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National Aeronautics and Space Administration

Goddard Space Flight Center

Astrophysics Science Division | Sciences and Exploration

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Acronyms & Glossary
acronyms, glossary

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Aberration - Property of an optical system that causes an image to have certain easily recognizable flaws. Aberrations are caused by geometrical factors such as the shapes of surfaces, their spacing and alignments. Image problems caused by factors such as scratches or contamination are not called aberrations.

ACE - Actuator Control Electronics

ACK - Acknowledge Message

ACS - Advanced Camera for Surveys - This third generation instrument located in the -V2 Aft Shroud Axial Bay, consists of three electronic cameras/channels, the Wide Field Channel (WFC), the High Resolution Channel (HRC) and the Solar Blink Channel (SBC), and a complement of filters and dispersers that detect light from the ultraviolet at 1200 angstroms to the near infrared at 10,000 angstroms, with 10 times the efficiency of current instruments. ACS suffered a power supply failure in January 2007, leaving only the SBC channel functional. SM4 will attempt a repair of ACS. ACS Website

Acoustic Test Chamber - The 6-foot-wide horns in this 42-foot-tall chamber can produce noise at levels as high as 150 dB simulating payloads to the noise of launch.

Acquisition, target - Orienting the HST line of sight to place incoming target light in an instrument's aperture

Actuator - Small, high-precision, motor-driven device that can adjust the location and orientation of an optical element in very fine steps, making fine improvements to the focus of the image

AD - Aperture Door

AFM - Adjustable Fold Mirror

Aft - Rear of the spacecraft

Alignment - Process of mounting optical elements and adjusting their positions and orientations so that light follows exactly the desired path through the instrument and each optical element performs its function as planned altitude Height in space

Altitude - Height in space, height above a datum, the datum usually being mean sea level. Not the same as elevation, for altitude generally refers to points above the Earth's surface rather than those on it.

AMA - Actuator Mechanism Assembly

AME - Actuator Mechanism Electronics

Angstrom - Å - A unit of length equal to 0.00000001 centimeters - Wavelength

Anomoly - something out of the ordinary or unexpected, different from the norm.

APE - Articulating PFR Extender

Aperture - Opening that allows light to fall onto an instrument's optics. Hubble's aperture door is approximately 10 feet (3 m) in diameter covers the opening to the Telescope's light shield.

Aplanatic - Image corrected everywhere in the field of view

Apodizer - Masking device that blocks stray light

Apogee - The point farthest from the Earth in a body circling the Earth.

ARB - Anomoly Review Board

Arc minute (am) - 1/60th of a degree. Measure of angle. A full circle equals 360 degree. 1 degree equals 60 arcminutes. 1 arcminute equals 60 arcseconds.

Arcsecond (as) - A wedge of angle, 1/3600th of 1 degree, in the 360-degree "pie" that makes up the sky. An arcminute is 60 seconds; a degree is 60 minutes. - For the measurement of small angles, where degrees are too large. 1 Degree = 60 arc minutes; 1 arc minute = 60 arc seconds. Further divisions are possible by using the SI prefixes e.g. mas = milli arc second or 1/1000th of 1 arc second. The "arc" in arc second assures that this is a measure of angle, not time.

AS - Aft Shroud

ASCS - Aft Shroud Cooling System

ASE - Airborne Support Equipment

ASLR - Aft Shroud Latch Repair (kits) - This kit will fix door latches in the aft shroud area of the telescope that have been damaged by extreme temperature changes and high torques.

ASIPE - Axial Scientific Instrument Protective Enclosure

ASIS - Axial Scientific Instrument Simulator

Astigmatism - Failure of an optical system, such as a lens or a mirror, to image a point as a single point

Astrometry - Geometrical relations of the celestial bodies and their real and apparent motions

ATM - Auxiliary Transport Module

Attitude - Orientation of the spacecraft's axes relative to Earth.

  • Roll is the deviation from the vertical (the angle between the z-axis of the vehicle and the vertical axis, or angular rotation around the y-axis).
  • Pitch is the angular rotation around the x-axis.
  • Yaw is rotation around the z-axis.

AURA - Association of Universities for Research in Astronomy

Axial - Along the same line as an axis or centerline

Axial Bay - One of four ports for axial instruments on HST.

Axial Instrument - Science Instruments for HST are classified according to their shape and how they are mounted in the telescope. Axial instruments are about the size and shape of a telephone booth. There are a total of four of these instruments in HST at any one time. Currently the four axial instruments are: STIS, NICMOS, ACS and COSTAR. Axial instruments are mounted in the rear of the telescope along the optical axis. Light from the telescope's mirrors enters these instruments directly.

Axis - (plural = axes) In astronomy, the imaginary line through the poles about which a body (planet, star, moon, etc.) rotates.


Baffle - Material that extracts stray light from an incoming image. In the Hubble telescope, there are three principal baffles. The telescope tube itself makes sure that only light from the region of the sky of interest reaches the primary mirror (and not earth, moon or sunlight). There is also a baffle around the secondary mirror, so that stray light from the telescope tube doesn't reach the secondary mirror. Finally there is a baffle that projects up from the center of the primary mirror, and stops unwanted light from getting through into the instruments.

BAPS - Berthing and Positioning System - This is the unit that holds and maneuvers HST while it is berthed to the orbiter. The BAPS can orient the telescope at a variety of angles and can rotate the HST a full 360 degrees to bring any part of HST within the reach of the astronauts and the RMS, or robot arm.

Blue-shift - An apparent shift toward shorter wavelengths of spectral lines in the radiation emitted by an object caused by motion between the object and the observer which decreases the distance between them.

BPS - BAPS Support Post - This post is installed to keep the BAPS immobile during activities which generate high torques on the HST, for example reboost.

Bunny Suit - An outfit that is worn in the Clean room to protect the room and it's contents from the those who work inside of it, because workers present the major source of contamination. The suit includes a special coverall, boots, a hood (full coverage or with built-in facemask), and gloves. This gear helps protect the sensitive flight hardware from particles that could impede performance.


C - Celsius

C&DH - Command & Data Handling

Calibration - A process for translating the signals produced by a measuring instrument (such as a telescope) into something that is scientifically useful. This procedure removes most of the errors caused by environmental and instrumental instabilities.

CAPCOM - Capsule Communicator

Carriers - the means by which new instruments and tools are transported on board the Space Shuttle. As of 2008 there are four types of payload carriers; MULE, ORUC, FSS and SLIC. There are also various box carriers; SOPE, LOPE, SIPE, RSIPE, FSIPE.

Cassegrain - Popular design for large, two-mirror reflecting telescopes in which the primary mirror has a concave parabolic shape and the secondary mirror has a convex hyperbolic shape. A hole in the primary allows the image plane to be located behind the large mirror.

CASH - Cross Aft Shroud Harness

CAT - Crew Aids and Tools - as known as Space Support Equipment (SSE) hardware, range from a simple bag for carrying some of the smaller tools to sophisticated, battery-operated power tools.

CCC- Charge Current Controller

Charge Coupled Device (CCD) - Charge coupled devices are the detectors used in modern video cameras and in digital cameras. Like all electronic photosensors, they turn light (photons) into electric charges (electrons). CCD's have many characteristics which are desirable for astronomy. A good CCD has a better dynamic range than film, enabling a CCD based camera to record many levels of light and dark. CCD's are linear over much of their range, meaning that the intensity of the image may be measured. They are fast, and respond to a wide range of wavelengths.

Coronagraph - region within a telescope that blocks a bright object so the material close to it can be views.

CCS - Control Center System

CDI - Command data interface

CEA - Central Electronics Assembly

CEB - CCD Electronics Box

CEIT - Card Extraction/Insertion Tool, (2) Crew Equipment Interface Test - a hands-on preflight exercise

Change-out - Exchanging a unit on the satellite

Clean Room - In a clean room, the air quality, humidity and temperature are controlled to prevent contamination of sensitive material. Clean rooms, which are used in a variety of fields from health care to the electronics industry, are rated on a scale that ranges from 100,000 to one, indicating the amount of contamination by dust and other airborne particles that may be expected to be found within. Class 1 clean rooms are the most sanitary. Prior to entry, workers go through rigid procedures to remove contaminates and must wear special head to toe clothing, called a bunny suit.

cm - Centimeter

Collecting area - The amount of area a telescope has that is capable of collecting electromagnetic radiation. Collecting area is important for a telescope's sensitivity: the more radiation it can collect (that is, the larger its collecting area), the more likely it is to detect dim objects.

Collimate - To straighten or make parallel two light paths

Coma - Lens aberration that gives an image a "tail"

COMCON - Communication Contact

Concave - Mirror surface that bends outward to expand an image

Convex - Mirror surface that bends inward to concentrate on an image

Coronagraph - region within a telescope that blocks a bright object so the material close to it can be views.

COS - Cosmic Origins Spectrograph - Fourth Generation Spectrometer. COS is an ultraviolet spectrograph optimized for observing faint point sources with moderate spectral resolution.

Cosmology - The astrophysical study of the history, structure, and dynamics of the Universe.

COSTAR- Corrective Optics Space Telescope Axial Replacement - Second Generation Corrective Optics located in the -V2 Aft Shroud Axial Bay. COSTAR is not an actual instrument, it consists of mirrors which refocus the abberated light from the HST optical system for first generation instruments. Has not been used since the last first generation instrument was removed in 2002. COSTAR is to be replaced in SM4 with COS.

CPL - Capillary Pumped Loop

CPM - Central Processor Module

CPU - Central Processing Unit

Crew Aids - fixed-in-place or portable equipment items, other than hand tools, used to assist crew members in accomplishing servicing mission tasks. Crew aids permit the astronauts to maneuver safely or to anchor themselves in one location while working in the weightlessness of space. They also help in moving Orbital Replacement Units (ORUs) and Scientific Instruments (SIs), protect equipment and crew during the changeout activities, and provide temporary stowage of equipment during Extra Vehicular Activities (EVAs). Examples of crew aids are: handrails, handholds, transfer equipment, protective covers, tethering devices, grapple fixtures, foot restraint sockets, and stowage and parking fixtures.

CSS - Coarse Sun Sensor; Center Support Structure

CSSA - Coarse Sun Sensor Assembly

CU/SDF - Control Unit/Science Data Formatter

CVL - NICMOS Cryo Vent Line


Dark Energy - The residual energy in empty space which is causing the expansion of the Universe to accelerate.

Dark Matter - Mass whose existence is deduced from the analysis of galaxy rotation curves and other indirect evidence but which has so far escaped direct detection.

dB - Decibel

DBA - Diode Box Assembly (Solar Array)

DBC - Diode Box Controller

degree - Measure of angle. A right angle equals 90 degree. The apparent diameter of the full moon is about half a degree.

Diffraction grating - Device that splits light into a spectrum of the component wavelengths, usually glass or a polished metal surface having a large number of very fine parallel grooves or slits. A diffraction grating has thousands of narrow lines ruled onto a glass surface. It reflects rather than refracts light so no photons are lost. A common example of a diffraction grating is a CD where the pits encoding the digital information act as a grating and disperse light into a colourful spectrum.

DIU - Data Interface Unit

DMS - Data Management Subsystem

DMU - Data Management Unit

Doppler effect - The apparent change in wavelength of sound or light caused by the motion of the source, observer or both. Waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching, redshifted (elongated) if receding. It occurs both in sound and light. How much the frequency changes depends on how fast the object is moving toward or away from the receiver.

Drag, atmospheric - Effect of atmosphere that slows a spacecraft and forces its orbit to decay


ECA - Electronics Control Assembly

Echelle grating - a type of diffraction grating which is characterised by a relatively low grooves density but is optimized for high diffraction orders. Echelle gratings are used in spectrometers and similar instruments, such as High Accuracy Radial Velocity Planet (HARPS), and numerous other astronomical instruments.

ECU - Electronics Control Unit

Egress - Emerge, exit, to go out

EMI - Electromagnetic Interference

Electromagnetic Spectrum (EMS) - Light is an electromagnetic wave. Different colors of light correspond to different wavelengths. The Electromagnetic spectrum is divided into seven sections. From the longest wavelengths (least energetic) to the shortest (most energetic): radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma ray radiation. The Hubble Telescope has instruments which can see wavelengths from Near Ultra-violet (NUV) to the Near Infra-Red (NIR).

Electron - Low-mass particle carrying negative charge.

Ellipsoid - Surface whose intersection with every plane is an ellipse (or circle)

EPDSU - Enhanced Power Distribution and Switching Unit

EPS - Electrical Power Subsystem

EP/TCE - Electrical Power/Thermal Control Electronics

ERO - Early Release Observation

ESA - European Space Agency

Echelle Spectroscopy - In "traditional" spectrographs a dispersing element - typically a diffraction grating or prism - is used to produce the spectrum. This results in a single spectrum which can be imaged using a CCD or other type of camera. The data can then be extracted using a suitable program. The recordable part of the wavelength range covered by this type of spectrograph is limited by the size of available image sensors, i.e., CCDs. An echelle is a diffraction grating in which the rulings are much further apart than usual. This leads to spectra of very high dispersion, but only over a short wavelength range in each order. As well as being 'short', the high orders will overlap. Echelle spectrographs for astronomy are designed so that the wavelength coverage in one order will overlap the coverage of the adjacent orders.

EMI - electromagnetic interference

ESM - Electronics Support Module

E/STR -Engineering/Science Data Recorders

External Tank - the large orange fuel tank contains the liquid hydrogen fuel and liquid oxygen oxidizer and supplies them under pressure to the three space shuttle main engines in the orbiter during lift-off and ascent. + Read More

ETIF - Environmental Test and Integration Facilities - houses Goddard's centrifuge, acoustics and thermal vacuum chambers

EUV - Extreme Ultraviolet

eV - Electron Volt. The energy an electron has after being accelerated by a 1 volt potential. Quanta of visible light (photons) have energies of a few eV.

EVA - Extra Vehicular Activity - Outside the spacecraft; activity in space conducted by suited astronauts, also known as a spacewalk.


F - Fahrenheit

FGE - Fine Guidance Electronics

FGS -Fine Guidance Sensors - Science/Guidance instruments. The FGS's are used in a "dual-purpose" mode serving to lock on to "guide stars" which help the telescope obtain the exceedingly accurate pointing necessary for observation of astronomical targets. These instruments can also be used to obtain highly accurate measurements of stellar positions

FHST - Fixed Head Star Tracker

FITS -Flexible Image Transport Sytem- a standardized data format which is widely used in astronomy.

FLT - Flight

FM - Frequency Modulation; Flight Model; Folding Mirror

FOC - Faint Object Camera - First generation imaging camera. FOC is used to image very small field of view, very faint targets. FOC was replaced in 2002 by ACS.

Focal Length - The focal length of a lense is the distance from the lense to the point where it focuses parallel rays of light.

Focal Plane - Axis or geometric plane where incoming light is focused by the telescope. The imaginary surface at which the light to all of the HST instrument is focused.

Focal Point - The point toward which light rays converge to form an image after passing through a lens or having been reflected by mirrors. The condition of sharpest imagery.

FOS - Faint Object Spectrometer - First Generation Spectrometer. FOS was used to obtain spectra of very faint or far away sources. FOS also had a polarimeter for the study of the polarized light from these sources. FOS was replaced in 1997 by NICMOS.

FOSR - Flexible optical solar reflector

FOV - Field of View - The field of view of an astronomical instrument is the angular size of an image. Although the WFC3 instrument will have a large field of view, this is large compared to other instruments. The entire field of view of the Hubble telescope is comparable to the "O" in "One Dime" held at arm's length. Although this seems like a small area, images from astronomical targets are frequently smaller than this.

FPA - Focal Plane Assembly

FPS - Focal Plane Structure

FPSA - Focal Plane Structure Assembly

FRB - Fastener Retention Block; Failure Review Board

FRR - Flight Readiness Review

FS - Forward Shell

FSIPE - FGS Scientific Instrument Protective Enclosure

FSS - Flight Support System (Structure) - This is the name for the structure which holds HST and provides power and computer interfaces while it is berthed to an Orbiter during Servicing.

FUV - Far ultraviolet


GEO - Geosynchronous orbit. - A satellite in circular orbit around the Earth's equator at 23000 miles altitude (36000 km) will remain stationary over the same location on Earth (i.e., the spacecraft goes around once in its orbit for every revolution of the Earth). This feature is very useful for communications satellites (allowing one satellite to provide continual coverage to a given area of the Earth's surface). As a result, the majority of satellites in geostationary orbit are commsats. Some satellites have orbits slightly higher or lower than GEO, but for simplicity sake, all satellites with perigees and apogees between 30000 and 40000 km are termed GEO.

GHRS - Goddard High Resolution Spectrograph -First Generation Spectrograph. GHRS was used to obtain high resolution spectra of bright targets. It was removed during SM2 in 1997 and STIS now occupies that location. Parts of GHRS are being reused for COS.

GHz - Gigahertz (1,000,000,000 Hz)

GGM - Gravity Gradient Mode

Grating - A piece of material in which numerous microscopic parallel lines are scribed. Light encountering a grating is dispersed to form a spectrum.

GSE - Ground support equipment

GSFC - Goddard Space Flight Center - located in Greenbelt, Maryland, was established on May 1, 1959 and named after the father of modern rocket propulsion. GSFC is the home of the Hubble Space Telescope Project. For more information visit the GSFC website.

GSSS - Guide Star Selection System

GSTDN - Ground Spaceflight Tracking and Data Network


HEO - Highly Elliptical Orbit. This class covers orbits which have large eccentricities (are highly elliptical). Molnyia orbits are a common example of this type of orbit. Although definitions of HEO vary from source to source, MSL defines HEO as orbits having perigees below 3000 km with apogees above 30000 km.

HGA - High Gain Antenna - a parabolic reflector (dish) mounted on a mast with a two-axis gimbal mechanism and electronics to rotate it 100 degrees in either direction. The higher signal gain is required when transmitting high-data-rate scientific data. Because of their narrow beam widths, the HGAs must be pointed at the TDRSs. The HGA transmit over two frequencies: 2255.5 MHz or 2287.5 MHz.

HIS - HST Instrument Servicing

HOST mission - Hubble Orbital Systems Test (1998) - This mission tested several pieces of equipment in preparation for SM3A and SM3B. It is also known as the John Glenn mission.

HRC - ACS High Resolution Channel/Camera - Provides high angular resolution over a small field of view, covering a broad wavelength range.

HSP - High Speed Photometer - First Generation Photometer. This instrument was used to measure very fast brightness changes in diverse objects. HSP was removed in SM1 in 1993 to make room for COSTAR.

HST - Hubble Space Telescope - Hubble orbits 600 kilometers (375 miles) above Earth, using excellent pointing precision, powerful optics, and state-of-the-art instruments to provide stunning views of the Universe that cannot be made using ground-based telescopes or other satellites. Hubble was originally designed in the 1970s and launched in 1990. Designed to be serviced in orbit, there have been four servicing mission to date. The fifth and last planned servicing mission, SM4, is targeted for 2009.

Hubble constant - A measure of the rate of expansion of the universe. The average value of velocity of recession divided by distance. Determined by the Hubble project team to be between 70 km/s/Mpc ±7 km/s/Mpc.

Hubble time - The age of the universe, equivalent to 1 divided by the Hubble constant. The Hubble time is the age of the universe if it has expanded since the big bang at a constant rate.

Hyperbolic - extravagant exaggeration

Hyperboloidal - Slightly deeper curve, mathematically, than a parabola; shape of the primary mirror

Hz - Hertz (cycles per second)


Ingress - Spacecraft passageway, opening, a place of entering

IGM - InterGalactic Medium - the gas between galaxies

in. - Inch

Interglactic - Between galaxies

Interstellar - Between celestial objects; often refers to matter in space that is not a star, such as clouds of dust and gas

Intravehicular (IV) - Inside the spacecraft

IR - Infrared - Light that is so red humans cannot see it. A band of the electromagnetic spectrum between the visible and the radio waves on the electromagnetic spectrum. Photons of infrared light are less energetic than photons of visible light.

ISM - InterStellar Medium - the material which fills the space between the stars. Approximately 99% of the interstellar medium is composed of interstellar gas, and of its mass, about 75% is in the form of hydrogen (either molecular or atomic), with the remaining 25% as helium.

IVA - Intravehicular Activity

Instruments - The Hubble Space Telescope has two kinds of instruments: (1) imagers, which take pictures; and (2) spectrographs, which analyze light. Instruments are classified as either Axial or Radial based on their shape and location within the telescope.


Jitter - Small rapid variations in a variable (such as a waveform) due to deliberate or accidental electrical or mechanical disturbances or to changes in the supply voltages in the characteristics of components, etc.

JPL - Jet Propulsion Laboratory - located in Pasadena, California and born within the California Institute of Technology (CALTECH), was established in the mid 1930's prior to the formation of NASA and was considered part of the United States Army Air Corps and tasked with developing US rocket technology prior to World War II. In December 1958, the Army formally transferred JPL to NASA, although it remains under Caltech management. For more information visit the JPL website.

JSC - Johnson Space Center, Houston Texas. established in 1961 as the Manned Spacecraft Center. In 1973, the Center was renamed in honor of the late President and Texas native, Lyndon B. Johnson. JSC is home to the NASA astronaut corps. The Mission Control Center (MCC) at JSC directs all space shuttle missions and also manages all activity onboard the international space station. JSC is the home of the Neutral Bouyance Lab. For more information visit the JSC website.


k - Kilo - a thousand

kb - Kilobytes

Kelvin (K) - One Kelvin degree is equivalent to one Celsius degree. The difference between the two temperature scales: All motion within an atom ceases at zero Kelvin - this point is called absolute zero.

keV - kilo electron Volt - A unit of energy equal to one thousand. X-ray photons have energies of 0.1-100 keV.

kg - Kilogram - The fundamental unit of mass.

km - Kilometer

KSC - Kennedy Space Center, Cape Canaveral, FL - In July 1962, NASA established its Launch Operations Center on Florida's east coast, and renamed it in late 1963 to honor the president who put America on the path to the moon. This spaceport has served as the departure gate for every American manned mission and hundreds of advanced scientific spacecraft. For more information visit the KSC website.

K band - A portion of the electromagnetic spectrum in the microwave region of frequencies ranging between 12 and 93 gigahertz (GHz). The 18-40 GHz range is used for radar and general communications. The 12-18 GHz region is used for satellite communications.

KU-Band - 10.90 to 36.00 GHz


L band - A frequency between 390 MHz and 1.55 GHz which is used for satellite communications and for terrestrial communications between satellite equipment.

Latch - Mechanical device that attaches one component, such as a science instrument, to the structure of the telescope and holds it in precisely the right place

lb - Pound

LCC - Launch Control Center - is the electronic "brain" of Launch Complex 39 at KSC. + Read More

LCH - Launch

LEO - Low Earth Orbit - Although definitions of LEO vary from source to source, MSL defines LEO as orbits having apogees and perigees below 3000 km. The large majority of all satellites are in Low Earth Orbit. Hubble orbits 600km (375 miles) above earth and is therefore a LEO satellite.

LGA - Low Gain Antenna - Spiral cones that provide spherical (omnidirectional) coverage. They are set 180 degrees apart on the light shield and aft bulkhead of the spacecraft. The LGAs receive ground commands and transmit engineering data. These antennas are used for all commanding of the Telescope and for low-data-rate telemetry and operate in a frequency range of 2100 MHz to 2300 MHz.

LGA PC - Low Gain Antenna Protective Cover

Light gathering (light collecting) power - The ability of a telescope to collect light. Proportional to the area of the telescope's objective lense or mirror.

Light speed - 186,000 mps

Light year - The distance traveled by light in 1 year, approximately 6 trillion miles (10 trillion kilometers)

LON - Launch On Need

Long Slit Spectroscopy - A technique used to obtain both spatial and spectral information at the same time, a long slit which blocks most of the viewing field, allowing only a narrow strip of light to pass through. The light then passes through a dispersive device such as a prism or diffraction grating which breaks it into its component wavelengths.

LOPE - Large ORU Protective Enclosure

LOS - Line of sight

LS - Light Shield

Luminosity - Intensity of a star's brightness, The total amount of energy a star radiates in on second.

LVPS - Low-Voltage Power Supply


m - Meter - the fundamental unit of length; Million

M - Absolute visual magnitude

µm - Micrometer; 1 millionth of a meter

mm - Millimeter

MA - Multiple access

Magnitude, absolute - How bright a star appears without any correction made for its distance

Magnitude, apparent - How bright a star would appear if it were viewed at a standard distance

Magnitude scale - The astronomical brightness scale. The larger the number, the fainter the star.

MAMA - Multi-Anode Microchannel Array- MAMAs are high-technology photom-counting devices developed specifically for low-noise FUV/NUV applications.

MAT - Multiple Access Transponder

MCC - Mission Control Center

MCP - Microchannel plate

MEB - Main Electronics Box

MEO - Medium Earth Orbit - Although definitions of MEO vary from source to source, MSL defines MEO as orbits having apogees greater than 3000 km but less that 30000 km. These are sometimes used by navigation (e.g., GPS) and communications (e.g., Odyssey) missions.

MET - Mission Elapsed Time

Metrology - Process of making extremely precise measurements of the relative positions and orientations of the different optical and mechanical components

MeV - Mega electron Volt - A unit of energy equal to one million eV. Gamma-ray photons are those with energies greater than 0.1 MeV, equal to 100 keV.

MFR - Manipulator Foot Restraint

MHz - Megahertz - 1 million Hertz

min - Minute

MLI - Multi-layer insulation - This is the highly reflective material which is attached to spacecraft to act as a heat barrier. It consists of many sheets of 25 mm thick polyester or polyamide layers sewn together. It is often silver or gold in color.

MLP - Mobile Launch Platform - Link to more information

Mpc - Megaparsec - 1 million parsecs

MOPE - Multimission ORU Protective Enclosure

MPS - miles per second

ms - Millisecond (1 thousandth of a second)

MSFC - Marshall Space Flight Center - located at the Redstone Arsenal in Huntsville, Alabama, established in the summer of 1960 and named for the late General George C. Marshall. It was the home of the US rocket experts after WWII. Shortly before it opened, NASA described it as "The only self-contained organization in the nation which was capable of conducting the development of a space vehicle from the conception of the idea, through production of hardware, testing and launching operations." For more information visit the MSFC website.

MSL - Mission and Spacecraft Library

MSM - Mode Selection Mechanism - part of the STIS instrument optical system

MSS - Magnetic Sensing System

MT - Magnetic torquer

MTA - Metering Truss Assembly

MTL - Multisetting Torque Limiter - tool to prevent damage to hardware due to the application of torque which may exceed design limits. Multisetting torque limiters are used in conjunction with the power tools or hand tools that interface with bolts and latches on the telescope.

MTS - Metering Truss Structure

MULE - Multi-Use Lightweight Equipment carrier


NASA - National Aeronautics and Space Administration - created on October 1, 1958, "to provide for research into the problems of flight within and outside the Earth's atmosphere and for other purposes." Headquartered in Washington D.C. and with multiple facilities located throughout the US. View Center/Facilities Map.

NBL - Neutral Buoyancy Laboratory - located within the Sonny Carter Training Facility (SCTF) at JSC, provides controlled neutral buoyancy operations to simulate the zero-g or weightless condition that is experienced by spacecraft and crew during space flight. It is an essential tool for the design, testing and development of the International Space Station and future NASA programs. For the astronaut, the facility provides important pre-flight training for extravehicular activities (EVA) and with the dynamics of body motion under weightless conditions.

NASCOM - NASA Communications Network

NCC - Network Control Center; NICMOS Cryo-Cooler

NCS - NICMOS Cooling System - This mechanical cooler which was tested during the HOST mission and installed during SM3B, allowing NICMOS to resume science operations.

Nebula - Mass of luminous interstellar dust and gas, often produced after a stellar nova

Neutron - An atomic particle with no charge and about the mass of a proton.

NGST - Next Generation Space Telescope

NICMOS - Near Infrared Camera and Multi-Object Spectrometer - Second Generation Imager/Spectrograph located in the +V2 Aft Shroud Axial Bay. NICMOS is HST's only NIR instrument.

NIR - Near Infrared - Near Infrared light is not visible to human eyes, but many celestial objects shine brightly with this light. Typically associated with heat, NIR images show the presence of molecules and complex compounds. The preferred term for the shorter wavelengths in the infrared region (the entire infrared region extends from about 0.7 µm (visible red) to about 3 µm). The longer wavelength end grades into the middle infrared, sometimes called solar infrared, as it is only available for use during the daylight hours. Also known as the shortwave infrared (SWIR).

nm - Nanometers - 1/1,000,000,000 of a meter - When wavelengths of light are extremely small, units like nanometers (billionths of meters) are frequently used. Visible light is 400-700 nm

NM- Nautical miles

NOBL - New Outer Blanket Layer - The NOBL are stainless steel panels covered with a protective thermal coating. These panels fit over existing, degraded insulation on Hubble's exterior surface, to control Hubble's internal temperature. In 1997, astronauts on Servicing Mission 2 discovered damaged areas of insulation and performed temporary repairs. The NOBL are permanently mounted.

Nova -From the Latin, meaning "new", a Star that suddenly becomes explosively bright

NPE - NOBL Protective Enclosure

NSSC-I - NASA Standard Spacecraft Computer, Model-I - manages the Hubble science instruments. It controls the commanding to the instruments and controls their data output. It also monitors the instruments for problems.

NSTS - NASA Space Transportation System (Shuttle)

NT - NOBL Transporter

NUV - Near UltraViolet - The longest wavelengths of the ultraviolet region, nominally 300 to 400 nm.


OBSS - Orbiter Boom Sensor System - The OBSS consists of sensors and cameras on the end of a boom system that is launched installed on the starboard sill of the shuttle. The boom is used in conjunction with the Shuttle Remote Manipulator System (SRMS), also referred to as the Robotic Arm, to inspect the shuttle for damage while in space.

Occultation - Eclipsing one body with another

OCE - Optical Control Electronics

OCE-EK - OCE Enhancement Kit

OCS - Optical Control Subsystem

OPF - Orbiter Processing Facility - Located at KSC is where the shuttle and its payloads are processed both before and after launch. + Read more

OPS - Operations

Orbit - The path a body takes around another object or point in space under the influence of various physical forces, including gravity. There are several categories of orbits. (See - LEO, MEO, HEO, VHO, GEO).

Orbiter - What most call the Shuttle is about the same size and weight as a DC-9 aircraft, the Orbiter contains the pressurized crew compartment, the huge cargo bay, and the three main engines mounted on its aft end. + Read More. Visit this link for more information on the Orbiter fleet.

Orientation - Position in space relative to Earth

ORU/ORI Orbital Replacement Unit / Orbital Replacement Instrument - Since HST was designed to be serviced on-orbit, many of the systems, and all of the instruments were designed and built as replaceable units. Over 70 items on HST can be replaced in-orbit and range in size from a shoebox to a telephone booth, most of the items can be removed or installed with the aid of wrenches and screwdrivers. Visit this link for a list of a few ORUs

ORUC - Orbital Replacement Unit Carrier - Special boxes that provide stowage and environmental protection and isolate the delicate instruments and other ORUs from the forces and vibrations of launch and ascent to orbit. See also SOPE and LOPE.

OSS - Office of Space Science, NASA Headquarters

OTA - Optical Telescope Assembly - two mirrors (primary and secondary) and associated structures that collect light from celestial objects and concentrates the incoming light in the focal plane for use by the science instruments.


PAO - Public Affairs Office

Parabola - The set of all points (x,y) that are the same distance from a fixed line (called the directrix) and a fixed point (focus) not on the directrix.

Parallax - Change in the apparent relative orientations of objects when viewed from different positions

Parsec - The distance to an object that has a parallax of one arcsecond (equivalent to 3.26 light years).
A kiloparsec (kpc) is equal to 1000 parsecs. A megaparsec (Mpc) is equal to a million parsecs.

Payload (PL) - Cargo being transported by a vehicle. Context - The Hubble Telescope was a payload carried in the Space Shuttle's cargo bay, removed from the cargo bay and placed into its own orbit around the Earth. The formal designation as a "payload" indicates that the experiment will be accorded top priority in crew time and energies during the entire flight, along with all other experiments carrying the same"payload" designation.

PC - Planetary camera

PCB - Printed Circuit Board

PCEA - Pointing Control Electronics Assembly

PCS - Pointing Control System/Subsystem

PCU - Power Control Unit - Hubble's power switching station. The PCU controls and distributes electricity from the solar arrays and batteries to other parts of the telescope.

PDA - Photon Detector Assembly

PDM - Primary Deployment Mechanism (Solar Array)

PDU - Power Distribution Unit

Perigee - The point nearest to the Earth in a body circling the Earth, opposite of apogee.

PFR - Portable Foot Restraint; Prime Focal Ratio

PGT - Pistol Grip Tool - a self-contained, microprocessor-controlled, battery-powered 3/8 inch drive hand-held power tool with a pistol-style handle. It also can be used as a non-powered ratchet wrench. The PGT's micro processor can be programmed to control limits for torque, speed, number of turns and angle.

Plasma - A fourth state of matter -- not a solid, liquid or gas. In a plasma, the electrons are pulled free from the atoms and can move independently. The individual atoms are charged, even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality.

Photon - The smallest (quantum) unit of light/electromagnetic energy. Photons are generally regarded as particles with zero mass and no electric charge.

PIT - Post Installation Test

Pixel - Single picture element of a detection device

PKG - Package

PO - Pick Off

POCC - Payload Operations Control Center

Polarity - Light magnetized to move along certain planes. Polarimetric observation studies the light moving along a given plane.

Primary mirror (PM) - Large mirror in a reflecting telescope the size of which determines the light-gathering power of the instrument. The primary mirror of the Hubble telescope measures 2.4 m (8 ft) in diameter and weighs about 826 kg (1820 lbs). It is constructed of ultra-low expansion silica glass and coated with a thin layer of pure aluminum to reflect visible light. A thinner layer of magnesium fluoride is layered over the aluminum to prevent oxidation and to reflect ultraviolet light.

Prism - Device that breaks light into its composite wavelength spectrum

PRT - Power Ratchet Tool - powered by a 28-volt battery. Made of titanium and aluminum, the 17-inch (43 cm) tool will be used for tasks requiring controlled torque, speed or turns, and can be used where right-angle access is required.

PSI - Pounds per Square Inch

PSEA - Pointing/Safemode Electronics Assembly

PSO - HST Project Science Office at GSFC


Quasar - Quasi-stellar object of unknown origin or composition. Enormously bright objects at the edge of our Universe that emit massive amounts of energy and are likely powered by black holes.


RAC - Rigid Array Carrier - One of the transport modules used in the shuttle's bay.

Radial - Perpendicular to a plane (i.e., instruments placed at a 90-degree angle from the optical axis of the HST)

Radial Bay - One of four ports for radial instruments on the HST.

Radial Instrument - Science Instruments for HST are classified according to their shape and how they are mounted in the telescope. 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, rather than directly like axial instruments. There are a total of four radial instruments on HST. Three of the radial instruments are the FGS's and the fourth is WFPC2. WFC3 will replace WFPC2 in SM4.

RAM -Random-Access Memory

RBM - Radial Bay Module

RDA - Rotary Drive Actuator

Reboost - To boost a satellite back into its original orbit after the orbit has decayed because of atmospheric drag

Red-shift - An apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by the emitting object moving away from the observer.

Reflecting telescope - Telescope that uses mirrors to collect and focus incoming light

Refracting telescope - Telescope that uses lenses to collect and focus light

Resolution - Ability to discriminate fine detail in data. A measure of the ability to separate observable quantities. In the case of imagery, it describes the area represented by each pixel of an image or the ability to distinguish two objects very close together in space. In a spectrum, it is the ability to measure closely separated wavelengths. High Resolution has a large amount of fine detail.

Resolution, spectral or frequency - Determines how well closely spaced features in the wavelength spectrum can be detected. Spectral Resolution is the ability of the telescope to differentiate two light signals which differ in frequency by a small amount. The closer the two signals are in frequency while still allowing the telescope to separate them as two distinct components, the higher the spectral resolution of the telescope.

Resolution, angular - Determines how clearly an instrument forms an image

RF - Radio frequency

RGA - Rate Gyro Assembly

RIAF - Radial Instrument Alighment Fixture (WFPC)

Ritchey-Chretien - A modern optical design for two-mirror reflecting telescopes. It is a derivative of the Cassegrain concept in which the primary mirror has a hyperbolic cross section.

RIU - Remote Interface Unit

RMGA - Retrieval Mode Gyro Assembly

RMS - Remote Manipulator System - the shuttle robotic arm, about the size of a Greyhound bus - 15.2-meter (50-foot) long articulating arm remotely controlled from the flight deck of the Orbiter

RNS - Relative Navigation System

Rollback - when the Space Shuttle must be rolled back from the launch pad atop the Mobile Launcher Platform and Crawler-Transporter to the Vehicle Assembly Building (VAB)

ROM - Read-only memory

RS - Reed-Solomon Error Correction Encoding

RSIPE - Radial Science Instrument Protective Enclosure

RSU - Rate Sensor Unit - The gyroscopes are part of Hubble's pointing system. They provide a frame of reference for Hubble to determine where it is pointing and how that pointing changes as the telescope moves across the sky. They report any small movements of the spacecraft to Hubble's pointing and control system computers. The computers then command the spinning reaction wheels to keep the spacecraft stable or moving at a desired rate. The gyroscopes work by comparing Hubble's motion relative to the axis of the spinning masses inside the gyroscopes. In the absence of external forces, these axes remain stable relative to the fixed stars in the sky. By keeping Hubble fixed relative to these axes, Hubble stays stable relative to the stars.

RWA - Reaction Wheel Assembly


S band - A radio frequency band extending from approximately 2.0 to 4.0 gigahertz. It is part of the microwave portion of the electromagnetic spectrum. 1500 - 5200 Mhz

SA - Solar Array

SA3 - Solar Array 3 - Hubble's new solar arrays installed on servicing mission 3B. Although 45 percent smaller than its predecessors, they will produce 25 percent more power.

SAA - South Atlantic Anomaly - The SAA is a broad region, extending roughly from the west coast of South America, across the Atlantic ocean to the southern tip of Africa, in which the Earth's magnetic field dips downward to relatively low altitudes. This results in a much higher than average number of charged particles (mostly electrons) encountered by satellites in low Earth orbit as they pass through the SAA. This trapped particle radiation typically induces high levels of background noise in light sensors and other electronic components.

SAC - Second Axial Carrier - One of the transport modules used in the shuttle's bay; Solar Array Carrier

SADA - Solar Array Drive Assembly

SADE - Solar Array Drive Electronics

SADM - Solar Array Drive Mechanism

SAGA - Solar Array Gain Augmentation

SBA - Secondary Baffle Assembly

SBC -ACS Solar Blind Channel/Camera - designed for observing faint UV emission from sources bright at visible wavelengths

SCM - Soft Capture Mechanism - To be installed on Hubble's aft bulkhead for use in future deorbiting mission

SCP - Stored Command Processor

SCRS - Soft Capture and Rendezvous System - Comprised of two separate pieces the Soft Capture Mechanism (SCM) and the Relative Navigation System (RNS)

SDAS - Science Data Analysis Software

SDI - Science Data Interface

SDM - Secondary Deployment Mechanism

sec - Second - the fundamental unit of time

Secondary mirror - In a two-mirror reflecting telescope, the secondary mirror sits in front of the larger primary mirror and reflects light to the point at which it will be detected and recorded by an instrument. In simple telescopes, the secondary mirror is flat and bounces the light out the side of the ube to an eyepiece. In more complex and larger telescopes like Hubble, it is convex and reflects light through a hole in the primary mirror to the focal plane.

Servicing Mission (SM) - A servicing mission is a Space Shuttle flight during which repairs and upgrades are performed on the Hubble Space Telescope. Originally planned to occur at approximately three-year intervals to replace scientific instruments, as well as batteries and other limited-life items, several factors have affected mission scheduling including unexpected instrument failure, shifting NASA priorities and shuttle flight tragedies.

Servicing Mission Integrated Timeline (SMIT) - a detailed, minute-by-minute schedule of mission activities.

SES - Space Environment Simulator

SI - Science Instrument

SIC&DH - SI Control and Data Handling (subsystem)

SIFIG - Science instrument Fine Guidance System

SIPE - Science Instrument Protective Enclosure

SLIC - Super Lightweight Interchangeable Carrier

SMA - Secondary Mirror Assembly (OTA)

Servicing Mission Orbital/Observatory Verification (SMOV) - procedures to recommision HST; involving thorough engineering checkout of all serviced subsystems, optical alignment and focus, and initial calibration of instruments; beginning upon HST release and lasting approximately 18 weeks.

SM1 - First HST Servicing Mission, December 1993

SM2 - Second HST Servicing Mission, February 1997

SM3A - HST Servicing Mission 3A, December 1999

SM3B - HST Servicing Mission 3B, February 2002

SM4 - HST Servicing Mission 4

Selectable Optical Filter Assembly (SOFA) - This is the name of the carousel which was removed form WF/PC (1) and which held the filters used on orbit in this instrument. By re-using legacy equipment in this way, development costs are held down for WFC3.

SOGS - Science Operations Ground System

SOL - Speed of Light - The speed of light in a vacuum (the way the term is normally used) is defined as about 300,000,000 m/s (186,000 miles/second). It is considered to be the fastest speed anything can reach. It is expressed as "c" in Einstein's famous equation: E=mc2

SOPE - Small ORU Protective Enclosure

SPA - Solar Panel Assembly

Spectral band - An interval in the electromagnetic spectrum defined by two wavelengths, frequencies, or wave numbers. With Landsat, bands designate the specific wavelength intervals at which images are acquired. I.e., a band is a slice of wavelengths from the electromagnetic spectrum.

Spectral class or type - A star's position in the temperature classification system O, B, A, F, G, K, M. Based on the appearance of the spectrum. The following table shows some of the spectral characteristics of stars, and how they're related to temperature and color:

Spectral Class
Spectral lines
Temperature (K)
Ionized helium, other ionized atoms
> 25,000
some hydrogen
10,000 - 25,000
some ionized metals
7,500 - 10,000
hydrogen and ionized metals (like calcium)
6,000 - 7,500
ionized calcium, neutral and metals (like iron)
5,000 - 6,000
neutral metals, some molecular bands
3,500 - 5,000
strong molecular bands (like titanium oxide)
< 3,500
Note: 1) here, "metals" are elements heavier than helium. 2) most star colors are pale, which is characteristic for hot, glowing objects.

Spectral devices - These include spectrographs, instruments that photograph the spectrum of light within a wavelength range; spectrometers, which measure the position of spectral lines; and spectrophotometers, which determine energy distribution in a spectrum.

Spectrograph - An optical device for breaking light down into a spectrum and recording the results photographically. Light entering a spectrograph can be split or dispersed into a spectrum by one of two means, using a prism or a diffraction grating.

Spectroscope - A telescope instrument that breaks light up into its constituent wavelengths and allows quantitative measurements of intensity to be made.

Spectrum - The distribution of energy wavelengths and frequencies. If white light is passed through a prism, it is broken into many colors, like a rainbow, called a spectrum. The spectrum of a star or luminous source can tell us many things about it, like temperature, composition, and velocity. See Electromagnetic Spectrum. (plural = spectra)

Spherical aberration - Image defect caused by a mismatch in the shapes of the reflecting surfaces of the primary and secondary mirrors. Light from different annular regions on the primary mirror comes to a focus at different distances from the secondary mirror, and there is no one position where all of the light is in focus.

SRB - Solid Rocket Booster - the two SRBs provide the main thrust to lift the space shuttle off the pad. + Read More

SRMS - Shuttle Remote Manipulator System - also referred to as the Robotic Arm - see RMS

SSAT - S-band Single-Access Transmitter - This replacement transmitter uses radio waves to send data to the ground. The older unit it replaces will be returned to Earth and refurbished for a later flight.

SSC - Science Support Center

SSDF - SSDIF - Spacecraft Systems Development (and Integration) Facility - This building is located at GSFC. The SSDIF is a 7,989 m2 (86,000 ft2) facility designed to provide support for the integration and testing of spacecraft hardware. It is unique in the fact that it contains a 36,816m3 (1.3M ft3) horizontal, unidirectional High Bay Clean Room. This facility houses the world¡¦s largest Class-10,000 clean room, this clean room plays an important role in Hubble servicing. Additional features include: Automated Data Processing Area, Shipping/Receiving Area, Flight Hardware Storage Area, and Precision Cleaning facilities.

SSE - Space Support Equipment - The Carriers, Tools and Crew Aids used by the crew members to carry out the servicing of HST.

SSM - Support Systems Module - The SSM encloses the aft portion of the OTA and contains all of the structures, mechanisms, communications devices, electronics and electrical power subsystems needed to operate the telescope.

SSME - Space Shuttle Main Engine - reusable, high-performance, liquid-propellant rocket engines with variable thrust. Part of the Main Propulsion System that also includes the Solid Rocket Boosters and the External Tank. Each orbiter has 3 SSMEs. + Read More

SSM-ES - SSM Equipment Section

SSR - Solid State Recorder - Hubble's original data recorders were mechanical, reel-to-reel tape recorders with many moving parts that wear out over time. The digital SSR has no moving parts or tape to break, so it is much more robust. This next-generation recorder is faster and more reliable, and it can store 10 times as much data as a mechanical recorder

SSRF - Shell/Shield Repair Fabric - - Sheets of flexible, aluminized TeflonR fabric that fit over the original multi-layer insulation on Hubble's forward shell and light shield to add thermal protection. In 1997, astronauts on Servicing Mission 2 discovered damaged areas of insulation and performed temporary repairs. The SSRF will be permanently mounted over these temporary coverings.

SSS - Space Shuttle System - consists of four primary elements: an orbiter spacecraft, two Solid Rocket Boosters (SRB), an external tank to house fuel and oxidizer, and three Space Shuttle main engines.

STDN - Space (flight) Tracking and Data Network

STIS - Space Telescope Imaging Spectrograph -Second Generation Imager/Spectrograph located in the +V2 Aft Shroud Axial Bay. STIS is used to obtain high resolution spectra of resolved objects. The special ability of STIS is to simultaneously obtain spectra from many different points along the target. .

STOCC - Space Telescope Operations Control Center - Located at Goddard Space Flight Center, the STOCC is where all commanding to the HST originates.

STS - Space Transportation System - name for the overall Shuttle program. The SM4 is designated STS-125.

STSci - Space Telescope Science Institute - The STSci is located in Baltimore, Maryland at the John's Hopkins University and is the home of HST scientists. The STSci maintains all operations for the telescope including proposal selection, scheduling, data calibration, distribution, and archiving. - The STSci is responsible for allocating observing time and for calibration, data storage, retrieval, and distribution of science data. STSci also provides software tools for manipulating the data.


TA - Translation Aids

TAG - Two-Axis Gimbal

TAV - Target Acquisition and Verification

TCE - Thermal Control Electronics

TCS - Thermal Control Subsystem

TDRS - Tracking and Data Relay Satellite


TECI - Thermoelectric-cooled inner (shield)

TECO - Thermoelectric-cooled outer (shield)

Thermal band - A general term for intermediate and long wavelength infrared-emitted radiation, as contrasted to short wavelength reflected (solar) infrared radiation. In practice, generally refers to infrared radiation emitted in the 3 to 5 £gm and 8 to 14 £gm atmospheric windows.

TLM - Telemetry - Radio signals (data and commands) sent to and from a spacecraft used to encode and exchange data with a ground station.

Trajectory - The curve described by a projectile in flight.

Thermal Vacuum Chamber (GSFC) - Aside from removing all but the smallest trace of air, Goddard's Thermal Vacuum Chamber can chill a payload down to minus 310 F, or heat it to a sizzling 302 F.


UDM - Umbilical disconnect mechanism

UHF - Ultra High Frequency

ULE - Ultralow expansion

UV - Ultraviolet - A band of the electromagnetic spectrum between the visible and the X-ray. Photons of ultraviolet light are more energetic than photons of visible light. So named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet.

UVIS - Ultraviolet Imaging Spectrograph


V - Volts

V1, V2, V3 - HST axes -The primary axis, V1, runs through the center of the Telescope. The other two axes parallel the SA masts (V2) and the high gain antenna (HGA) masts (V3). Pointing instruments use references to these axes to aim at a target in space, position the SA or change Telescope orientation in orbit.

VAB - Vehicle Assembly Building - located at KSC its where the integration and stacking of the complete Space Shuttle vehicle is done. + Read More

VCS - Vapor-cooled shield

Velocity - The speed and direction in which an object moves.

VEST - Vehicle Electrical Systems Test (Facility) - Located in the GSFC cleanroom, the Hubble high fidelity mock-up trains astronauts on installation and removal of flight hardware components.

VHF - Very High Frequency

VHO - Very High Orbit. This type of orbit includes all orbits which have perigees at or above GEO and apogees above GEO, yet remain in orbit around the Earth (or Earth-Moon system). Orbits in this class are often highly elliptical, with apogees several hundred thousand kilometers in altitude.

VIK Voltage/Temperature Improvement Kit - These protect Hubble's batteries from overcharging and overheating when in safe mode


W- Watt

WAG - "Wild Ass" Guess

Wavelength - Spectral range of light in an image, The distance between successive peaks or troughs of a wave. Usually represented by the greek letter lambda.

WFC - ACS Wide Field Channel/Camera - designed for efficient wide-area surveys at visible wavelength

WF/PC - Wide Field / Planetary Camera - First Generation Imaging camera. WF/PC operated in either Wide Field mode, capturing the largest images, or Planetary mode with higher resolution.

WFPC2 - Wide Field Planetary Camera 2 -. Second-generation instrument. This dual-channel (visible and near-IR) camera was installed during Servicing Mission 1 in December 1993. It replaced WFPC1 and was built with optics to compensate for the spherical aberration of the primary mirror.

WFC3 - Wide Field Camera 3 - Fourth Generation Imaging camera. This camera will supplement ACS and guarantee imaging capability for HST after the fourth Servicing Mission. .


X band - A radio frequency band extending from approximately 8.0 to 12.5 gigahertz. It is part of the microwave portion of the electromagnetic spectrum and is used for some communications satellites and by X-band radar primarily for science and technology applications.

X-ray: Light that is so blue humans cannot see it. A band of the spectrum between the ultraviolet and the gamma-ray. Photons of X-ray light are more energetic than photons in the ultraviolet but less energetic than photons in the gamma-ray. X-radiation can go through human skin tissue but is stopped by dense bones. This property thus makes X-rays valuable in medicine.