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    Apollo spacecraft sextant
    From: Jim Thompson
    Date: 2004 May 2, 08:47 -0300

    Here is a picture of the sextant used on the Apollo spacecraft in the
    Appolo's guidance and navigation system is described here, in an excerpt
    from the Apollo operations manual:
    "The sextant is a highly accurate optical instrument capable of measuring
    the included angle between two targets. Angular sightings of two targets are
    made through a fixed beam splitter and a movable mirror located in the
    sextant head. The sextant lens provides 1.8-degree true field of view with
    28X magnification. The movable mirror is capable of sighting a target to 57
    degrees LOS from the shaft axis. The mechanical accuracy of the trunnion
    axis is twice that of the LOS requirement due to mirror reflection which
    doubles any angular displacement in trunnion axis."
    They used the sextant for determining position in space, spacecraft
    attitude, measuring angles on the moon, and for taking pictures of potential
    landing spots.
    That page explains the role of the sextant ("SXT") within the spacecraft's
    G&N system:
    The optics provide accurate star and landmark angular measurements.
    Sightings are accomplished by the navigator using the SXT (sextant) and SCT
    (scanning telescope). The optics are positioned by drive motors commanded by
    the optics hand controller or manually using a universal tool, as desired.
    The shaft axes are parallel. Trunnion axes may be operated in parallel or
    offset, as desired. The SCT is a unity power instrument providing an
    approximate 60-degree field of view. It is used to make landmark sightings
    and to acquire and center stars or landmarks prior to SXT use. The SXT
    provides 28-power magnification with a 1.8-degree field of view. The SXT has
    two lines of sight, enabling it to measure the included angle between two
    objects. This requires two lines of sight which enable the two viewed
    objects to be superimposed. For a star-landmark sighting, the landmark line
    of sight is centered along the SXT shaft axis. The star image is moved
    toward the landmark by rotating the shaft and trunnion axes until the two
    viewed objects are superimposed. The shaft and trunnion angles are repeated
    by the optic CDUs. When the navigator is satisfied with image positions, he
    issues a mark command to the AGC (Apollo guidance computer). The AGC reads
    the optics CDU angles, IMU CDU angles, and time, and computes the position
    of the spacecraft. The AGC bases the computation on stored star and landmark
    data which may also be used by the AGC to request specific stars or
    landmarks for navigational sightings. Two or more sightings, on two or more
    different stars, must be taken to perform a complete position determination.
    Now THAT's astronavigation.  This is a highly significant milestone in the
    evolution of the sextant, and in the history of human navigation.  It means
    that humans first navigated to the moon in a similar way to how Cook opened
    up the Pacific Ocean centuries ago -- by doing celestial navigation with an
    optical instrument.  Although their peripherals were vastly more
    sophisticated than Cook's.
    Jim Thompson
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