Instrument Host Overview
  ========================
    The Hubble Space Telescope (HST) was the first and flagship
    mission of NASA's Great Observatories program.  Designed to
    complement the wavelength capabilities of the other spacecraft
    in the program (CGRO, AXAF, and SIRTF), HST was a 2.4 m, f/24
    Ritchey-Chretien telescope capable of performing observations
    in the visible, near-ultraviolet, and near-infrared (1150 A to
    1 mm).
 
    Placed into a low-earth orbit by the space shuttle, HST was
    designed to be modular so that on subsequent shuttle missions
    it could be recovered, have faulty or obsolete parts replaced
    with new and/or improved instruments, and be re-released.  HST
    was roughly cylindrical in shape, 13.1 m end-to-end and 4.3 m
    in diameter at its widest point.
 
    HST used an elaborate scheme for attitude control to improve
    the stability of the spacecraft during observations.
    Maneuvering was performed by four of six gyros, or reaction
    wheels.  Pointing could be maintained in this mode (coarse
    track) or the Fine Guidance Sensors (FGSs) could be used to
    lock onto guide stars (fine lock) to reduce the spacecraft
    drift and increase the pointing accuracy.
 
    Power to the two on-board computers and the scientific
    instruments was provided by two 2.4 x 12.1 m solar panels.  The
    power generated by the arrays was also used to charge six
    nickel-hydrogen batteries which provided power to the
    spacecraft during the roughly 25 minutes per orbit in which HST
    was within the Earth's shadow.
 
    Communications with the satellite were maintained with the TDRS
    satellites.  Observations taken during the time when neither
    TDRS was visible from the spacecraft were recorded on tape
    recorder and dumped during periods of visibility.  The
    spacecraft also supported real-time interactions with the
    ground system during times of TDRS visibility, enabling
    observers to make small offsets in the spacecraft pointing to
    perform their observations.  HST was the first scientific
    spacecraft designed to utilize the full capabilities of TDRS,
    communicating over either multiple-access or single-access
    channels at any of the supported transmission rates.
 
    HST was operated in three distinct phases.  During the first phase
    of the mission (Orbital Verification or OV), responsibility for
    the spacecraft was given to the Marshall Space Flight Center.  OV
    consisted of an extended, eight-month checkout of the spacecraft,
    including test of the on-board computers, pointing control system,
    solar arrays, etc.  This phase was followed by the Science
    Verification (SV) phase, lasting nearly another year, during which
    each of the six science instruments was tested to verify their
    capabilities and set limits on their safe operations during the
    remainder of the mission. Responsibility for the spacecraft during
    SV was given to Goddard Space Flight Center.  The last phase of
    the mission, known as the General Observer (GO) phase, was planned
    to last from the end of SV through the end of the mission and was
    the responsibility of the Space Telescope Science Institute.
    General observations were phased in gradually, however, during the
    SV phase because the OV and SV portions of the mission were
    considerably longer than expected prior to deployment.
 
    The mission was troubled soon after launch by the discovery that
    the primary mirror was spherically aberrated [sic].  In addition,
    problems with the solar panels flexing as the spacecraft passed
    from the Earth's shadow into sunlight caused problems with the
    pointing stability.  Steps were taken to correct these problems,
    including replacement of the solar panels, replacement of the Wide
    Field and Planetary Camera with a second-generation version with
    built-in corrective optics, and replacement of the High-Speed
    Photometer with COSTAR (Corrective Optics Space Telescope Axial
    Replacement) to correct the aberration for the remaining
    instruments. Subsequent servicing missions (1997, 1999 2002)
    updated or repaired onboard systems and replaced some of the
    orignal instruments with new instruments.