Technicians in the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center help guide the Fine Guidance Sensor or FGS.

Technicians in the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center help guide the Fine Guidance Sensor, or FGS, as it is lifted over the crossbar of the stand at right. The sensor will be installed on the Orbital Replacement Unit Carrier or ORUC, below. An FGS consists of a large structure housing a collection of mirrors, lenses, servos, prisms, beam splitters and photomultiplier tubes. There are three fine guidance sensors on Hubble located at 90-degree intervals around the circumference of the telescope. Along with the gyroscopes, the optical sensors are a key component of Hubble’s highly complex but extraordinarily effective “pointing control system.” The ORUC is one of three carriers that are being prepared for the integration of telescope science instruments, both internal and external replacement components, as well as the flight support equipment to be used by the astronauts during the fifth and final Hubble servicing mission, STS-125, on space shuttle Atlantis. Launch is targeted for Oct. 8. Photo credit: NASA/Jim Grossmann