The Pointing Control System (PCS) aligns Hubble so that the telescope
points to and remains locked on a target. The PCS is
designed for stable pointing to within .01 arcsec and is capable of
holding a target for up to 24 hours while Hubble continues to orbit
the Earth at 17,500 mph. If the telescope were in Los Angeles, it could
hold a beam of light on a dime in San Francisco without
the beam straying from the coin's diameter.
Hubble employs a variety of sensors to detect
its own orientation and position. All work in tandem to send the correct
information to the actuators to adjust Hubble's position on command.
Fine Guidance Sensors (FGS) (3)
These sensors are locked onto two guide stars to keep Hubble
in the same relative position of these stars.
Coarse Sun Sensors (CSS) (4)
Measure Hubble's orientation to the sun. Also assist in deciding
when to open and close the aperture door.
Magnetic Sensing System (MSS) (2)
Measures Hubble's orientation relative to Earth's magnetic field.
Rate Sensor Units (RSU) (3)
Each RSU contains two gyroscopes, used for maintaining stable
Fixed Head Star Trackers (FHST) (3)
An electro-optical detector that maps areas of the sky to determine
Hubble's orientation in space.
Receiving information from the sensors, the actuators physically
adjust Hubble's position and orientation so that Hubble can view the
required celestial bodies.
Reaction Wheel Actuators (RWA) (4)
The reaction wheels work by rotating a large flywheel up
to 3000 rpm or braking it. The resulting changes in angular
momentum serve to move the telescope from one target to another.
Magnetic Torquers (MT) (4)
The torquers are used primarily to manage reaction wheel
speed. Reacting against Earth's magnetic field, the torquers
reduce the reaction wheel speed, thus managing angular momentum.