Hubble's Fine Guidance Sensors help keep the telescope fixed on its target by sighting on guide stars.

The spacewalkers then will swap one of the telescope's three Fine Guidance Sensors with a refurbished unit.

The Hubble Space Telescope has three Fine Guidance Sensors (FGSs).

The mission also replaced a Fine Guidance Sensor, six gyroscopes, and two battery unit modules to allow the telescope to continue to function at least through 2014.

In 2004, astrometric measurements with the Fine Guidance Sensors on the Hubble Space Telescope suggest that the planet's orbit is inclined by around 53 with respect to the plane of the sky.

As far as possible, binary stars and non-stellar objects have been excluded or flagged as not meeting the requirements of Fine Guidance Sensors.

In addition to replacing all six gyroscopes on the December flight, the crew replaced a Fine Guidance Sensor (FGS) and the spacecraft's computer.

The astronauts also expect to swap one of Hubble's three Fine Guidance Sensors for an improved version that should enhance the observatory's pointing and measurement capacity.

Fine Guidance Sensor (FGS)

Harbaugh and Tanner replaced a degraded Fine Guidance Sensor and a failed Engineering and Science Tape Recorder with new spares.