APPENDIX
Glossary
AC
Advanced Camera: a generic name for a number of different proposals to develop a third-generation camera to replace one of the Hubble Space Telescope’s first or second-generation instruments in the late 1990s. In 1995, NASA selected the proposal called the Hubble Advanced Camera for Exploration.
Active optics
A technique to take account of slowly varying forces, such as gravitational deflections and temperature drifts, that can distort a mirror on time scales of minutes to hours, resulting in imperfect images. This is usually implemented by means of a lookup table that provides values for actuator signals as a function of temperature and telescope orientation. These tables are initially generated by making observations of wavefronts from a distant source at a variety of temperatures and orientations.
Adaptive optics
A set of techniques to adjust the shape of mirrors on time scales of a fraction of a second to correct for rapid fluctuations in image quality. In the astronomical community, this term is used for systems that correct for distortions in images of ground-based telescopes due to atmospheric turbulence (“seeing”). In the DOD community there can be other local sources of distortion, such as in laser weapons systems, and the term is used to cover a wider range of applications. In both communities, the essential feature of the implementation is use of real-time sensing of the wavefront from a distant source to provide the signals to the actuators that control the shape of the mirror.
Afocal
An optical system that receives parallel rays of light from a distant object and outputs parallel rays of light at a different magnification
ALOT
Adaptive Large-Optics Technologies: a DOD-sponsored project at Itek Optical Systems that developed a complete, lightweight, 4-meter space telescope equipped with an advanced adaptive-optics system
Anastigmat
An optical system that does not suffer from common optical defects such as coma, astigmatism, or spherical aberration
Aperture
The diameter of the primary lens or mirror of a telescope; hence, the simplest single measure of the light-gathering power of a telescope
Aphelion
The point at which a body in orbit around the Sun reaches its farthest distance from the Sun
Apogee
The point at which a body in orbit around Earth reaches its farthest distance from Earth
Arc minute
A unit of angle corresponding to 1/60th of a degree. The full moon is 30 arc minutes in diameter.
Arc second
A unit of angle corresponding to 1/3600th of a degree; 1/60th of an arc minute. An arc second is approximately the size of a penny viewed from about 2.5 miles.
Array
Astronomical instruments that have recently been fabricated using new electronic components called detector arrays that consist of thousands of individual detectors constructed on centimeter-sized wafers of silicon, or other materials
Asteroid
An object orbiting the Sun that is smaller than a major planet (sub-kilometer to about 1,000 km in diameter) but shows no evidence of an atmosphere or other types of activity associated with comets. Most asteroids are located in a belt between Mars and Jupiter ranging from 2.2 to 3.3 AU from the Sun.
ATD
Advanced Technology Demonstrator
AU
Astronomical unit: the mean distance between Earth and the Sun, about 150 million km or 93 million miles
BMDO
Ballistic Missile Defense Organization: the arm of the U.S. Department of Defense charged with developing missile-defense systems
Byte
A unit of information used in reference to computers and quantities of data. A byte consists of 8 bits (0s and 1s) and may correspond to a single character or number.
CCD
Charge-coupled device: an electronic chip that has in recent decades replaced photographic emulsions as the primary recording medium for astronomical images. The recording portion of the chip is divided into discrete photosensitive elements (pixels) arranged in a rectangular array of rows and columns. The HST’s CCDs have 640,000 pixels arranged in 800 rows and 800 columns. For various applications, some pixels are masked. An astronomical CCD has no masked area. At the end of an exposure, a shutter is closed to keep more light from coming in while the array is being read. The entire array of pixels is shifted, one column at a time, into a so-called column register at the edge, and after each shift the column register is read out slowly, row by row, through amplifiers and other circuitry.
Clementine
A small-spacecraft technology demonstration mission launched by the DOD’s BMDO in January 1994. The mission mapped the entire Moon but failed before flying by asteroid 1620 Geographos.
Coma
The spherical envelope of gas and dust surrounding the nucleus of an active comet
Comet
A volatile-rich body that develops a transient atmosphere as it orbits the Sun Observed comets usually have highly elliptical orbits, many approaching parabolic, although many comets in the Kuiper Disk are likely to have nearly circular orbits, as are many of the currently unobservable comets in the Oort cloud. When a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas, and often a tail.
Cosmic ray
A charged, subatomic particle of matter (not radiation), usually an atom stripped of all its orbiting electrons and accelerated to extremely high energies by unknown processes in space
Deceleration parameter
A measure of the rate at which universal expansion is retarded by gravity
Diffraction limit
The finest detail that can be discerned with a telescope. The physical principle of diffraction limits this to a value proportional to the wavelength of the light observed divided by the diameter of the telescope.
Discovery
A relatively new program within NASA’s Solar System Exploration Division to fly frequent, relatively cheap missions, primarily in the inner solar system. NEAR (Near Earth Asteroid Rendezvous) will be the first Discovery mission flown. Mission costs are capped at $150 million, excluding launch costs. NASA’s plan is to cancel any mission that can not stay within the cost limit.
DOD
Department of Defense
Ecliptic
The mean plane of Earth’s orbit around the Sun
ESA
European Space Agency
Figure
The exact shape of the surface of a mirror or other optical component
FOV
Field of view: the region of sky visible to a detector at any one time
Frame-transfer CCD
A type of CCD in which half the chip is masked. At the end of an exposure, the entire image is shifted rapidly to the masked half of the array. Then the masked portion is shifted, one column of pixels at a time, into the column register at the edge, and, after each 1-column shift, the register is slowly read out through an amplifier and other circuitry.
F-SAT
Frugal satellite: a generic satellite bus currently under development by Lockheed. It will be able to provide basic services, such as power, guidance, and communications, to a range of different payloads.
FSM
Fast steering mirror
FTE
Full-time equivalent: a measure of the workload imposed by a particular activity
FWHM
Full width at half maximum: a measure of the degree of concentration of light from a source produced by an optical system. It is equal to the width of a plot of intensity of light versus distance from the center of the image at a point where the intensity has fallen to half its maximum value.
GEO
Geostationary Earth orbit
Gigabyte
One billion (109) bytes: a unit of information used to describe quantities of data or the storage capacity of computers
HACE
Hubble Advanced Camera for Exploration: a third-generation instrument to be installed in the Hubble Space Telescope toward the end of this decade. This particular camera was recently chosen from among several proposals to build an Advance Camera (AC) for the HST.
HgCdTe array
An array used for sensing near-infrared radiation. The photo-sensitive material is composed of an alloy of mercury (Hg), cadmium (Cd), and tellurium (Te). The spectral range extends from the red to an infrared cutoff that is adjustable in manufacture between 2.5 and 5 microns with trade-offs for poorer sensitivity in exchange for the wider spectral range. The arrays are addressable, storing signals in each pixel as a capacitor, from which the voltage is read. Thus the readout is fundamentally different from that of optical CCDs.
High-Z telescope
A proposed 4-meter space telescope optimized for cosmological observations
HST
Hubble Space Telescope: a 2.4-meter-aperture, optical/ultraviolet telescope developed by NASA and the European Space Agency
Infrared astronomy
The study of astronomical objects using intermediate-wavelength radiation to which the atmosphere is mostly opaque and the human eye insensitive. Humans sense infrared energy as heat. The infrared part of the electromagnetic spectrum generally corresponds to radiation with wavelengths from 1 µm to 1000 µm (1 µm is one-millionth of a meter). Objects with temperatures around room temperature or lower emit most of their radiation in the infrared.
InSb array
An array used for sensing near-infrared radiation. The photo-sensitive material is indium (In) antimonide (Sb). The spectral range extends from the far red to approximately 5 microns. Currently available InSb devices have greater sensitivity than HgCdTe devices when a response to 5 microns is required, but they also require operation at very low temperatures to achieve this sensitivity. Both technologies are still under active development.
IPSRU
Inertial Pseudo-Stellar Reference Unit: a device developed at Charles Stark Draper Laboratories to provide an inertially stable beam of light that can be tracked by an optical system in a spacecraft to maintain stable pointing
IRAS
Infrared Astronomical Satellite: a cryogenically cooled, infrared satellite, developed as a cooperative program among the United States, the United Kingdom, and the Nether
lands, which between January and November 1983 made a survey of the entire sky at several different infrared wavelengths
IRTF
Infrared Telescope Facility: a 3-meter telescope located on Mauna Kea, Hawaii, operated by NASA to study planets and other astronomical objects
ISO
Infrared Space Observatory: a 0.6-meter-aperture, cryogenically cooled, infrared space telescope to be launched by the European Space Agency in late 1995
IUE
International Ultraviolet Explorer: a joint U.S.-ESA-UK orbiting telescope to study ultraviolet radiation
JFCs
Jupiter-family comets
Kuiper Airborne Observatory
A 0.9-meter-diameter telescope for infrared and submillimeter observations that is carried above most of Earth’s water vapor in a C-141 aircraft
Kuiper Disk
The extension of the solar system beyond the Neptune-Pluto region to hundreds of astronomical units. Originally hypothesized by Edgeworth and by Kuiper to avoid a discontinuity in the surface density at Neptune-Pluto, this disk is now invoked as the source of the short-period comets of the inner solar system, all of which have dynamical lifetimes much less than the age of the solar system.
LAMP
Large Active Mirror Program: a DOD-sponsored project at Itek Optical Systems to develop a 4-meter-aperture, actively controlled, segmented mirror for use in a space-based, laser weapon
Line-transfer CCD
A type of CCD in which alternate rows of pixels are masked to facilitate rapid readout. To read out the array, each row of pixels is shifted (simultaneously) to the adjacent, masked row. The unmasked rows can then begin integrating again immediately, while the masked rows are read out slowly. The masked rows are read out along the row, one column of pixels at a time, to a column register at the end of the chip. After each shift along the rows by one column, the entire end-column register is shifted out sequentially to the read amplifier.
LOS
Large Optical Segment: a DOD-sponsored project at Itek Optical Systems to develop an 11-meter aperture, actively controlled, segmented mirror
Magnitude
A logarithmic unit of brightness for stars and other astronomical objects. Fainter stars have numerically larger magnitudes. The brightest stars, excluding the Sun, are about magnitude 0; the faintest star visible to the unaided eye is about magnitude 6. A star with V = 15 is one-millionth as bright as the half-dozen brightest stars with V = 0. Stars as faint as magnitude 28 can be seen with powerful terrestrial or spaceborne telescopes.
mas
Milliarc second: one-thousandth of an arc second
Molniya orbit
A highly eccentric and inclined orbit with a nominal period of 12 hours used mainly by Russian telecommunication satellites
NASA
National Aeronautics and Space Administration
NEO
Near-Earth orbit: object whose orbit brings it near Earth’s orbit: specifically, certain asteroids and comets
New Millennium
A NASA program to develop and fly a series of ultra-lightweight spacecraft to demonstrate the advanced technology required for future space science missions
NICMOS
Near-Infrared Camera and Multi-Object Spectroscope: a second-generation instrument for the Hubble Space Telescope to be installed in 1997. This camera uses a HgCdTe array as its sensor.
NTOT
New Technology Orbital Telescope
Occultation
The obscuration of one celestial body by another of greater apparent diameter; especially the passage of the Moon in front of a star or planet, or the disappearance of a star behind an asteroid or comet.
Oort cloud
A spherical cloud of comets having semimajor axes between 1,000 and 50,000 AU. Comets in this cloud, most of which have orbits with low eccentricity, can be sufficiently perturbed by passing stars or giant molecular clouds so that a fraction of them acquire orbits that take them within the orbits of Jupiter and Saturn.
Optical astronomy
The study of astronomical objects using light waves with wavelengths from about 0.3 to 1 µm. The human eye is sensitive to most of these wavelengths.
Parallax
The apparent shift in position of a nearby object relative to a more distant object, as the observer changes position. Using basic trigonometry, it is possible to derive the distance of a star from its parallax as observed from opposite points on Earth’s orbit.
Perigee
The point at which a body in orbit around the Earth most closely approaches the Earth
Perihelion
The point at which a body in orbit around the Sun most closely approaches the Sun
Pixel
The smallest element of a digital image. A typical image from a single chip in the Hubble Space Telescope is a square with 800 × 800 discrete pixels.
Point-spread function
The variation of intensity with distance from the center of an image of a true point source. The PSF describes the observing instrument ’s effect on the light source. A detected image is a convolution of the true brightness distribution on the sky with the PSF of the instrument.
Polar Stratospheric Telescope
A proposed project in which a 4-meter telescope is suspended beneath a tethered aerostat
Proton
A family of large Russian launch vehicles
Protoplanetary or protostellar disk
A disk of gas and dust surrounding a young star or protostar out of which planets may form
Protostar
The earliest phase in the evolution of a star, in which most of its energy comes from the infall of material, or accretion, onto the growing star. A protostellar disk probably forms around the star at this time.
PSF
Point-spread function
PtSi
A platinum (Pt) silicide (Si) infrared array detector
Quadrature
The direction at right angles to the direction toward the Sun as seen from Earth. When another body of the solar system is at quadrature, its apparent angular motion is entirely due to its own motion. At any other point in the body’s orbit, the motion of the Earth is also reflected in the apparent motion of the body.
Redshift
The increase in the wavelength of a spectral line from an astronomical body relative to its value measured in a terrestrial laboratory. The wavelength shift is then given by the factor (1 + z). The redshift is usually interpreted as being due to the Doppler effect (motion away from the observer), and for nearby objects relative motions lead to both blueshifts and redshifts. At very large distances the redshift is interpreted in many cosmologies, but not all, as being due to the expansion of the universe.
Refigure
Changing the shape of an optical surface
Resolution
The ability of an instrument to discern fine detail
SDIO
Strategic Defense Initiative Organization: the predecessor of BMDO
SIRTF
Space Infrared Telescope Facility: NASA’s proposed 1-meter-aperture, cryogenically cooled, infrared space telescope
STIS
Space Telescope Imaging Spectrograph: a second-generation instrument that will be installed in the Hubble Space Telescope in 1997
Strehl ratio
A measure of the quality of an optical instrument equal to the ratio of the amplitude of the point-spread function to that of an equivalent, ideal instrument
STScI
Space Telescope Science Institute
Supernova
An explosive death of a star whose tremendous energy output causes its expanding debris to glow brightly enough to be seen at extragalactic distances for weeks or months thereafter
TGBNTOO
Task Group on BMDO New Technology Orbital Observatory
Titan IV
A large U.S. launch vehicle used primarily by the Department of Defense
Type I Supernova
A type of supernova that occurs in a binary star system containing a white dwarf. All such events are believed to have the same intrinsic brightness and so can be used as distance indicators.
ULE
Ultra-low expansion: a type of glass ceramic used to make telescope mirrors
Van Allen Belts
Zones of intense radiation surrounding Earth’s mid-section, caused by charged particles trapped in Earth’s magnetic field
Wavefront
A surface of constant phase, that is, an imaginary surface linking the crests or troughs of the rays of light from a common source
WFPC
Wide Field/Planetary Camera: the primary camera used by the Hubble Space Telescope before its first servicing mission
WFPC2
Wide Field/Planetary Camera 2: the primary camera currently being used on the Hubble Space Telescope
YSOs
Young stellar objects
z
Redshift: the increase in the wavelength of a spectral line from an astronomical body relative to its value measured in a terrestrial laboratory. The wavelength shift is then given by the factor (1 + z).
Zodiacal emission
A faint glow caused by sunlight scattering off interplanetary dust near the plane of the ecliptic
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