photo shows a team of astronauts replacing one
of three Fine Guidance Sensors on Hubble during
a servicing mission.
sensors are located at 90-degree intervals around
the circumference of the focal plane structure.
Each Fine Guidance Sensor is just over five feet
wide and three feet long and weighs 485 pounds.
Each Fine Guidance Sensor enclosure houses a very
precise optical system called an interferometer.
The telescope's high pointing accuracy and stability
is due largely to the Fine Guidance Sensors. Two
Fine Guidance Sensors are normally used in observations
to locate and lock onto a target star while observations
are made with a science instrument. From time
to time one of the Fine Guidance Sensors is also
used to perform scientific measurements, determining
highly precise positions and motions of stars,
and detecting the positions of companion stars.
The process of determining the positions of stars
is called astrometry.
re-certified Fine Guidance Sensor was installed
as a replacement during the Second Servicing Mission
and another re-certified Fine Guidance Sensor
was installed as a replacement during Servicing
Mission 3A. The Fine Guidance Sensors were built
by Hughes Danbury Optical Systems in Danbury,