab measurements on the spare FOS
Blue detector shows higli sensitivity to external rnagnetic fields of
167 tnicrons per gauss. This over sensitivity to external magnetic
fields is similar to that for the orbiting Red detector of 147 m~crons
per gauss. A study of the fabrication history of the FQS detector
build shows that this spare Blue detector magnetics is from the same
1985 vintage as is the orbital FOS Red detector niagnetics. Further we
find from our laU nleasurements tliat the FQ8 detector‘s
susceptability to image displacement by external magnetic fields is
strongly affected by FOS degaussing activity. The amount of image
shift caused by & degauss is proportional to the difference in the
external magnetic field that has o ccurred since the previous
degauss. For the spare FOS Blue detector, the proportionality constant
is 61 microns per gauss. The sign of image shift caused by an FOS
degauss is opposite to the direction of the external magnetic field
change between the degausses. This results in an apparent detector
external field sensitivity factor of 106 microns/gauss with degaussing
hefore each positional measurement for the FOS spare Blue
detector. Tliis new understanding of the FQ8 geomagnetic image niotion
phenomena should allow the optimal removal of the effect from FO8
data. Also our lab test results show that the GHRS degauss is
considerably more effective than the EOS degauss procedure in
suppressing tbe Digicon Detector‘s past magnetic history.