Glossary

131 terms from instrumentation, observing, analysis and archives, defined for readers from physics, engineering and data science. Elsewhere on the site, a dotted underline marks a term defined here.

Sky and coordinates

arcsecondarcsec · milliarcsecond · microarcsecond · mas: 1/3600 of a degree (″). A milliarcsecond (mas) is 10⁻³″ and a microarcsecond (µas) 10⁻⁶″. Atmospheric seeing limits ground telescopes to about 0.5–1″ without adaptive optics.See also angular resolution, seeing
continuous viewing zoneCVZ: The region of sky a spacecraft can observe without interruption through its whole orbit or year, usually around the ecliptic poles. Targets there accumulate the longest time series.See also ecliptic
declinationDec: Celestial latitude in the equatorial system: the angle north (+) or south (−) of the celestial equator, from −90° to +90°. A ground site at latitude φ sees declinations up to roughly 90° − |φ| beyond its own pole.See also right ascension, equatorial coordinates
eclipticecliptic plane · ecliptic pole · ecliptic latitude · ecliptic hemisphere: The plane of Earth's orbit projected on the sky; the Sun, Moon and planets stay close to it. Solar-system bodies and zodiacal light concentrate there, and spacecraft that must keep the Sun at a fixed angle often observe in strips perpendicular to it, so the ecliptic poles get the deepest coverage.See also continuous viewing zone
equatorial coordinatesequatorial frame · ICRS: The standard celestial frame, aligned with Earth's rotation axis and realised today as the International Celestial Reference System (ICRS). Positions are given as right ascension and declination.See also right ascension, declination, Galactic coordinates, reference frame
Galactic coordinatesGalactic longitude · Galactic latitude: A frame aligned with the Milky Way: latitude b is measured from the Galactic plane and longitude l from the direction of the Galactic centre. Natural for anything concentrated in the disc.See also Galactic plane, equatorial coordinates
Galactic planeGalactic disc · Galactic bulge: The plane of the Milky Way's disc, a band around the sky at Galactic latitude b ≈ 0°. Dense in stars, gas and dust: rich for stellar work, crowded and heavily extincted for extragalactic surveys, which usually avoid it.See also Galactic coordinates, extinction, crowding
HEALPix: Hierarchical Equal Area isoLatitude Pixelisation: a scheme that divides the sphere into 12 × 4ᵏ cells of equal area. The standard grid for all-sky maps, CMB data and coverage maps.See also MOC, HiPS
Lagrange pointSun–Earth L2 · Sun–Earth L1 · L2 · L1: A point where the gravity of two bodies and the orbital motion balance. Sun–Earth L2, 1.5 million km beyond Earth, keeps Sun, Earth and Moon on one side: a stable thermal environment for passive cooling and an unobstructed view of most of the sky.
right ascensionRA: Celestial longitude in the equatorial system, measured eastward along the celestial equator from the March equinox, usually in hours (0–24 h) or degrees.See also declination, equatorial coordinates
square degreedeg²: Unit of solid angle: one degree by one degree on the sky. The whole sky is about 41,253 deg²; the full Moon covers about 0.2 deg².See also field of view

Optics and detectors

adaptive opticsAO: Correcting atmospheric distortion in real time with a deformable mirror driven by a wavefront sensor, at kilohertz rates, using a bright natural star or a laser guide star. Brings large ground telescopes close to their diffraction limit in the infrared.See also seeing, angular resolution
angular resolutiondiffraction limit · diffraction-limited · spatial resolution: The smallest angular separation at which two point sources can be told apart. For a filled aperture of diameter D at wavelength λ the diffraction limit is about 1.22 λ/D; for an interferometer, D is the longest baseline.See also point-spread function, baseline, seeing, arcsecond
bolometertransition-edge sensor · TES: A detector that measures incoming power through the heating of an absorber. Transition-edge sensors, held at the edge of a superconducting transition, are the standard for CMB and submillimetre cameras.
CCDcharge-coupled device · pn-CCD: Charge-coupled device: a silicon pixel array that stores photo-electrons and shifts them to a readout amplifier. The workhorse optical detector; in X-rays, each photon's charge also measures its energy.See also charge transfer efficiency, HgCdTe detector
charge transfer efficiencyCTE: The fraction of charge a CCD keeps at each pixel-to-pixel transfer. Radiation damage in orbit lowers it, leaving trails behind sources; pipelines correct for it (Hubble's flc and drc products are CTE-corrected).See also CCD
Cherenkov lightCherenkov · imaging atmospheric Cherenkov telescope · IACT · Cherenkov telescope: Light emitted when a charged particle moves faster than light's phase velocity in a medium. Ground gamma-ray telescopes image the nanosecond flash from the particle shower a TeV photon makes in the atmosphere; neutrino detectors see it from secondary particles in ice or water.See also neutrino
coded-mask imagingcoded-mask · coded mask · coded-aperture: Imaging hard X-rays and gamma rays, which cannot be focused, by placing a patterned mask above a position-sensitive detector. Each source casts a shifted shadow of the pattern; deconvolution recovers the sky over a wide field at modest resolution.
effective areacollecting area: Geometric collecting area multiplied by the efficiency of optics and detector, as a function of energy or wavelength. The figure of merit for photon-starved X-ray and gamma-ray instruments, in cm².
étendueetendue: Collecting area times field of view (A·Ω). Sets how fast a telescope can survey the sky to a given depth; survey telescopes such as Rubin are designed to maximise it.See also field of view, effective area
field of viewFoV · fields of view: The patch of sky an instrument images at once, in square degrees for wide-field cameras or in steradians (sr) for all-sky monitors; 4π sr is the whole sky.See also square degree, étendue
grazing-incidence opticsgrazing-incidence · Wolter-I · Wolter · nested shells: X-rays reflect only at angles of a degree or less, so X-ray telescopes use nested shells of paraboloid and hyperboloid mirrors seen almost edge-on (the Wolter-I design). More shells add effective area; figure accuracy sets the PSF.See also effective area, point-spread function, lobster-eye optics
HgCdTe detectorHgCdTe · H2RG · H4RG: Mercury-cadmium-telluride infrared arrays (Teledyne H2RG and H4RG) used by JWST, Euclid and Roman. Each pixel is read non-destructively, so an exposure is sampled many times up the ramp.See also up-the-ramp sampling
integral field unitIFU · integral-field spectroscopy: A spectrograph front end that slices a small field into many elements and takes a spectrum of each, giving a data cube with two spatial axes and one wavelength axis.See also data cube
lobster-eye opticslobster-eye · micropore optics: Arrays of tiny square channels on a spherical surface that focus soft X-rays by grazing reflection, as a lobster's eye focuses light. They trade PSF quality for a very wide field of view, suited to monitoring.See also grazing-incidence optics, field of view
microcalorimetermicrocalorimeters: A detector cooled to about 50 mK that measures each X-ray photon's energy from the tiny rise in temperature it causes. Reaches energy resolution of a few eV, far finer than an X-ray CCD.See also spectral resolution
multi-object spectroscopyfibre positioners · robotic fibres · multi-object spectrograph · microshutter array: Taking spectra of many chosen targets in one field at once, through fibres placed by robotic positioners (DESI, SDSS-V) or through programmable slits and shutters (JWST NIRSpec). The basis of redshift surveys.See also redshift
multilayer opticsdepth-graded multilayers · multilayer coatings: Grazing-incidence mirrors coated with hundreds of alternating thin layers whose spacing varies with depth, so they reflect hard X-rays by Bragg reflection. What lets NuSTAR focus up to 79 keV.See also grazing-incidence optics
pair-conversion telescopepair conversion · pair-conversion: Above about 20 MeV a gamma ray converts into an electron–positron pair in a heavy foil; tracking the pair and measuring its energy in a calorimeter gives the photon's direction and energy. The design of the Fermi LAT.
point-spread functionPSF: The image a system makes of a point source. Its width (often quoted as FWHM, or half-energy width for X-ray optics) sets the effective resolution; its shape matters for photometry, deblending and weak-lensing shape measurement.See also angular resolution, weak gravitational lensing
polarimetrypolarisation · polarimeter · polarimetric: Measuring the polarisation of incoming radiation. It encodes magnetic-field geometry, scattering and emission mechanisms; in the CMB it separates E- and B-mode patterns.See also B-modes
Schmidt telescopeSchmidt: A wide-field design: a spherical primary mirror with a thin corrector plate at its centre of curvature, giving sharp images over several degrees. Used by survey instruments such as ZTF and Kepler.
scintillatorscintillation detector: A crystal (NaI, BGO, CsI) that emits a flash of visible light when a gamma ray deposits energy in it, read by photomultipliers or photodiodes. Simple, large and omnidirectional, so used for burst monitors.
seeing: Blurring of starlight by turbulence in the atmosphere, usually quoted as the FWHM of a star's image in arcseconds. Good optical sites reach 0.5–0.8″, far worse than the diffraction limit of a large mirror.See also adaptive optics, angular resolution
segmented mirrorsegmented primary: A primary mirror built from many smaller segments phased to act as one surface. The only practical route beyond about 8 m (JWST has 18 hexagonal segments, the ELT 798).
slitless spectroscopygrism · grisms · slitless: Placing a dispersing element (often a grism, a prism–grating combination) in the beam with no slit, so every source in the field gets a spectrum at once. Efficient for surveys; spectra of neighbours can overlap.See also spectral resolution

Interferometry and radio

21 cm lineHI · neutral hydrogen · hydrogen intensity mapping · intensity mapping: The 1420 MHz hyperfine line of neutral hydrogen (HI). It traces gas in galaxies; redshifted to lower frequencies, its summed brightness over large volumes (intensity mapping) maps large-scale structure without resolving individual galaxies.See also baryon acoustic oscillations, epoch of reionisation
aperture arrayphased array · phased-array · aperture arrays · dipole antennas: Fixed antennas (often simple dipoles) combined electronically rather than steered mechanically, so a beam can point anywhere, and many at once. Used at low frequencies by LOFAR and SKA-Low.
baselinebaselines: The separation between two antennas of an interferometer, projected onto the plane perpendicular to the source. Long baselines give fine resolution; short ones recover extended emission.See also interferometry, uv-coverage
continuumcontinuum emission · continuum surveys: Broadband emission, such as synchrotron or thermal dust, as opposed to spectral lines. Continuum surveys sum over a wide bandwidth for sensitivity.See also spectral line
correlatorFX correlator: The digital system that cross-multiplies the signals from every pair of antennas to form visibilities. Its cost grows with the square of the number of antennas.See also visibilities
drift scantransit telescope · drift-scan: A stationary telescope that lets Earth's rotation sweep the sky through its beam. CHIME has no moving parts and sees every northern declination once a day.
fast radio burstFRB: A millisecond radio pulse of extragalactic origin. Dispersion by free electrons along the path delays low frequencies, measuring the column of ionised gas between us and the source. Some repeat; the sources are thought to be mainly magnetars.
interferometryinterferometer · interferometric: Combining waves from separate receivers coherently. In radio and optical astronomy, correlating signals from separated telescopes gives the resolution of one as large as their separation. In gravitational-wave detection, a laser Michelson interferometer measures tiny differential changes in its arm lengths.See also baseline, visibilities, correlator, strain
janskyJy · mJy · µJy · µJy/beam: Unit of spectral flux density: 1 Jy = 10⁻²⁶ W m⁻² Hz⁻¹. Radio survey depth is quoted as rms noise in µJy per beam.
pulsarmillisecond pulsar · radio pulsar: A rotating, magnetised neutron star whose beamed emission sweeps past Earth once per turn. Millisecond pulsars, spun up by accretion, keep time to tens of nanoseconds over decades.See also pulsar timing array, neutron star
spectral linemolecular line · molecular-line · emission line · emission lines: Emission or absorption at a specific frequency from an atomic or molecular transition. Its strength gives abundance and temperature; its Doppler shift gives line-of-sight velocity.See also 21 cm line, redshift
uv-coverageuv plane · uv coverage: The set of spatial frequencies an interferometer has sampled, set by its baselines and Earth's rotation. Gaps in it limit image fidelity and fix the largest structure the array can recover.See also visibilities, baseline
visibilitiesvisibility data: The complex correlation of the signals from each pair of antennas: samples of the Fourier transform of the sky brightness. An image is reconstructed from them by inverse transform and deconvolution (CLEAN).See also uv-coverage, correlator
VLBIvery-long-baseline interferometry · very long baseline interferometry: Very-long-baseline interferometry: telescopes thousands of kilometres apart record signals against atomic clocks and correlate them afterwards. Earth-sized baselines at 1.3 mm give the EHT about 20 µas resolution.See also interferometry, baseline

Observing and surveys

alertalert stream · alerts · public alerts: A machine-readable notice that a source has appeared, changed or been detected, issued within seconds to days so other facilities can follow up. Optical alerts reach users through brokers; high-energy, neutrino and gravitational-wave alerts through GCN.See also alert broker, time-domain astronomy
alert brokerbrokers · community brokers: A service that ingests an alert stream (from ZTF or Rubin, up to millions of alerts a night), cross-matches and classifies each alert, and lets users filter for what they need. Examples: ALeRCE, Fink, Lasair, ANTARES.See also alert, cross-match
all-sky surveyall-sky scanning · scanning survey: Observing the entire celestial sphere to uniform depth, typically by scanning great circles or tiling fields. Produces catalogues and maps others select targets from.See also footprint
cadencecadences: The interval between successive measurements of the same field or target. Sets which variability timescales a time series can resolve: TESS reaches 20 s, Rubin revisits each field every few nights.See also light curve, time-domain astronomy
commissioning: The period after launch or construction when an observatory is tested, aligned and calibrated before routine science operations start.
data rightsdata-rights holders: Access to a survey's proprietary data products granted to scientists in the funding countries or member institutions, ahead of or instead of a fully public release.
exclusive access periodproprietary period · exclusive period · exclusive access: The time, often 12 months, during which only the proposing team can use new data before they become public in the archive. Survey and community missions increasingly have none.
first lightscientific first light · telescope first light: The first image or signal a new telescope or instrument records from the sky. Science operations usually begin months later, after commissioning.See also commissioning
footprintfootprints · survey footprint: The region of sky a survey or observation covers, usually stored as a polygon list or a MOC.See also MOC
pointed observationpointed observations · pointed follow-up: Observing chosen targets, usually selected by peer review in annual proposal cycles, as opposed to surveying the sky systematically. Gives depth on few objects; the total area observed stays small.See also exclusive access period
sector: TESS's unit of observation: one 24° × 96° strip of sky observed continuously for about 27 days (two 13.7-day orbits) before the spacecraft re-points.See also continuous viewing zone, cadence
time-domain astronomytime-domain · transient · transients: Studying how sources vary or appear over time: supernovae, tidal disruptions, flares, variable stars and moving objects. Needs wide fields, high cadence and fast data processing.See also alert, cadence, difference imaging

Photometry, astrometry and spectroscopy

asteroseismologystellar oscillations: Inferring a star's interior, mass, radius and age from the frequencies of its oscillation modes, seen as ppm-level brightness variations in long, uninterrupted light curves.See also ppm, light curve
astrometryastrometric · epoch astrometry: Measuring positions and their changes on the sky: parallax, proper motion and orbital motion. Gaia reaches tens of µas for bright stars.See also parallax, proper motion, reference frame
crowdingcrowded field · crowded fields · blending · blended: Sources close enough that their PSFs overlap, so flux is shared between them. Pipelines with large pixels (TESS's are 21″) need careful deblending or difference imaging in dense fields.See also point-spread function, difference imaging
difference imagingdifference image · difference images · image subtraction: Subtracting a PSF-matched reference image from a new one so only what changed remains. The core of transient detection, and robust in crowded fields.See also crowding, alert, time-domain astronomy
extinctiondust extinction · reddening: Dimming and reddening of light by interstellar dust, strongest at short wavelengths. Severe toward the Galactic plane, where infrared and radio see through it.See also Galactic plane
light curvelight-curve: Flux of one source as a function of time. The basic product of transit searches, variable-star work and transient follow-up.See also photometry, transit, cadence, systematics
magnitudemag · limiting magnitude · AB magnitude: A logarithmic flux scale, m = −2.5 log₁₀(F/F₀): five magnitudes is a factor of 100, and larger is fainter. The naked eye reaches about 6; single Rubin exposures reach about 24 in r.See also photometry, passband
microlensinggravitational microlensing: Brightening of a background star as a foreground star passes almost in front of it and bends its light. A planet around the lens adds a short anomaly, revealing planets at several AU independent of their brightness.See also light curve
parallax: The apparent annual shift of a nearby star as Earth orbits the Sun. A parallax of 1″ means a distance of 1 parsec (3.26 light years); distance in pc is 1/parallax in arcseconds.See also astrometry, parsec
parsecpc · kpc · Mpc · Gpc: The distance at which 1 AU subtends 1″: 3.086 × 10¹⁶ m, or 3.26 light years. Galaxies are measured in Mpc.See also parallax
passbandbandpass: The range of wavelengths a filter or channel transmits, together with its transmission curve. Rubin's ugrizy and WISE's W1–W4 are passband sets.
photometryphotometric · aperture photometry · forced photometry · epoch photometry: Measuring the flux of a source in a passband. Aperture photometry sums pixels inside a region; PSF photometry fits the point-spread function; forced photometry measures at a position fixed in advance, whether or not a source is detected there.See also passband, magnitude, light curve, point-spread function
ppmparts per million: Parts per million of a star's flux; the unit of precision for transit and asteroseismic photometry.See also transit, asteroseismology
proper motionproper motions: A star's angular motion across the sky, in mas per year, from its velocity transverse to our line of sight.See also astrometry
radial velocityradial velocities · RV: Velocity along the line of sight, from the Doppler shift of spectral lines. Planet searches measure the star's reflex motion to below 1 m s⁻¹.See also spectral line
redshiftspectroscopic redshift · photometric redshift: z = λ_observed/λ_emitted − 1, the stretching of light by cosmic expansion (or by recession). For distant sources it stands in for distance and look-back time: z ≈ 1 is about 8 Gyr ago, z ≈ 10 about 13.3 Gyr.See also baryon acoustic oscillations, multi-object spectroscopy
reference frameGaia-CRF · ICRF · celestial reference frame: The set of objects with fixed, precisely known positions that realises a coordinate system. The current optical frame is Gaia-CRF3, tied to distant quasars; other catalogues align their astrometry to it.See also equatorial coordinates, astrometry
spectral resolutionresolving power · energy resolution: How finely a spectrograph separates wavelengths, quoted as resolving power R = λ/Δλ (optical, infrared) or energy resolution ΔE in eV (X-rays). R ~ 100 measures redshifts of faint galaxies; R ~ 100,000 measures stellar radial velocities.See also microcalorimeter, radial velocity
systematicssystematic errors · instrumental systematics: Signals introduced by the instrument or observing conditions rather than the source, such as pointing jitter, thermal drift or scattered light. Removing them without removing astrophysical signal is most of light-curve processing.See also light curve
transittransiting · transit method: A planet passing in front of its star and blocking a fraction (R_p/R_*)² of its light, about 84 ppm for an Earth across the Sun. Repeated transits give period and radius; spectra taken during transit probe the planet's atmosphere.See also ppm, transmission spectroscopy, light curve
transmission spectroscopytransmission spectra · transmission spectrum · time-series spectroscopy: Comparing a star's spectrum in and out of a planet's transit: wavelengths where the planet's atmosphere absorbs make the transit slightly deeper, revealing its molecules.See also transit

High-energy astrophysics

afterglowafterglows: The fading X-ray, optical and radio emission from the shock a gamma-ray burst's jet drives into its surroundings, lasting hours to months. Localising it gives the host galaxy and redshift.See also gamma-ray burst
electronvolteV · keV · MeV · GeV: The energy an electron gains across 1 volt, 1.602 × 10⁻¹⁹ J. High-energy astronomy labels photons by energy: optical light is about 1.7–3.1 eV, X-rays 0.1–100 keV, gamma rays from about 100 keV to beyond 100 TeV. A 1 keV photon has wavelength 1.24 nm.
event listphoton list · event file · evt2: The basic X-ray and gamma-ray data product: a table with one row per detected photon (arrival time, detector position, sky position, energy and quality flags). Images, spectra and light curves are all binned from it.See also electronvolt
gamma-ray burstGRB · gamma-ray bursts: A flash of gamma rays lasting milliseconds to minutes, followed by a fading afterglow at longer wavelengths. Long bursts come from collapsing massive stars; short ones from neutron-star mergers.See also afterglow, multi-messenger astronomy
hard X-rayshard X-ray · soft X-ray · soft X-rays · soft gamma rays: The higher-energy end of the X-ray band, conventionally above about 10 keV; soft X-rays are below about 2 keV. Hard X-rays penetrate absorbing gas, so they reveal obscured black holes.See also electronvolt, multilayer optics
multi-messenger astronomymulti-messenger: Observing one event through more than one carrier: electromagnetic radiation, gravitational waves, neutrinos or cosmic rays. GW170817, a neutron-star merger, was seen in gravitational waves, gamma rays and across the spectrum.See also neutrino, strain, gamma-ray burst
neutrinoneutrinos · astrophysical neutrinos: A nearly massless, neutral particle that interacts only weakly, so it escapes dense sources and crosses the Universe undeflected. High-energy astrophysical neutrinos point back to where cosmic rays are accelerated.See also Cherenkov light, multi-messenger astronomy
neutron starneutron stars: The collapsed core of a massive star: about 1.4 solar masses in a 12 km radius, at above nuclear density. Its mass–radius relation tests the equation of state of dense matter.See also pulsar, X-ray timing
supernova remnantsupernova remnants: The expanding debris and shocked interstellar gas left by a supernova, bright in X-rays for thousands of years. X-ray spectra give its temperature and the elements the explosion made.
X-ray timingpulse-profile modelling · pulse profile: Measuring photon arrival times precisely enough to follow pulsations and variability. Modelling the pulse shape of hotspots on a rotating neutron star, bent by gravity, constrains its mass and radius.See also neutron star

Gravitational waves

chirp masschirp: The combination of the two masses, (m₁m₂)^{3/5}/(m₁+m₂)^{1/5}, that sets how fast an inspiral's frequency rises. It is the best-measured parameter of a compact-binary signal.See also compact binary coalescence
compact binary coalescencecompact binary · merger · mergers · binary black hole: The inspiral and merger of two black holes, two neutron stars or one of each. The signal sweeps up in frequency and amplitude (a chirp) until the objects merge; its shape gives their masses, spins and distance.See also chirp mass, strain
false-alarm rateFAR · false alarm rate: How often detector noise alone would produce a candidate at least as significant, e.g. once per 100 years. The main significance statistic for gravitational-wave alerts.
observing runobserving runs · O4 · O5: A planned period of coordinated LIGO–Virgo–KAGRA data taking (O1 in 2015, O4 in 2023–2025), separated by breaks for detector upgrades.
parameter estimationposterior samples · parameter-estimation: Bayesian inference of a signal's source properties by comparing waveform models with the strain data. The result is a set of posterior samples that encodes uncertainties and correlations between parameters.
pulsar timing arrayPTA · pulsar timing · Hellings–Downs: A set of millisecond pulsars timed for years. Nanohertz gravitational waves perturb pulse arrival times by nanoseconds, correlated between pulsar pairs in a specific way with their separation on the sky (the Hellings–Downs curve).See also pulsar, strain
sky localisationlocalised · localisation · credible area · credible areas: Where on the sky a source probably lies, published as a HEALPix probability map; its size is quoted as the area containing 90% probability. Two detectors give long arcs of hundreds to thousands of deg²; a third and fourth detector shrink it by triangulation.See also HEALPix, superevent
strainh(t): The fractional change in length, ΔL/L, a passing gravitational wave induces; the calibrated output of a detector. Signals from merging black holes reach about 10⁻²¹, a change of a few × 10⁻¹⁸ m over a 4 km arm.See also interferometry, compact binary coalescence
supereventsuperevents: GraceDB's grouping of all candidate triggers from different search pipelines that correspond to one astrophysical signal, named like S250206dm.See also alert, sky localisation

Cosmology

B-modesB-mode · B-mode polarisation · primordial B-modes: The curl-like component of CMB polarisation. On degree scales, primordial gravitational waves from inflation would produce it; its amplitude is quoted as the tensor-to-scalar ratio r.See also inflation, tensor-to-scalar ratio, polarimetry
baryon acoustic oscillationsBAO: A preferred separation of about 150 Mpc (comoving) in the galaxy distribution, imprinted by sound waves in the early Universe. Measuring it at many redshifts traces the expansion history like a standard ruler.See also redshift, dark energy
cosmic microwave backgroundCMB: Thermal radiation released 380,000 years after the Big Bang, now at 2.7255 K and peaking near 160 GHz. Its temperature and polarisation anisotropies, at the 10⁻⁵ level, constrain the content and geometry of the Universe.See also B-modes, foregrounds, ΛCDM
dark energy: Whatever drives the accelerating expansion of the Universe, about 69% of its energy density today. Measured through expansion history (supernovae, BAO) and the growth of structure (weak lensing, clustering).See also baryon acoustic oscillations, weak gravitational lensing, ΛCDM
epoch of reionisationreionisation · cosmic dawn: The period, roughly 150 million to 1 billion years after the Big Bang (z ≈ 20–6), when the first stars and galaxies ionised the neutral hydrogen filling the Universe. Probed by redshifted 21 cm emission and high-z galaxies.See also 21 cm line, redshift
foregroundsforeground · component separation · component-separated: Emission between us and the signal of interest; for the CMB, Galactic synchrotron, free–free and dust. Component separation uses their different spectra across many frequencies to remove them.See also cosmic microwave background
inflationcosmic inflation: A proposed epoch of exponential expansion in the first fraction of a second, which would have seeded the density fluctuations seen in the CMB and galaxy distribution.See also B-modes, primordial non-Gaussianity
primordial non-Gaussianitynon-Gaussianity · f_NL: Departures of the initial density field from Gaussian statistics, parameterised by f_NL. Different inflation models predict different values; large-volume galaxy surveys such as SPHEREx can measure it.See also inflation
redshift-space distortionsRSD: Anisotropy in the clustering of galaxies mapped by redshift, caused by their peculiar velocities falling into structures. Measures the growth rate of structure.See also redshift, baryon acoustic oscillations
tensor-to-scalar ratio: The ratio of primordial gravitational-wave power to density-fluctuation power, which sets the energy scale of inflation. BICEP/Keck (2021) set r < 0.036; LiteBIRD and Simons Observatory aim at ~0.001–0.003.See also B-modes, inflation
weak gravitational lensingweak lensing · weak-lensing · cosmic shear · shear: Coherent distortion of distant galaxy shapes, at the percent level, by mass along the line of sight. Averaged over millions of galaxies it maps dark matter and the growth of structure; it needs exquisite PSF control.See also point-spread function, dark energy
ΛCDMLambda-CDM · standard cosmological model: The standard cosmological model: a flat universe with dark energy as a cosmological constant (Λ), cold dark matter and ordinary matter, fit by six parameters.See also dark energy, cosmic microwave background

Data and archives

ADQLAstronomical Data Query Language: Astronomical Data Query Language: SQL with spherical-geometry functions (CONTAINS, POINT, CIRCLE), sent to archives through the TAP protocol.See also TAP
calibrationflat field · flat-fielded · flat-fielding · dark current: Removing instrument signatures and putting data in physical units: subtracting bias and dark current, dividing by the flat field (pixel-to-pixel sensitivity), correcting non-linearity and fitting the astrometric solution. Reference files hold these calibrations and are updated as the instrument is understood.See also data level
coaddcoadds · coadded · co-added · mosaic: Many exposures of the same field resampled onto a common grid and combined, reaching deeper than any single exposure and rejecting artefacts. A mosaic stitches adjacent fields.See also data level
cross-matchcross-matching · cross-identification · cross-identifications: Associating entries in different catalogues with the same astrophysical source, by position and, when needed, proper motion, brightness or probability.
data cubeimage cube · image cubes · cube · cubes: A three-dimensional array with two sky axes and one spectral axis (wavelength, frequency or velocity), produced by integral field units and radio interferometers.See also integral field unit, visibilities
data levelprocessing level · Level 0 · Level 1 · Level 2: How far processing has taken a data product, from raw telemetry (Level 0) to calibrated exposures, measurements and combined catalogues. Each mission numbers its levels differently; see the Data levels page.
data releasedata releases: A versioned, documented publication of a survey's processed data (DR1, DR2…). Later releases usually reprocess everything with improved calibration, so their values supersede, rather than add to, earlier ones.
FITSFlexible Image Transport System: Flexible Image Transport System, astronomy's standard file format since 1981: headers of keyword–value pairs followed by images or binary tables.See also world coordinate system
full-frame imageFFI · FFIs · full-frame images: A calibrated image of a camera's entire detector, kept at a longer cadence than selected targets. For TESS, the source of light curves for millions of stars no one pre-selected.See also target pixel file, sector
high-level science productHLSP · HLSPs · high-level science products: A data product made by a science team or the community, rather than the mission pipeline, and hosted by the archive: custom light curves, mosaics, catalogues. MAST hosts hundreds.
HiPSHierarchical Progressive Survey: Hierarchical Progressive Survey: an all-sky image cut into HEALPix tiles at increasing resolution, so a viewer like Aladin Lite fetches only the tiles in view at the zoom level needed.See also HEALPix, MOC
IVOAVirtual Observatory · International Virtual Observatory Alliance: International Virtual Observatory Alliance, which sets the interoperability standards (TAP, HiPS, MOC, VOTable) that let tools query and combine archives worldwide.See also TAP, HiPS, MOC
MOCMulti-Order Coverage map · MOCs: Multi-Order Coverage map: an IVOA standard describing any region of sky as a set of HEALPix cells of mixed sizes. Compact, and fast to intersect, which makes it ideal for footprints.See also HEALPix, footprint
TAPTable Access Protocol: Table Access Protocol, the IVOA standard for querying archive databases with ADQL over HTTP. Gaia, MAST, IRSA, ESO and CDS all offer TAP services.See also ADQL, IVOA
target pixel fileTPF · target pixel files · postage stamps: A time series of small cutouts of pixels around one selected star, at short cadence. Kepler and TESS light curves are extracted from them.See also full-frame image, light curve
up-the-ramp samplingramp · ramps · non-destructive reads: Reading an infrared detector repeatedly during one exposure without resetting it, so charge accumulates as a ramp. Fitting the slope gives a count rate and rejects cosmic-ray hits.See also HgCdTe detector
world coordinate systemWCS · world coordinates: Header keywords in a FITS file that map pixel positions to sky coordinates (and wavelength or time), including projection and distortion.See also FITS