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National Aeronautics and Space Administration

Goddard Space Flight Center

Astrophysics Science Division | Sciences and Exploration

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Pointing Control System
Scientific Instruments

Hubble Space Telescope graphic with red outline around instrument location

Hubble's scientific instruments work either together or individually to bring us stunning images from the farthest reaches of space. Presently, Hubble can accommodate five scientific instruments and three fine guidance sensors. The instruments are housed in the bottom third, or aft section, of the telescope. Science instruments for Hubble are classified according to where they are mounted in the telescope.

Axial instruments are about the size and shape of a telephone booth. Axial instruments are mounted in the rear of the telescope along the optical axis. Light from the telescope's mirrors enters these instruments directly.

Radial instruments are mounted above the axial instruments and to the side. Radial instruments receive light by use of "pickoff mirrors" which redirect the light path into the instrument through apertures.

axial bays (4)

The axial bay is the bottom most area of Hubble and where four of the scientific instruments are housed. Double doors in the aft section on each side of Hubble provide access to this area. These four instruments are aligned with the main optical axis and are mounted just behind the primary mirror. Each science instrument has an entrance aperture, located in different portions of the Hubble's focal plane. As of mid-2008 they consists of:

ACS (Advanced Camera for Surveys): the newest camera (2002) with a wider field of view, and better light sensitivity than WFPC2. It effectively increases Hubble's discovery power by 10x.

COSTAR (Corrective Optics Space Telescope Axial Replacement): contains corrective optics for spherical aberration in the primary mirror (technically, not an instrument). COSTAR is to be replaced by the Cosmic Origins Spectrograph (COS) instrument in Servicing Mission 4.

COS (Cosmic Origins Spectrograph) (Post SM4) is an ultraviolet spectrograph optimized for observing faint point sources with moderate spectral resolution.

NICMOS (Near Infrared Camera and Multi-Object Spectrometer): Infrared instrument that is able to see through interstellar gas and dust.

STIS (Space Telescope Imaging Spectrograph): separates light into component wavelengths, much like a prism.

Hubble with doors open showing STIS and NICMOS in instrument positions 1 & 2 respectively Hubble with doors open showing ACS and COSTAR/COS in instrument positions 3 & 4 respectively

radial bay (4)


The radial bay is a donut-shaped reservour sitting above the axial bay and houses the remaining scientific instruments. Presently, Wide Field Planetary Camera 2 (WFPC2) occupies one of the four radial bays available on Hubble. WFPC2 is responsible for taking many of Hubble's famous pictures. WFPC2 is to be replaced by the next generation of camera, Wide Field Camera 3 (WFC3), in Servicing Mission 4.

Hubble bottom from V1 Axis showing location of WFPC2 in radial bay

The remaining three radial bays house the Fine Guidance Sensors (FGS), used primarily to control pointing of the Telescope. The sensors are in the focal plane structure, at right angles to the optical path of the Telescope and 90 degrees apart and have pick-off mirrors to deflect incoming light into their apertures. The sensors lock onto guide stars and measure relative positions, providing data to the spacecraft's targeting system and gathering knowledge on the distance and motions of stars. When two FGSs lock on guide stars to provide pointing information for the Telescope, the third FGS serves as a science instrument to measure the position of stars in relation to other stars.

Drawing of Hubble base showing location of the three FGS in relation to the other Pre SM4 science instrumentsDrawing of Hubble base showing location of the three FGS in relation to the other Post SM4 science instruments