The FUSE Archival Data Handbook - MAST - STScI

one SiC channel, each producing a spectrum which falls onto a single detector. Each channel
has a bandpass of about 200 ˚A. Thus, at least two channels are required to cover the ∼290 ˚A
wavelength range of the instrument.
Figure 2.1 also shows the orientation of the instrument prime coordinate system (X,Y ,Z).
The two LiF channels are on the +X side of the instrument, which is always kept in the shade
(i.e., the −X side was always facing toward the sun). This orientation minimizes the amount
of sunlight that can make its way down the baffles surrounding the LiF channels. Minimizing
stray light in the LiF channels was crucial to the operation of the Fine Error Sensor (FES)
guidance camera, which operated at visible wavelengths (see Chapter 5). The orientation of
the satellite was biased by several degrees in roll around the Z axis in order to keep the radiator
of the operational FES in the shade.
2.2.1 Focal Plane Assemblies
Each telescope focused on an FPA that served as the entrance aperture for its spectrograph. An
FPA was a flat mirror mounted on a two-axis adjustable stage which contained three apertures.
The apertures were not shuttered, so whatever light fell on them was dispersed by the gratings
and focused onto the detectors.
The orientation of the three apertures is depicted in Fig. 2.2, and their properties are summarized
in Table 2.1. The HIRS aperture ensured maximum spectral resolution, minimum sky
background and sometimes allowed for spatial discrimination for extended objects or crowded
stellar fields. However, it blocked part of the point spread function, and small, thermally induced
mirror motions often led to the loss of all light from one or more channels during an
observation. The MDRS aperture, with stable mirror alignment, provided maximum throughput
and a spectral resolution comparable to the HIRS aperture for a point source with only
slightly more airglow contamination than HIRS. However, it too, was susceptible to thermal
drifts of the separate channels. The LWRS aperture was the least sensitive to alignment issues,
but it allowed the largest sky background contamination. Although image motion could degrade
the resolution of LWRS spectra, this was largely corrected by the pipeline processing for
TTAG data. The LWRS aperture was also used to observe faint extended objects, producing
a filled-aperture resolution of ∼100 km s −1 . LWRS was the default aperture throughout most
of the mission because of the thermal image motions that occurred on orbit.
In addition, a target could also be placed outside the apertures at the reference point
(RFPT), which did not transmit light. When a target was placed at the RFPT, the three
apertures sampled the background sky nearby.
Each FPA could be moved independently in two directions. Motion tangential to the Rowland
circle, which is roughly in the dispersion direction, and perpendicular to the apertures.
These motions allowed co-alignment of the channels and permitted focal plane splits (FP splits,
see Chapter 7) for high signal-to-noise ratio observations of bright targets. Motion in Z enabled
focusing of the apertures with respect to the mirrors, spectrograph grating, and detector.
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