Glossary items by topic: Hubble Space Telescope

Advanced Camera For Surveys (ACS)

An optical camera aboard the Hubble Space Telescope that uses CCD detectors to make images. The camera covers twice the area, has twice the sharpness, and is up to 10 times more efficient than the telescope’s Wide Field and Planetary Camera 2. The ACS wavelength range spans from ultraviolet to near-infrared light. The camera’s sharp eye and broader viewing area allow astronomers to study the life cycles of galaxies in the remotest regions of the universe. Astronauts installed the camera aboard the telescope in March 2002, but the camera experienced an electrical short in 2007 that shut down all but one data channel. During Servicing Mission 4 in 2009, astronauts replaced the failed circuit boards and added a new power supply box to restore power to the camera.

Association of Universities for Research in Astronomy, Inc. (AURA)

A consortium of educational and other non-profit institutions that operates world-class astronomical observatories. Members include five international affiliates and 29 U.S. institutions, including the Space Telescope Science Institute in Baltimore, Maryland, the science operations center for NASA’s Hubble Space Telescope.


Batteries provide all the electrical power to support Hubble operations during the night portion of its orbit, when the telescope is in Earth’s shadow. The telescope’s orbit is approximately 97 minutes long. Roughly 61 minutes of Hubble’s orbit are in sunlight and 36 minutes are in Earth’s shadow. During Hubble’s sunlight or daytime period, the solar arrays provide power to the onboard electrical equipment. The solar arrays also charge the spacecraft’s batteries so they can power the spacecraft during the night portion of Hubble’s orbit. Hubble has six nickel-hydrogen batteries. These batteries, which had been onboard Hubble since the telescope was launched in 1990, were replaced during Servicing Mission 4.

Corrective Optics Space Telescope Axial Replacement (COSTAR)

An apparatus installed during the 1993 First Servicing Mission. By placing small and carefully designed mirrors in the telescope, COSTAR successfully improved restored Hubble’s vision to its original design goals. All the new instruments installed during the servicing missions have internal corrections for spherical aberration and do not require the services of COSTAR. Hubble’s last original instrument, the Faint Object Camera, was replaced by the Advanced Camera for Surveys during SM3B. COSTAR was replaced by the Cosmic Origins Spectrograph during Servicing Mission 4 and returned to Earth in the space shuttle.

Cosmic Origins Spectrograph (COS)

A spectrograph that detects ultraviolet light. A spectrograph works by breaking up light from an object into its individual wavelengths so that its composition, temperature, motion, and other chemical and physical properties can be analyzed. COS will study the structure of the universe and how galaxies, stars and planets formed and evolved. Astronauts installed COS during SM4.

Digital Image

A visible image that is recorded by an electronic detector and subdivided into small picture elements (pixels). Each element is assigned a number that corresponds to the brightness recorded at its physical location on the detector. Computer software converts the numerical information into a visual image. The Hubble Space Telescope records digital images.

European Space Agency (ESA)

A fifteen-member consortium of European countries for the design, development, and deployment of satellites. The Space Telescope — European Coordinating Facility (ST-ECF) supports the European astronomical community in exploiting the research opportunities provided by the Earth-orbiting Hubble Space Telescope. The ESA members are Austria, Belgium, Denmark, Finland, France, Germany, Ireland, Italy, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, and the United Kingdom, with Canada as a cooperating state.

Faint Object Camera (FOC)

An instrument aboard the Hubble Space Telescope that recorded high-resolution images of faint celestial objects in deep space. Built by the European Space Agency, the camera collected ultraviolet and visible light from celestial objects. The camera served as Hubble’s “telephoto lens” — recording the most detailed images over a small field of view. The FOC’s resolution allowed Hubble to single out individual stars in distant star clusters. The instrument was replaced in March 2002 during Servicing Mission 3B.

Faint Object Spectrograph (FOS)

An instrument aboard the Hubble Space Telescope that acted like a prism to separate light from the cosmos into its component colors, providing a wavelength “fingerprint” of the object being observed. Such information yields clues about an object’s temperature, chemical composition, density, and motion. Spectrographic observations also reveal changes in celestial objects as the universe evolves. The instrument was replaced in February 1997 during the Second Servicing Mission.


A type of window that absorbs certain colors of light while allowing others to pass through. Astronomers use filters to observe how celestial objects appear in certain colors of light or to reduce the light of exceptionally bright objects. For example, a pair of sunglasses acts as a type of filter, reducing the amount of incoming light while still allowing some light to pass through to the eyes.

Filter Wheels

Rotating wheels in a telescope instrument that allow specific colors of light from a celestial object to pass through and form an image on the detector. The Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope has 12 filter wheels, each of which holds four filters.

Fine Guidance Sensors (FGS)

Targeting devices aboard the Hubble Space Telescope that lock onto “guide stars” and measure their positions relative to the object being viewed. Adjustments based on these precise readings keep Hubble pointed in the right direction. The sensors also are used to perform celestial measurements.

Fixed Head Star Trackers (FHST)

Small telescopes with wide fields of view that are aboard the Hubble Space Telescope and used in conjunction with the Fine Guidance Sensors. The star trackers locate the bright stars that are used to orient the telescope for scientific observations.

Goddard High Resolution Spectrograph (GHRS)

A science instrument aboard the Hubble Space Telescope that made finely detailed spectroscopic observations of ultraviolet sources. The GHRS was removed from Hubble in February 1997 and replaced with the Space Telescope Imaging Spectrograph.

Goddard Space Flight Center (GSFC)

NASA’s flight control center in Greenbelt, Maryland, which receives data from orbiting observatories such as the Hubble Space Telescope (HST). HST digital data are then relayed to the Space Telescope Science Institute in Baltimore, Maryland, where they are interpreted into pictures. Goddard also conducts scientific investigations, develops and operates space systems, and works toward the advancement of space science technologies.

Guide Star

A star that a telescope’s guidance system locks onto to ensure that a celestial object is followed and observed as the telescope moves, owing either to the Earth’s rotation or the telescope’s orbital trajectory. The Hubble Space Telescope uses two of its three Fine Guidance Sensors to detect and lock onto guide stars. The telescope’s science operations center has more than 15 million guide stars in its database — the Guide Star Catalogue.


A gyroscope is a spinning wheel mounted on a movable frame that assists in stabilizing and pointing a space-based observatory. Gyroscopes are important because they measure the rate of motion as the observatory moves and help ensure the telescope retains correct pointing during observations. The gyroscopes provide the general pointing of the telescope while the fine guidance sensors provide the “fine tuning.” Gyroscopes are used in navigational instruments for aircraft, satellites, and ships. The Hubble Space Telescope has six gyroscopes for navigation and sighting purposes.

High Speed Photometer (HSP)

An original science instrument aboard the Hubble Space Telescope that made very rapid photometric observations of celestial objects in near-ultraviolet to visible light. The instrument was removed in December 1993 during the First Servicing Mission.

Hubble Space Telescope (HST)

An orbiting telescope that collects light from celestial objects in visible, near-ultraviolet, and near-infrared wavelengths. The telescope was launched April 24, 1990 aboard the NASA Space Shuttle Discovery. The 12.5-ton (11,110-kg), tube-shaped telescope is 13.1 m (43 ft) long and 4.3 m (14 ft) wide. It orbits the Earth every 96 minutes and is mainly powered by the sunlight collected by its two solar arrays. The telescope’s primary mirror is 2.4 m (8 ft) wide. The telescope is operated jointly by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). HST is one of the many NASA Origins Missions, which include current satellites such as the Far Ultraviolet Space Explorer (FUSE) and future space observatories such as the James Webb Space Telescope (JWST).

Image Intensifier

A device capable of intensifying light from a faint source so that it may be more easily detected.

Marshall Space Flight Center (MSFC)

NASA center overseeing the research, development, and implementation of three primary areas essential to space flight: reusable space transportation systems, generation and communication of new scientific knowledge, and management of all space lab activities. Located in Huntsville, Alabama, the center aided in the design, development, and construction of the Hubble Space Telescope.

Multi-Layer Insulation (MLI)

A “skin” or blanket of insulation covering the Hubble Space Telescope, which protects the observatory from temperature extremes. This insulation protects the telescope from the cold of outer space and also reflects sunlight so that the telescope does not become too warm. The MLI on Hubble is made up of many layers of aluminized Kapton, with an outer layer of aluminized Teflon.

National Aeronautics And Space Administration (NASA)

A Federal agency created on July 29, 1958 after President Dwight Eisenhower signed the National Aeronautics and Space Act of 1958. NASA coordinates space exploration efforts as well as traditional aeronautical research functions.

Near Infrared Camera And Multi-Object Spectrometer (NICMOS)

An instrument that sees objects in near-infrared wavelengths, which are slightly longer than the wavelengths of visible light. (Human eyes cannot see infrared light.) NICMOS is actually three cameras in one, each with different fields of view. Many secrets about the birth of stars, solar systems, and galaxies are revealed in infrared light, which can penetrate the interstellar gas and dust that blocks visible light. In addition, light from the most distant objects in the universe “shifts” into infrared wavelengths due to the universe’s expansion. By studying objects and phenomena in this spectral region, astronomers probe our universe’s past, present, and future; and learn how galaxies, stars, and planetary systems form. Astronauts installed NICMOS aboard the Hubble Space Telescope in February 1997 during the Second Servicing Mission.

New Outer Blanket Layer (NOBL)

Covers that protect Hubble’s damaged external blankets and help to maintain the telescope’s normal operating temperatures. The covers are made of specially coated stainless steel foil, which is trimmed to fit each particular equipment bay door.

Pickoff Mirror

One of four flat mirrors inside the Hubble Space Telescope. Each mirror is tilted at a 45-degree angle to the incoming light, diverting a small portion of it to the optical detectors or to one of the fine guidance sensors.


A light-sensitive picture element on a charge-coupled device (CCD) or some other kind of digital camera. A pixel is a tiny cell that, placed together with other pixels, resembles the mesh on a screen door. The Hubble Space Telescope’s Wide Field and Planetary Camera 2 has four CCDs, each containing 640,000 pixels. Each pixel collects light from a celestial object and converts it into a number. The numbers (all 2,560,000 of them) are sent to ground-based computers, which convert them into an image. The greater number of pixels, the sharper the image.

Primary Mirror

A large mirror in a reflecting telescope that captures light from celestial objects and focuses it toward a smaller secondary mirror. The primary mirror in the Hubble Space Telescope measures 94.5 inches (2.4 meters) in diameter.

Prime Focus

The location where light reflected from the primary mirror of a reflecting telescope comes into focus. Placing a secondary mirror in the light path allows the light to be focused elsewhere, in a more convenient location for the science instruments.

Rate Sensor Units (RSUs)

Boxes that house Hubble’s gyroscopes. Each rate sensor unit contains two gyroscopes. Astronauts remove the rate sensor units when they replace gyroscopes, so gyroscopes are always replaced two at a time.

Reaction Wheel

One of four spinning flywheels aboard the Hubble Space Telescope. The flywheels work together to make the observatory rotate either more rapidly or less rapidly toward a new target.

Secondary mirror

A small mirror in a reflecting telescope that redirects light from the larger primary mirror toward the light-sensitive scientific instruments. In a Cassegrain-type telescope like the Hubble Space Telescope, the secondary mirror is slightly convex and directs light from the primary mirror back through a hole in the center of the primary mirror.

Servicing Missions

Hubble was the first space telescope designed to be serviced in space. Scientists believed that periodic servicing missions would extend Hubble’s operating life and keep the observatory up-to-date. Astronauts have visited Hubble five times. The first servicing mission was in December 1993 and the second in February 1997. The third mission was split into two visits. Part A took place in December 1999 and part B in March 2002. The final servicing mission visit occurred in May 2009.

Soft Capture Mechanism

When Hubble reaches the end of its mission, NASA must be able to safely return the telescope to Earth. When that time comes, the space shuttle will no longer be operating, so another means of capturing the telescope must be available. The soft capture mechanism is a compact device that, when attached to the Hubble Space Telescope, will assist in its safe de-orbit. This device has structures and targets that will allow a next generation space vehicle to more easily capture and guide the telescope into a safe, controlled re-entry.

Solar Arrays

Two rigid, wing-like arrays of solar panels that convert sunlight directly into electricity to operate the Hubble Space Telescope’s scientific instruments, computers, and radio transmitters. Some of the energy generated is stored in onboard batteries so the telescope can operate while in Earth’s shadow (which is about 36 minutes out of each 97-minute orbit). The solar arrays are designed for replacement by visiting astronauts during servicing missions.

Space Shuttle

A reusable U.S. spacecraft operated by astronauts and used to transport cargo, such as satellites, into space. The spacecraft uses rockets to launch into space, but it lands like an airplane. A space shuttle carried the Hubble Space Telescope into space in 1990. Astronauts aboard subsequent space shuttles have visited the telescope to service it.

Space Telescope Imaging Spectrograph (STIS)

The Space Telescope Imaging Spectrograph (STIS) is a general-purpose spectrograph that spans ultraviolet, visible, and near-infrared wavelengths. It was installed in February 1997 during the Second Servicing Mission. A spectrograph works by breaking up light from an object into its individual wavelengths so that its composition, temperature, motion, and other chemical and physical properties can be analyzed. STIS stopped functioning in 2004 due to a power supply failure, but astronauts replaced a low-voltage power supply board during Servicing Mission 4.

Space Telescope Science Institute (STScI)

The astronomical research center responsible for operating the Hubble Space Telescope as an international scientific observatory. Located in Baltimore, Maryland, STScI is managed by AURA (Association of Universities for Research in Astronomy) under contract to the National Aeronautics and Space Administration (NASA) and the National Science Foundation (NSF).

Tracking and Data Relay Satellite System (TDRSS)

A network of four communication satellites used to relay data and commands to and from U.S. spacecraft, including the Hubble Space Telescope. The Goddard Space Flight Center provides the day-to-day management and operations of TDRSS, the first space-based global tracking system.

Wide Field Camera 3 (WFC3)

This new camera replaced the Wide Field and Planetary Camera 2 during Servicing Mission 4. WFC3 has the latest CCD (charge-coupled device) technology and optical coatings which provide a broader range of colors, spanning ultraviolet, visible, and near-infrared wavelengths. WFC3 will greatly enhance Hubble’s observational capabilities by studying a diverse range of objects and phenomena, from early and distant galaxy formation to nearby planetary nebulae, and finally our own backyard — the planets and other bodies of our solar system.

Wide Field / Planetary Camera (WF/PC)

A collection of eight separate, yet interconnected, cameras originally used as the main optical instrument on the Hubble Space Telescope. Four cameras were used in tandem to observe in either wide-field, low-resolution mode or narrow-field, high-resolution (“planetary”) mode. The Wide Field and Planetary Camera 2 replaced the WF/PC during the December 1993 servicing mission.

Wide Field and Planetary Camera 2 (WFPC2)

The Hubble Space Telescope’s “workhorse” instrument, WFPC2 snapped high-resolution images of faraway objects. Its 48 filters allowed scientists to study precise wavelengths of light and to sense a range of wavelengths from ultraviolet to near-infrared light. The instrument’s four CCDs (charge-coupled devices) collected information from stars and galaxies to make photographs. WFPC2 was installed aboard the Hubble telescope during the December 1993 servicing mission and was replaced by Wide Field Camera 3 in 2009 during Servicing Mission 4.

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