The Hubble Space Telescope Second Servicing Mission (SM-2)

Fine Guidance Sensor (FGS)

The Hubble Space Telescope (HST) uses sensors that accurately point the telescope to its targets and are capable of measuring the motion of stars to great accuracy. One Fine Guidance Sensors (FGS) will be installed on the telescope during the Second Servicing Mission (SM-2) scheduled for early 1997.

A 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 the HST located at 90-degree intervals around the circumference of the telescope. Two FGSs are used to point the telescope at an astronomical target and then hold that target in a scientific instrument's field of view. The third FGS can be used as a scientific instrument for celestial measurements (astrometry).

Pointing Control:

The fine guidance sensors are one of five different types of sensors used by the HST's pointing control system to point the telescope at a target with an accuracy of 0.01 arcsec. The guidance sensors lock on to a star and then measure any apparent motion to an accuracy of 0.0028 arcsec. This gives the HST the ability to remain pointed at that target with no more than 0.007 arcsec of deviation over long periods of time. This level of stability and precision is comparable to being able to hold a laser beam focused on a dime 200 miles away (about the distance from Washington D.C. to New York).

Astrometry science:

Astrometry is the science that deals with the determination of precise positions and motions of stars, and other celestial objects. The FGSs can provide star positions that are about 10 times more precise than those observed from a ground-based telescope.

When used for astrometric science, the fine guidance sensors will let the HST:

Search for a wobble in the motion of nearby stars which could indicate the presence of a planetary companion.

Determine if certain stars really are double stars.

Measure the angular diameter of stars, galaxies, etc.

Refine the positions and the absolute magnitude scale for stars.

Help determine the true distance scale for the universe.

Servicing:

On-orbit servicing of the HST by Shuttle astronauts makes it possible to upgrade the telescope's capabilities. A spare FGS is being refitted with a new mechanism to accomplish better focusing. Once this spare is exchanged for one of the original units during the servicing mission, telescope operators will be able to compensate for changes due to on-orbit conditions, and optimize its performance by keeping the FGS more finely tuned.

The Fine Guidance Sensors were originally built by Perkin-Elmer Corp., Danbury, Conn. under contract to NASA. The FGS for the SM-2 is a spare unit, being recertified by Hughes Danbury Optical Systems, Inc. (formerly Perkin-Elmer). The astrometry science program is managed through the Space Telescope Science Institute in Baltimore, Md. and is open to scientists throughout the world in the same manner as all other HST science.

FGS PHYSICAL CHARACTERISTICS

Size 1.6 x 3 x 5.4 ft (0.5x1x1.6 m)

Weight 485 lb (220kg)

Wavelength Range 467 to 700 nm

Fields of View 60 x 60 arcmin 5 x 5 arcsec

For additional information contact:

Tammy Jones

Goddard Space Flight Center

Office of Public Affairs

(301) 286-5566

Internet: http://www.gsfc.nasa.gov

Space Telescope Science Institute

Office of Public Outreach

(410) 338-4707

Internet: http://www.stsci.edu