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CAPE CANAVERAL, Fla. – The Fine Guidance Sensor, or FGS, is being prepared for backlight inspection and cleaning in the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center. The FGS is part of the payload for the fifth and final Hubble servicing mission, STS-125, aboard space shuttle Atlantis. 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.” Atlantis is targeted to launch Oct. 8. Photo credit: NASA/Jack Pfaller