Far Ultraviolet Imaging from the IMAGE Spacecraft: 1. System Design 

S. B. Mende, H. Heetderks, H. U. Frey, M. Lampton, S. P. Geller
Space Sciences Laboratory, University of California Berkeley, Berkeley, CA
94720

S. Habraken, E. Renotte, C. Jamar, P. Rochus
Centre Spatiale de Liege, Liege, Belgium B-4031

J. Spann
George C. Marshall Spaceflight Center, Huntsville, AL 35812

S. A. Fuselier
Lockheed Martin Advanced Technology Center, Palo Alto, CA, 94304

J.-C. Gerard
University of Liege, Liege, Belgium B-4000

R. Gladstone
Southwest Research Institute, San Antonio, Texas 78228

S. Murphree, L. Cogger
University of Calgary, Calgary, Canada AB T2N 1N4

            Direct imaging of the magnetosphere by the IMAGE spacecraft 
will be supplemented by observation of the global aurora, the footprint of
magnetospheric regions. To assure the simultaneity of these observations and
the measurement of the magnetospheric background neutral gas density, the
IMAGE satellite instrument complement includes three Far Ultraviolet (FUV)
instruments. In the wavelength region 120-190 nm, a downward-viewing auroral
imager is only minimally contaminated by sunlight, scattered from clouds and
ground, and radiance of the aurora observed in a nadir viewing geometry can be
observed in the presence of the high-latitude dayglow. The Wideband Imaging
Camera (WIC) will provide broad band ultraviolet images of the aurora for
maximum spatial and temporal resolution by imaging the LBH N2 bands of the
aurora.  The Spectrographic Imager (SI), a monochromatic imager, will image
different types of aurora, filtered by wavelength.  By measuring the
Doppler-shifted Lyman-a, the proton-induced component of the aurora will be
imaged separately.  Finally, the GEO instrument will observe the distribution
of the geocoronal emission, which is a measure of the neutral background
density source for charge exchange in the magnetosphere.  The FUV instrument
complement looks radially outward from the rotating IMAGE satellite and,
therefore, it spends only a short time observing the aurora and the Earth
during each spin.  Detailed descriptions of the WIC, SI, GEO, and their
individual performance validations are discussed in companion papers.  This
paper summarizes the system requirements and system design approach taken to
satisfy the science requirements.  One primary requirement is to maximize
photon collection efficiency and use efficiently the short time available for
exposures.  The FUV auroral imagers WIC and SI both have wide fields of view
and take data continuously as the auroral region proceeds through the field of
view.  To minimize data volume, multiple images are taken and electronically
co-added by suitably shifting each image to compensate for the spacecraft
rotation.  In order to minimize resolution loss, the images have to be
distortion-corrected in real time for both WIC and SI prior to co-adding.  The
distortion correction is accomplished using high speed look up tables that are
pre-generated by least square fitting to polynomial functions by the on-orbit
processor.  The instruments were calibrated individually while on stationery
platforms, mostly in vacuum chambers as described in the companion papers. 
Extensive ground-based testing was performed with visible and near UV
simulators mounted on a rotating platform to estimate their on-orbit
performance.  The predicted instrument system performance is summarized and
some of the preliminary data formats are shown. 

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Space Science Reviews, IMAGE Special Issue, Vol. 91, pp. 243-270, February, 2000