ISR 066 : Geomagnetic Image Deflection Problem in the Faint Object
Spectrograph

Junkkarinen, V. T., Beaver, E.A., Cohen, R. D., Hier, R., Lyons, R.,
Rosenblatt, E.

In orbit HST Data taken with the red Digicon detector of the FOS show
an image drift correlated with the Earth's magnetic field. This drift
occurs because the mu-metal magnetic shield around the detector does
not shield properly. The amplitude of the drift for a typical orbit is 
roughly equal to the spectral resolution of the FOS. For data taken
every minute or two, resampling and shifting can be used to remove
most of the drift in the spectral direction. The drift will compromise 
accuracy at high signal to noise ratiosbecause the edge of the image
apperture can drift beyond the edge of the image array in the
direction perpendicular to the dispersion. The microprocessor in the
FOS can be reprogrammed to counter the drift by changing the
deflection current on a timescale of one minute or less. In orbit data 
from the blue digicon show some drift that does not correlate well
with the magnetic field. The blue digicon mu-metal shield works better 
than the red side shield by an factor of ~3.