causing the detector to reboot and turn off the detector high voltage…

causing the detector to reboot and turn off the detector high voltage as a safety measure. This currently requires ramping up the
high voltage via ground commands, which typically results in a one day loss of data for that detector; this process is now being
automated.

                                          4. MECHANICAL PERFORMANCE

         The FUSE gratings and mirrors are mounted via athermal optical mounts to a carbon fiber structure with an extremely
low coefficient of thermal expansion. This system was designed to minimize mechanical variations which might affect the ability
to hold the optics stable to several microns over a several meter distance. During the design and construction phase, thermal
models were made in order to predict the variations in temperature during an orbit and when changing attitude. Thermistors inside
the satellite monitor the temperatures, and the onboard computers control the temperatures to &1( C during an orbit. The
instrument focus was also adjusted before launch to compensate for the expected changes due to the difference between the
ground and on-orbit operating temperature, gravity release, and dimensional changes due to outgassing of water from the
structure.

          There are two time scales of temperature variations on orbit. The orbital variation is typically ~1( C internally, but can
be as high as 5( C on external surfaces such as the baffles. In addition, there are long-term temperature variations due to changes
in attitude, which lead to different parts of the satellite being illuminated by the sun. These variations can be as large as 30( C,
with a time constant that can be as long as 10 hours, but are more typically 6 hours. This long time constant means that before
equilibrium is reached after slewing to a new target, the observation may be completed, and another slew may have taken place.

         On orbit we have discovered that thermal changes are inducing higher than expected rotations of both the gratings and
mirrors. The rotation of the gratings causes the spectra to move in two dimensions on the detectors in a roughly sinusoidal fashion
over the course of an orbit; this degrades the spectral resolution. These motions are as high as 90 µm (0.09 Å in wavelength
space), depending on the channel. Algorithms have been developed to minimize these shifts during the ground processing of
data taken in TTAG mode. These corrections decrease the amplitude of the motion to less than 0.015 Å in wavelength space.

          The mirror rotation, which makes it difficult to keep the four channels coaligned, initially resulted in a misalignment
large enough for targets to drift out of the LWRS aperture in one or more channels in many instances. We have empirically
measured the behavior and limited the frequency of this occurrence to less than ~10% of observations. An understanding of the
drift in more detail is now being obtained as part of a plan to begin making regular observations in the smaller apertures.

          A number of workarounds were developed for these problems. These are primarily operational constraints imposed in
order to control the moderate thermal variation seen by certain parts of the satellite. Due to satellite power constraints, scattered
light, and safety issues, FUSE was designed to operate in the range of 15( < 5 < 105( , where 5 is the angle between the anti-sun
and the satellite optical axis. In order to minimize the mirror motions described above, further constraints have been imposed.
Normal operations are now limited to 30( < 5 < 85(. Exceptions are made for specific campaigns, such as a week of observations
in the Large Magellenic Cloud at 5 1 92( . During these campaigns, the instrument is allowed to reach thermal equilibrium
before a coalignment of the channels is made, and slews are limited to several degrees on the sky between targets in order to
minimize drifts of the optics. In addition, regular coalignment operations can be made if channels begin drifting. Characterization
is not complete, but it is believed that with the development of thermal models and careful scheduling of observations, this can
be improved in the future, allowing reasonable efficiency in the LWRS and MDRS apertures.

                                              5. OPTICAL PERFORMANCE

5.1. Telescope Performance

          The FUSE telescopes provide a complex, non-uniform point spread function at the FPAs. Preflight measurements and
analysis8 of a spare mirror, combined with metrology on the flight mirrors, showed that 88±5% of the encircled energy is within
a 1.5" diameter circle at 1000 Å. In flight, knife edge scans made using the FPAs showed that the telescope PSFs are consistent
with these ground measurements. This narrow PSF means that the spectral resolution of point source objects is not limited by
the FPA apertures, but rather by the ~0.3" satellite pointing stability and the alignment characteristics described above. The knife
edge scans were also used to optimize the mirror to FPA distances, which have been located to ±50 µm.