.title
Fine Guidance Sensors

.synonyms
pointing
stabilization control
orientation
fine guidance censors

.description
Instruments used to locate and lock on to a target object, maintaining
position of the telescope relative to the target.
.contents

.para
     Because of the very high spatial reolution of the Space Telescope
great pointing stability is required.  Two of the fine guidance sensors
will be used to locate and lock on to a target while the third is
free to view and precisely measure the relative positions of other stars
in the vicinity.


.para
     The three fine guidance sensors are expected to produce distance
measurements ten times more accurate than those done from .~ Earth~.  The
sensors work through a complex system of mirrors and light detectors to
locate the position of a star  against a background of other stars.  In
measuring the distance to a star, .~ ground controllers~ will focus the
sensors on a foreground star to transmit its position relative to the
stars in the background.  Similar measurements are made about six months
later.  Changes will indicate the distance of the star from the .~ earth~.


.para
     The precise measurement of star positions is called astrometry.
Information provided by the fine guidance sensors will assist in
resolving the open-versus-closed universe question.