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The GUVI instrument consists of three major functional elements: a scanning
imaging spectrograph (SIS) that obtains the spectral data, a detector
processor which converts information recorded at the detector into
wavelength and spatial information as required, and an interface to the
TIMED spacecraft. The SIS subassembly consists of a cross track scanning
mirror at the input to the telescope and spectrograph optics. The
spectrograph is a Rowland circle mount which gives reasonable spatial
imaging quality along the entrance slit axis. Lifetime studies and
experience with the SSUSI detectors indicated that it was prudent to have
two detectors at the focal plane of the instrument. The actual detector to
be used is determined by the location of a mirror which can be inserted into
the instrument’s optical path. The detectors are custom built micro-channel
plate intensified wedge-and-strip anode sealed tube units.
The imaging spectrograph builds multispectral images by scanning spatially
across the satellite track. One dimension of the detector array contains 14
spatial pixels (along the spacecraft track), and the other dimension
consists of 160 spectral bins over the range of 115 to 180 nm. In normal
imaging mode, selected regions of the spectrum are down-linked as “colors”.
These “colors” preserve the physics of the observations while limiting the
bandwidth required. The scan mirror sweeps the 14 spatial pixel footprint
from horizon to horizon perpendicular to the spacecraft motion, producing
one frame of 14 cross-track lines in 15 seconds. At an altitude of 625 km,
looking at nadir, the footprint covers a width of 108 km at a height of 100
km above the ground. The viewing point 100 km above the ground moves by 104
km in one cycle of the scan mirror, so successive scans overlap at nadir.
The detector processor bins the data into the selected colors.
The imaging mode scan cycle consists of a limb viewing section followed
by an Earth viewing section. Limb viewing pixels are collected from 80.0°
from nadir (the start of scan) to 67.2° from nadir. The limb viewing section
has a cross track resolution of 0.4° per pixel, and consists of 32 cross
track pixels by 14 along track pixels with five colors. At 80.0° from nadir
and a spacecraft altitude of 625 km, the spectrograph will view a tangent
point 519 km above the horizon, at a distance of 1215 km. At a more typical
limb viewing angle of 68.8, the spectrograph will view a tangent point 152
km above the horizon, at a distance of 2530 km. The variation in tangent
point with view angle is shown in the limb” portion. At a tangent point
altitude of 152 km, the footprint of the instrument covers a distance of 530
km, but the viewing point only moves by 105 km in one 15 s scan mirror
cycle. Therefore, the same pixel on the limb is re-sampled on five successive
sweeps of the scan mirror on each orbit. The Earth viewing section has a
cross track resolution of 0.8° per pixel, and contains 159 cross track
pixels by 14 along track pixels and five colors. Pixels are collected from
67.2 from nadir to -60° from nadir (the end of scan).
In the spectrograph mode, the scan mirror is held at a fixed viewing
angle (normally either the nadir direction for "ground truth" or on the limb
for star calibrations) and the entire spectrum is down-linked. The along
track dimension of the detector array is binned into 14 spatial pixels.
Spectral data from 176 bins (the 160 bins which the “colors” are chosen
from, plus 16 more on the long wavelength end) are produced for the 14
spatial pixels every 3.0 seconds.
There is, of course, a large range in the intensity of the scene that
GUVI is expected to see. The detector electronics have a large dynamic range
which can accommodate much of this range but some further flexibility was
desired as well as the ability to incorporate a closed position. While we
have some confidence in the first principles models used to predict the
upper atmospheric radiance and the throughput of the system, we did not know
the what the achieved quantum efficiency of the detectors would prove to be
to within a factor of two. This was achieved by implementing two independent
stepper motors which move metal vanes into position in front of the entrance
to the spectrograph. These two vanes reduce the instrument’s field of view
from 0.78 deg to 0.30 or 0.18 deg. By activating both vanes at the same time
a closed position is achieved. The intermediate width slit is intended for
use during normal imaging mode operation. The widest slit would be used in
imaging mode to increase the sensitivity should the optical efficiency of
the system decrease over time or to minimize the statistical error for low
count rate scenes such as when the FUV nightglow is to be observed. The
narrowest slit improves the spectral resolution and reduces the instrument
throughput. Any slit can be used in any mode of operation. Furthermore, the
slit mechanism is designed so that two motors must fail (they are
independent) in a specific (i.e. "closed") mode in order for the aperture to
be "shuttered". The expected "failure" mode would be one that would leave us
with a fixed slit.
The major elements of the SIS are shown as is the implementation of two
independent detectors. The TIMED spacecraft interface was specified to be
IEEE 1553. A large scan mirror provides the horizon-to-horizon view for a
telescope at the entrance to the spectrograph. There are two detectors,
identical in form, fit, and function, which convert the incoming far
ultraviolet photons into packets of charge which are located by the detector
electronics. A once-open door protects the scan mirror from contamination during ground
operations and launch. There are two redundant detectors, each with a focal
plane electronics box to amplify and process signals. These signals are then
sent to the Electronics Control Unit box inside the spacecraft bus. The SIS
electronics box provides control signals for the scan mirror, slit
selection, and pop-up mirror motors.
 
GUVI detectors come from two different manufacturers: Photek and
Siegmund Scientific. These photos illustrate the dimensions of the detector
and the spectrograph to detector interface. The slots in the mounting flange
are provided to account for uncertainty in the potted position of the tube
and the rotation of the anode at the instrument's exit plane. The tube on
the left was manufactured by Siegmund Scientific whereas the one on the
right was manufactured by Photek. The Photek photo shows the tube during a
test: this is not a flight configuration but does illustrate that the GUVI
tubes were specified to be identical in form, fit and function so they are
interchangeable.
Five SSUSI units and the GUVI unit have been thoroughly characterized at
the JHU/APL Optical Calibration Facility.6 The responsivity was
measured at 16 along-slit angles and 20 wavelengths for both detectors, with
the scan mirror set for nadir viewing.
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