Glossary¶

The word you didn't recognise in that paper, defined in a sentence or two. This is an A–Z of the radio-astronomy terms and jargon the course (and the literature) lean on. For the maths symbols see notation.md; for the maths itself — convolutions, Fourier transforms, coordinates — see math-preliminaries.md.

Where a term has a whole chapter behind it, we point you to it. Definitions are kept short on purpose; follow the chapter or References for depth. Terms cross-link to one another — click a highlighted term to jump straight to its entry.

Jump to: A · B · C · D · E · F · G · H · I · J · K · L · M · N · O · P · Q · R · S · T · U · V · W · X · Y · Z

A¶

Asinh stretch: An image display transform, \(y = \sinh^{-1}(x/a)\), that is linear near the noise and logarithmic for bright pixels — so a high-dynamic-range radio source shows its faint lobes and bright core at once (Lupton et al. 2004). The radio default in jansky.plotting.radio_norm (Chapter 46).
Active galactic nucleus (AGN): A compact, luminous core at the centre of a galaxy, powered by accretion onto a supermassive black hole. Many are bright radio sources, often with jets and lobes; quasars are the most luminous AGN.
AOFlagger / SumThreshold: The standard automated RFI-flagging package (AOFlagger) and its core algorithm (SumThreshold, Offringa et al. 2010): flag windows of increasing length whose mean exceeds a threshold that decreases with window size, so faint but extended interference accumulates above the noise where a single-sample cut would miss it (Chapter 39).
Antenna temperature (\(T_A\)): The power a receiver sees from a source, expressed as the temperature of a matched resistor that would deliver the same power: \(P = k_B T_A \,\Delta\nu\). It is a property of the antenna plus source, not of the source alone, and is the quantity you actually measure (see Chapters 3–4).
Aperture efficiency (\(\eta_A\)): The fraction of a dish's geometric area that is effective at collecting signal, after losses from surface errors, blockage, illumination taper, and spillover. The effective area is \(A_e = \eta_A A_{\text{geom}}\), with \(\eta_A\) typically 0.5–0.7.
Aperture synthesis: Building up the resolution of a single huge dish by combining many small antennas and using Earth's rotation to sample many baseline orientations. The core technique of Chapter 8.
Accretion: The infall of gas onto a compact object (white dwarf, neutron star, black hole), usually through an accretion disk, where released gravitational energy heats the gas and makes it radiate. It powers AGN and X-ray binaries — and the white-dwarf binary now thought to drive some long-period transients (Chapters 45, 47).
Angular resolution: The smallest angular separation a telescope can distinguish, set by diffraction at \(\theta \approx 1.22\,\lambda/D\) for a dish of diameter \(D\) — or by the longest baseline for an interferometer. It sharpens at shorter wavelength and larger aperture (Chapters 4, 7).
Arcminute / arcsecond: Subdivisions of angle on the sky: \(1° = 60'\) (arcminutes) \(= 3600''\) (arcseconds), with the milliarcsecond (mas) and microarcsecond (µas) finer still — the scale VLBI reaches. The natural units for source sizes and resolution (Chapter 19).

B¶

Bandpass: The frequency-dependent gain of the instrument across the observing band. Bandpass calibration divides it out using an observation of a source with a known, flat spectrum so that real spectral features aren't confused with instrumental shape.
Bandwidth (\(\Delta\nu\)): The width in frequency over which a receiver collects power. Wider bandwidth means more collected power and lower noise (see the radiometer equation).
Baseline: The vector between a pair of antennas in an interferometer, usually measured in wavelengths. Each baseline samples one spatial frequency of the sky; longer baselines give finer angular resolution (Chapters 7–8).
Beam: The angular response pattern of an antenna or array — how sensitive it is as a function of direction. In interferometry you meet several: the primary beam (the field of view of one dish), the synthesised beam (the effective resolution of the array, i.e. the point-spread function), the dirty beam (that PSF including sidelobes from incomplete uv coverage), and the clean beam (an idealised Gaussian fit to it). See Chapters 8–9.
Brightness temperature (\(T_B\)): The temperature a blackbody would need to emit the observed surface brightness at a given frequency. In the radio (Rayleigh–Jeans) regime, \(T_B \propto I_\nu\), which makes it a convenient, frequency-independent way to quote intensity (Chapter 2).
Black hole: A region where gravity is so strong that nothing, not even light, escapes. Supermassive black holes (\(10^6\)–\(10^{10}\,M_\odot\)) at galaxy centres power AGN and were imaged by the EHT; stellar-mass ones are forged in supernovae (Chapters 19, 24, 45).
Blackbody radiation: The thermal radiation of an opaque body in equilibrium, set only by its temperature and described by the Planck law \(B_\nu(T)\). At radio (low) frequencies it reduces to the Rayleigh–Jeans form \(B_\nu \approx 2k_B T\nu^2/c^2\); the CMB is the sky's near-perfect blackbody (Chapters 2, 22).
Blazar: A radio-loud AGN whose relativistic jet points almost straight at us, so Doppler beaming makes it exceptionally bright and rapidly variable — the most extreme viewing angle of the unified AGN model (Chapter 45).

C¶

CASA (Common Astronomy Software Applications): The NRAO software suite for calibrating and imaging interferometer data, used for VLA imaging in Chapter 12. It reads and writes Measurement Sets.
Cosmic dawn: The era (\(z \sim 15\)–30, observed at ~50–90 MHz) when the first stars switched on and their Lyman-α photons coupled the 21 cm spin temperature to the cold gas, producing the deep absorption trough in the global 21 cm signal that EDGES claimed to detect near 78 MHz (Chapter 42).
CLEAN: Högbom's iterative deconvolution algorithm: model the sky as a sum of point sources, repeatedly subtracting a scaled dirty beam at the brightest pixel, then restore with a clean beam. The workhorse of radio imaging (Chapter 9).
Closure amplitude: A ratio of four visibility amplitudes (\(|V_{ij}||V_{kl}| / |V_{ik}||V_{jl}|\)) that cancels antenna-based gain errors. With closure phase, the robust observable that lets sparse VLBI arrays like the EHT image black holes (Chapter 19).
Closure phase: The sum of visibility phases around a triangle of baselines, which cancels all antenna-based phase errors. It is therefore a robust observable, central to VLBI and to self-calibration (Chapter 19).
CMB (cosmic microwave background): The relic radiation of the hot early universe — a near-perfect 2.725 K blackbody peaking in the microwave. Discovered as excess antenna temperature by Penzias & Wilson (1965); see Chapter 22.
CNN (convolutional neural network): A neural network whose layers slide learned filters across the input, giving it translation invariance — well matched to image-like data such as the dynamic spectra used to classify FRBs vs RFI (Chapter 38).
Confusion matrix: A table of a classifier's predictions vs the true labels (true/false positives and negatives), from which accuracy, precision, and recall are read. The basic scorecard for a classification model (Chapter 38).
CME (coronal mass ejection): An eruption of plasma from the Sun whose shock drives a type II radio burst; its speed can be read off the burst's frequency drift (Chapter 23).
CO line: The rotational transitions of carbon monoxide (J=1→0 at 115 GHz and its harmonics), the standard millimetre tracer of cold molecular gas (Chapter 24).
Continuum: Broadband emission that varies smoothly with frequency, as opposed to narrow spectral lines. Synchrotron and thermal (free–free) radiation are continuum sources; continuum imaging is the subject of Chapter 12.
Correlator: The hardware (or software) that multiplies and time-averages the signals from every pair of antennas to produce visibilities. The beating heart of an interferometer.
Cross-correlation: Multiplying the (time-aligned) signals from two antennas and averaging. The result, as a function of baseline, is the visibility — the quantity an interferometer measures.
Coherent vs incoherent emission: Incoherent emission sums the power of many independent radiators (ordinary synchrotron, thermal); coherent emission sums their fields in phase, reaching brightness temperatures far above the \(\sim10^{12}\) K incoherent limit. Pulsars, FRBs, and long-period transients must be coherent (Chapters 13, 18, 47).

D¶

Declination (Dec, \(\delta\)): The celestial-sphere analogue of latitude: angular distance north (+) or south (−) of the celestial equator, from \(-90°\) to \(+90°\). See math-preliminaries.md for the coordinate machinery.
Deconvolution: Recovering the true sky from a dirty image by removing the effect of the dirty beam. Because uv coverage is incomplete, this is an ill-posed problem that needs an algorithm like CLEAN to constrain (Chapter 9).
De-dispersion: Removing the frequency-dependent dispersion delay by shifting each channel back in time so a broadband pulse re-aligns; the core of pulsar and FRB searches (Chapters 13, 18).
Doppler drift: The slow change in a narrowband signal's frequency from the relative acceleration of transmitter and receiver (e.g. a planet's spin and orbit). SETI searches integrate along trial drift rates to recover such signals (Chapter 21).
Dirty beam / dirty image: The point-spread function produced by the array's actual (incomplete) uv sampling, and the raw image you get by Fourier-transforming the visibilities before deconvolution. The dirty image is the true sky convolved with the dirty beam (Chapters 8–9).
Dispersion measure (DM): The integrated free-electron column along the line of sight to a pulsar, \(\mathrm{DM} = \int n_e\,dl\). Because lower frequencies are delayed more, DM tells you how far the signal has travelled through the interstellar medium and must be removed ("dedispersed") to recover the pulse (Chapter 13).
Dynamic range (imaging): The ratio of the brightest pixel to the background noise (RMS) in an image — often \(10^4\)–\(10^6\) for a radio map of an AGN. It is why a linear display shows a single white dot on black, and why the asinh/log stretches exist (jansky.plotting.dynamic_range, Chapter 46).
Dynamic spectrum: A two-dimensional plot of intensity against time and frequency. Pulses, RFI, and FRBs show up as characteristic shapes (a dispersed pulse sweeps from high to low frequency).
Dark matter: Non-luminous mass inferred from gravity — most directly from the flat rotation curves of galaxies, where HI 21 cm velocities stay high far beyond the visible disk. It outweighs ordinary matter roughly five to one (Chapter 11).
Doppler effect: The shift of an observed frequency when source and observer move relative to one another, \(\Delta\nu/\nu \approx -v/c\). It converts spectral-line frequencies into velocities — the basis of HI rotation curves, maser dynamics, and the SETI drift search (Chapters 11, 21, 24).
Drift scan: Observing with the telescope held fixed while the sky drifts through the beam as the Earth rotates. Jansky's rotating antenna worked this way, and it is still the simplest way to survey or to catch a source at transit (Chapter 1).

E¶

Death line (pulsars): A line on the period–period-derivative (\(P\)–\(\dot P\)) diagram below which a rotation-powered pulsar can no longer sustain the pair cascades that power coherent radio emission. Long-period transients sit far below it (as neutron stars they "shouldn't" emit), which is the central puzzle of Chapter 47 (jansky.transients.death_line_pdot).
Epoch folding: Finding an unknown period by folding a time series at many trial periods and scoring each folded profile (the Leahy \(\chi^2\) statistic); the score peaks sharply at the true period. How the minutes-to-hours periods of long-period transients are recovered (jansky.transients.epoch_folding_search, Chapter 47).
Effective area (\(A_e\)): The equivalent collecting area of an antenna for a matched source, \(A_e = \eta_A A_{\text{geom}}\). It sets the gain and, with \(T_{\text{sys}}\), the sensitivity via the SEFD.
Emission measure (EM): The line-of-sight integral of the squared electron density, \(\mathrm{EM} = \int n_e^2\, \mathrm{d}l\) (in pc cm⁻⁶). It sets the free-free optical depth and hence the whole thermal spectrum of an HII region — fitting it (with \(T_e\)) to a radio spectrum measures the density and depth of the ionized gas (Chapter 44).
Equipartition (minimum-energy) field: The magnetic field that minimises the total energy in relativistic particles plus field for an observed synchrotron source; near the minimum the particle and field energies are comparable. It is the standard way to estimate \(B\) (a few µG to tens of µG in radio lobes) from the radio luminosity and source size (Chapter 43).
Epoch of Reionization (EoR): The era (\(z \sim 6\)–10, observed at ~130–200 MHz) when ultraviolet light from the first galaxies reionized the neutral intergalactic hydrogen left over from recombination, extinguishing the redshifted 21 cm signal. Probed by the 21 cm power spectrum (LOFAR, MWA, HERA, SKA-Low) (Chapter 42).
Ephemeris: A table or model giving the predicted position (and for pulsars, the spin phase) of a source as a function of time. Pulsar timing uses an ephemeris to predict pulse arrival times (Chapter 13).

F¶

Fanaroff–Riley class (FR I / FR II): The morphological split of powerful radio galaxies: FR I sources are edge-darkened (brightest near the core, jets fading outward) and lower-luminosity; FR II sources are edge-brightened, with collimated jets feeding bright lobes and terminal hotspots, and higher luminosity. The transition sits near \(L_{178\,\mathrm{MHz}} \sim 10^{25}\) W Hz⁻¹ (Fanaroff & Riley 1974) (Chapter 45).
Faraday rotation: The rotation of a linearly polarised wave's plane of polarisation as it passes through a magnetised plasma, by an angle \(\Delta\chi = \mathrm{RM}\,\lambda^2\). The \(\lambda^2\) dependence is the signature that lets the rotation measure be recovered (Chapter 37).
Fast radio burst (FRB): A millisecond-duration, highly dispersed radio flash, mostly extragalactic in origin. Their large DMs make them probes of the otherwise-invisible diffuse cosmic baryons.
Flagging: Marking corrupted samples (RFI, dead antennas, edge channels) so they are excluded from averaging and imaging. Lossy but essential — too little leaves interference in the data, too much throws away signal and biases statistics (Chapter 39; Maths Lab C).
Flux density (\(S_\nu\)): Power received per unit area per unit frequency, integrated over the source — measured in janskys. The fundamental quantity describing how bright a radio source appears.
Free-free (thermal bremsstrahlung): The thermal continuum radiated by free electrons scattering off ions in ionized gas (HII regions, planetary nebulae, the thermal Galactic background). It is unpolarised and flat: optically thick at low frequency (\(T_B \to T_e\), \(S_\nu \propto \nu^2\)) and optically thin above the turnover, where the spectral index flattens to \(\alpha \approx -0.1\). The flat-vs-steep contrast with synchrotron is the standard thermal/non-thermal diagnostic (Chapter 44).
Fringe: The sinusoidal interference pattern an interferometer produces as a source moves through the beam. Its amplitude and phase are exactly the visibility; "fringe-fitting" solves for residual delays and rates (Chapter 7).
FWHM (full width at half maximum): The width of a peak (a beam, a spectral line, a pulse) measured between the points where it drops to half its maximum. A standard, robust measure of "how wide".

G¶

Gain: How efficiently an antenna converts incoming flux into measured signal — for a dish, \(G = A_e/(2k_B)\) in K/Jy. In calibration, "gains" also means the per-antenna complex amplitude and phase corrections solved for during reduction.
Gain calibration: Solving for each antenna's complex gain \(g_i\) (amplitude and phase, varying with time and frequency) — usually from a bright calibrator of known structure — and dividing it out of the data. Bandpass calibration is the frequency-dependent case; self-calibration uses the target's own model (Chapter 41, Maths Lab E).
GNU Radio: An open-source toolkit for building signal-processing "flowgraphs" from blocks (source → filter → FFT → sink); the basis of many amateur radio-astronomy receivers (Chapter 28).
GUPPI raw: A voltage-data file format (FITS-like ASCII header + 8-bit complex samples) from Green Bank / Breakthrough Listen backends; the input to the SETI search pipeline (Chapter 16).

H¶

Hanbury Brown–Twiss (HBT) / intensity interferometry: Correlating the intensity fluctuations of two detectors rather than their amplitudes; recovers a source's angular size while needing only modest timing — sidestepping the phase-coherence burden of ordinary interferometry (Chapter 25).
Hellings–Downs curve: The characteristic quadrupolar correlation between pulsar pairs as a function of their angular separation — the fingerprint of a gravitational-wave background in a pulsar timing array (Chapter 20).
HI / 21 cm line: The spectral line of neutral atomic hydrogen at 1420.4 MHz (wavelength 21 cm), from the hyperfine spin-flip transition. It traces cold gas across the Galaxy and, via its Doppler shift, the rotation of the Milky Way (Chapters 6 and 11).
Hour angle (HA): How far west of the local meridian a source has moved, measured in time or angle: \(\mathrm{HA} = \text{LST} - \alpha\). It encodes where a source is in its daily arc across the sky.
HII region: A cloud of ionized hydrogen (H⁺, "H-two") surrounding hot young stars, glowing in free-free continuum and radio recombination lines. Its sharp-edged extent is the Strömgren sphere (Chapter 44).
Hubble's law: The proportionality between a galaxy's recession velocity (redshift) and its distance, \(v = H_0 d\) — the signature of cosmic expansion. The Hubble constant \(H_0\) sets the distance scale underlying the Macquart relation (Chapters 18, 42).

I¶

Interferometer: An array of antennas whose signals are combined to act as one large telescope, achieving resolution set by the longest baseline rather than by dish size. The subject of Part III.
Inverse-Compton catastrophe: The runaway cooling that caps an incoherent synchrotron source's brightness temperature at ~\(10^{12}\) K: above it, the relativistic electrons inverse-Compton scatter their own synchrotron photons away. Apparent temperatures far higher signal relativistic (Doppler) beaming (Chapter 43).
IQ data: Complex baseband samples — In-phase (real) and Quadrature (imaginary) — that capture both amplitude and phase of a signal. The native output of an SDR and of digital receivers generally (Chapter 5).
Interstellar medium (ISM): The gas, dust, plasma, and magnetic field between the stars. Its free electrons disperse pulsar signals (the dispersion measure), its neutral hydrogen emits the 21 cm line, and its magnetic field rotates polarisation (Chapters 6, 13, 37).
Intergalactic medium (IGM): The diffuse, highly ionized plasma between galaxies, holding much of the universe's ordinary (baryonic) matter. Fast radio bursts weigh it through the dispersion it adds — the Macquart relation (Chapter 18).

J¶

Jansky (Jy): The unit of flux density, \(1\,\mathrm{Jy} = 10^{-26}\,\mathrm{W\,m^{-2}\,Hz^{-1}}\). Named for Karl Jansky, who in 1932 first detected radio waves from the Milky Way — and the namesake of this course.

K¶

KrakenSDR: A five-channel, phase-coherent RTL-SDR (all sharing one clock) that makes amateur interferometry and radio direction-finding practical (Chapter 17).
Keplerian orbit: Motion under inverse-square gravity, in which orbital speed falls as \(v \propto r^{-1/2}\). Water masers tracing a Keplerian disk weighed the NGC 4258 black hole, and a Keplerian binary sets the clock of some long-period transients (Chapters 24, 47).

L¶

Long-period radio transient (LPT): A radio source that pulses every few minutes to hours — orders of magnitude slower than any ordinary pulsar. Since 2022 (GLEAM-X J1627) about a dozen are known; several have now been identified as white-dwarf + M-dwarf binaries where the period is orbital, not a spin, with the rest debated between ultra-long-period magnetars and white-dwarf pulsars (Chapter 47).
log N–log S (source counts): The number of radio sources brighter than flux density \(S\) as a function of \(S\). In a static, uniformly-filled Euclidean universe the integral counts follow \(N(>S) \propto S^{-3/2}\) (differential \(\mathrm{d}N/\mathrm{d}S \propto S^{-5/2}\)), independent of luminosity. The observed departure from this slope — steeper at bright flux, flatter at faint flux — measures the cosmic evolution of the source population and helped end steady-state cosmology (Chapter 45).
LSR (Local Standard of Rest): A reference frame moving with the mean motion of stars in the Sun's neighbourhood around the Galaxy. Radial velocities of HI are usually quoted relative to the LSR to remove the Sun's peculiar motion (Chapter 11).
Luminosity: The total power a source radiates (in watts) — an intrinsic property, unlike flux density, which also falls off with distance. The radio luminosity function tallies sources by it (Chapters 43, 45).

M¶

Magnetar: A neutron star with an extreme magnetic field (\(B \sim 10^{14}\)–\(10^{15}\) G) whose decaying field powers X-ray/gamma-ray bursts and, in a few cases, radio emission. Magnetars sit at the top-right of the \(P\)–\(\dot P\) diagram; an ultra-long-period magnetar is one candidate explanation for long-period radio transients, and a Galactic magnetar (SGR 1935+2154) produced an FRB-like burst (Chapters 18, 47).
Machine learning (ML): Algorithms that learn a task from labelled (or unlabelled) examples rather than from hand-coded rules. In radio astronomy, used for FRB/RFI classification, source-finding, and parameter estimation — powerful but data-hungry, and only as good as its training set (Chapter 38).
Macquart relation: The roughly linear relation between a fast radio burst's dispersion measure and its redshift, which turns FRBs into probes of the universe's diffuse "missing" baryons (Chapter 18).
Maser: A microwave laser — naturally occurring, compact, very bright spectral-line emission (e.g. H₂O at 22 GHz). Masers orbiting in a Keplerian disk weighed the NGC 4258 black hole (Chapter 24).
Matched filter: The optimal linear detector for a known signal shape in white noise: correlate the data with a copy of the template. De-dispersion and the SETI drift search are matched filters (Chapters 18, 21; Maths Lab B).
Measurement equation: The model relating what an interferometer records to the true sky, antenna by antenna: \(V^\mathrm{obs}_{ij} = g_i\,V^\mathrm{true}_{ij}\,g_j^{*} + n_{ij}\) (and its full polarised, matrix form, Hamaker et al. 1996). Calibration inverts it for the gains \(g_i\) (Chapter 41).
Measurement Set (MS): CASA's on-disk data format for visibility data: a set of tables holding the correlated data plus all the metadata (antennas, frequencies, pointing). The standard input for Chapter 12.
Meteor scatter: Radio reflection off the ionised trail of a meteor; forward-scattering a distant transmitter lets amateurs count meteors as brief "pings" (Chapter 26).
Magnetosphere: The region around a planet or star where its magnetic field controls the motion of charged particles. Jupiter's drives decametric radio bursts; a pulsar's is where the coherent radio beam is generated (Chapters 23, 13).

N¶

Nanohertz gravitational waves: Gravitational waves with ~year periods, from supermassive-black-hole binaries — detectable only by pulsar timing arrays via the Hellings–Downs correlation (Chapter 20).
Noise temperature (\(T_N\)): The temperature of a fictitious resistor that would generate the same noise power as a given component (e.g. the receiver). Adding the noise contributions of every stage gives the system temperature.
Nyquist sampling: Sampling a band-limited signal at twice its bandwidth, the minimum rate that preserves all its information without aliasing. The reason an SDR's sample rate sets its usable bandwidth (Chapter 5).
Neutron star: The collapsed, city-sized remnant of a massive star — about \(1.4\,M_\odot\) crushed to nuclear density and threaded by an enormous magnetic field. We see them as pulsars and magnetars (Chapters 13, 47).

O¶

Optical depth (\(\tau\)): A dimensionless measure of how opaque a medium is along the line of sight; emergent intensity is attenuated by \(e^{-\tau}\). A source is optically thick (\(\tau \gg 1\)) or thin (\(\tau \ll 1\)), and the crossover shapes free-free, synchrotron-self-absorption, and 21 cm spectra (Chapters 43, 44).

P¶

Perceptually-uniform colormap: A colormap (viridis, inferno, cividis, magma) whose perceived lightness increases monotonically with the data, so equal data steps look like equal colour steps. The rainbow/"jet" map is not uniform — its non-monotonic luminance invents banding that isn't in the data — which is why it is avoided here (jansky.plotting.recommend_cmap, Chapter 46).
Parallactic angle: The angle between celestial north and the local vertical at a source, which changes as the source transits. It matters for alt-az telescopes and for tracking polarisation across an observation.
Phase centre: The point on the sky to which the interferometer's phases are referenced (the assumed centre of the field). Visibility phase measures position relative to the phase centre.
PICTOR: A free online hydrogen-line radio telescope you can point from a web browser; powered by the VIRGO package (Chapter 29).
Plasma frequency (\(f_p\)): The natural oscillation frequency of an ionised gas, \(f_p \propto \sqrt{n_e}\). Radio waves below it cannot propagate; in the solar corona it sets the frequency of solar bursts (Chapter 23).
Polarisation / Stokes parameters: The orientation state of the electromagnetic wave. The four Stokes parameters \((I, Q, U, V)\) fully describe it: \(I\) is total intensity, \(Q\) and \(U\) linear polarisation, \(V\) circular. Synchrotron emission is often linearly polarised, which probes magnetic fields (Chapter 37).
Polarisation fraction & angle: The linear polarisation fraction \(p = \sqrt{Q^2+U^2}/I\) (how polarised the emission is) and the polarisation (E-vector) angle \(\chi = \tfrac12\arctan(U/Q)\) (its orientation). The factor of two reflects that polarisation is a "headless vector" — a \(\pi\) rotation is the same state (Chapter 37).
Primary beam: The single-dish response pattern that sets an interferometer's field of view; sources far from the pointing centre are attenuated by it. Correcting for it ("primary-beam correction") restores true flux across the image (Chapter 12).
Pulsar: A rapidly rotating, highly magnetised neutron star whose beamed radio emission sweeps past us as regular pulses — a cosmic clock. Discovered by Bell and Hewish in 1967; the subject of Chapter 13.
Pulsar timing array (PTA): A set of millisecond pulsars timed for years as a galaxy-scale gravitational-wave detector. NANOGrav/EPTA/PPTA reported evidence for a nanohertz GW background in 2023 (Chapter 20).
Parsec (pc): The standard astronomical distance unit — the distance at which 1 AU subtends one arcsecond: \(1\,\mathrm{pc} \approx 3.26\) light-years \(\approx 3.086\times10^{16}\) m. Galactic distances run to kiloparsecs, extragalactic ones to megaparsecs (Chapter 11).
Plasma: An ionized gas of free electrons and ions — the state of most radio-relevant matter, from the ionosphere to the ISM to a stellar corona. Its plasma frequency fixes the lowest frequency that can propagate through it (Chapters 27, 23).
Point source: A source smaller than the telescope's resolution, so it appears simply as a copy of the beam (the PSF). The opposite of a resolved or extended source; CLEAN models the sky as a sum of point sources (Chapters 9, 12).

Q¶

Quasar: An extremely luminous AGN, so distant it appears starlike ("quasi-stellar"). Compact radio quasars make ideal calibrators and VLBI targets.

R¶

Radio window: The range of frequencies (roughly 10 MHz to ~1 THz) at which Earth's atmosphere is transparent enough for ground-based radio astronomy, bounded below by the ionosphere and above by molecular (mainly water) absorption (Chapter 1).
Radio luminosity function: The space density of radio sources per unit (logarithmic) luminosity. It is steeply declining: many faint star-forming galaxies, a break, and a rapidly thinning population of rare, powerful radio-loud AGN. Its convolution with cosmic evolution shapes the observed source counts (Chapter 45).
Radio-loud AGN: An active galactic nucleus whose accreting supermassive black hole launches powerful synchrotron-emitting jets, producing the radio galaxies and radio quasars that dominate the bright extragalactic radio sky. Orientation of the jet axis underlies the unified model (Chapters 45, 19).
Radio-quiet zone: A legally protected region where radio transmissions are restricted to shield a sensitive telescope — e.g. the US National Radio Quiet Zone around Green Bank, or the Murchison Radio-astronomy Observatory. A regulatory complement to on-line RFI flagging (Chapter 39).
Radio recombination line (RRL): A spectral line emitted when an electron recombines into a high-\(n\) level of an ion and cascades down (e.g. H109α at 5 GHz). Arising from the same ionized gas as free-free emission, RRLs give the gas velocity and — combined with the continuum — an independent electron temperature (Chapter 44).
ROC curve & AUC: The Receiver Operating Characteristic plots a classifier's true-positive rate against its false-positive rate as the decision threshold varies; the Area Under the Curve (AUC, 0.5 = chance, 1.0 = perfect) summarises it in one number. The standard way to compare detectors — e.g. a learned classifier vs a matched filter (Chapter 38).
RASDR (Radio Astronomy SDR): A LimeSDR-based amateur radio-astronomy receiver (myriadrf) that streams spectra to Radio-Sky Spectrograph over a TCP protocol (Chapter 30).
Radiometer equation: The relation giving the noise on a measurement, \(\Delta T \approx T_{\text{sys}} / \sqrt{\Delta\nu\,\tau}\) — sensitivity improves with more bandwidth and longer integration. The central tool of Chapter 3.
RFI (radio frequency interference): Human-made signals — phones, radar, satellites, microwave ovens — that contaminate radio data. Identifying and excising RFI ("flagging") is a routine and unglamorous part of every reduction.
RM synthesis (Faraday tomography): A Fourier-like transform of the complex polarisation \(P(\lambda^2) = Q + iU\) into Faraday depth \(\phi\), reconstructing emission as a function of \(\phi\) (Burn 1966; Brentjens & de Bruyn 2005). For a single Faraday-thin screen the result peaks at \(\phi = \mathrm{RM}\); its resolution is set by the \(\lambda^2\) coverage via the RMSF (Chapter 37).
Rotation measure (RM): The constant of proportionality in Faraday rotation, \(\Delta\chi = \mathrm{RM}\,\lambda^2\), equal to \(0.81\int n_e B_\parallel\,\mathrm{d}l\) (rad m\(^{-2}\), with \(n_e\) in cm\(^{-3}\), \(B_\parallel\) in µG, \(l\) in pc). It measures the line-of-sight magnetic field weighted by electron density — the workhorse probe of cosmic magnetism (Chapter 37).
Right ascension (RA, \(\alpha\)): The celestial analogue of longitude, measured eastward along the celestial equator, usually in hours (0–24h). With declination it fixes a source's position on the sky.
Rayleigh–Jeans law: The low-frequency (\(h\nu \ll k_B T\)) limit of the Planck blackbody law, \(B_\nu \approx 2k_B T\nu^2/c^2\) — linear in temperature, which is exactly why radio astronomers quote intensities as a brightness temperature (Chapter 2).
Recombination (cosmology): The epoch about 380,000 years after the Big Bang (\(z \approx 1100\)) when the cooling universe let electrons and protons combine into neutral hydrogen, releasing the CMB. (The same atomic process in an HII region produces radio recombination lines.) (Chapter 22).
Redshift (\(z\)): The fractional stretching of wavelength, \(1+z = \lambda_\mathrm{obs}/\lambda_\mathrm{emit}\), from cosmic expansion (or motion). It slides the 21 cm line of cosmic dawn down to metre wavelengths and calibrates the Macquart relation (Chapters 18, 42).
Rotation curve: A galaxy's orbital speed plotted against radius. Measured from the Doppler shift of HI 21 cm, the curves stay flat rather than falling off — the classic evidence for dark matter (Chapter 11).

S¶

SED (Saturn Electrostatic Discharge): An impulsive HF radio burst (1–40 MHz) from lightning in Saturn's storms, first seen by Voyager and studied in detail by Cassini — planetary lightning detected by exactly the dynamic-spectrum and polarimetry methods this course teaches (Chapter 40).
SEFD (system-equivalent flux density): The flux density of a source that would double the system noise, \(\mathrm{SEFD} = 2k_B T_{\text{sys}} / A_e\), in janskys. A single number summarising an antenna's sensitivity — lower is better.
Sferic: The broadband radio impulse ("radio atmospheric") from a lightning return stroke; its energy peaks in the VLF band (~5–10 kHz) and propagates globally in the Earth–ionosphere waveguide. Sferics are the signal lightning-location networks trilaterate, and a textbook broadband-RFI contaminant for radio telescopes (Chapters 40, 39).
SETI (search for extraterrestrial intelligence): The radio search for artificial narrowband signals (technosignatures), told apart from nature by being narrowband and Doppler-drifting (Chapter 21).
Self-calibration: Using a strong source in the field itself to solve for time-varying antenna gains and phases, then re-imaging — iterating image and calibration together. It dramatically improves dynamic range when closure relations constrain the solution.
Sidereal time: Time kept by the stars rather than the Sun; the Local Sidereal Time (LST) equals the RA currently on your meridian. A sidereal day is ~4 minutes shorter than a solar day — the same 4 minutes by which Jansky's hiss rose earlier each day.
SigMF: An open "Signal Metadata Format" for SDR recordings — a JSON metadata sidecar (.sigmf-meta) plus a raw binary data file (.sigmf-data) (Chapter 16).
Spectral index (\(\alpha\)): The power-law slope of a source's spectrum, \(S_\nu \propto \nu^{\alpha}\). A steep negative index signals synchrotron emission; a flatter or positive one suggests thermal or self-absorbed sources — the basis of separating emission mechanisms (Chapter 2).
Spectral kurtosis: A statistic that flags RFI by spotting signals too steady (continuous-wave) or too spiky to be natural Gaussian noise; it is ≈ 1 for clean noise (Maths Lab C).
Spin temperature (\(T_S\)): The excitation temperature of the 21 cm hyperfine levels, set by a weighted mean of the CMB, the gas kinetic temperature (collisions), and the Lyman-α colour temperature (the Wouthuysen–Field effect). Whether the cosmological 21 cm line is seen in absorption or emission against the CMB depends on \(T_S\) vs \(T_\mathrm{CMB}\) (Chapter 42).
Spectral line: Emission or absorption confined to a narrow range of frequencies, from a specific atomic or molecular transition (e.g. HI at 1420 MHz). Its Doppler shift gives velocity; its width gives temperature and turbulence.
Stokes parameters (\(I, Q, U, V\)): See Polarisation / Stokes parameters (under P) — the four numbers that fully describe a wave's polarisation state (Chapter 37).
Strömgren sphere: The roughly spherical zone of fully ionized hydrogen around a hot star, with a sharp edge at the Strömgren radius where the stellar ionizing-photon rate is balanced by recombinations, \(Q = \tfrac{4}{3}\pi R_s^3 n_e^2 \alpha_B\). It sets the size of an ionization-bounded HII region (Chapter 44).
Sudden ionospheric disturbance (SID): A jump in a VLF transmitter's received amplitude when a solar flare's X-rays ionise the ionospheric D-layer — the basis of SuperSID flare monitoring (Chapter 27).
Synchrotron radiation: Radiation from relativistic electrons spiralling in magnetic fields. It dominates the radio continuum of AGN, supernova remnants, and galaxies, with a characteristic power-law, often-polarised spectrum; a power-law electron distribution \(N(E)\propto E^{-p}\) radiates \(S_\nu\propto\nu^{\alpha}\) with \(\alpha = -(p-1)/2\) (Chapters 2, 43).
Synchrotron self-absorption (SSA): At low frequency a compact synchrotron source becomes optically thick to its own emission, turning the spectrum over to the universal \(S_\nu\propto\nu^{5/2}\) slope below the turnover — independent of the electron index. The turnover frequency probes the source's size and field (Chapter 43).
Spectral aging: The steepening of a synchrotron spectrum above a break frequency as radiative (synchrotron + inverse-Compton) losses deplete the highest-energy electrons first; the break frequency dates the source (Chapter 43).
Synthesised beam: The effective point-spread function of an interferometer, set by its uv coverage — the array's "resolution element". See Beam and Chapter 8.
System temperature (\(T_{\text{sys}}\)): The total noise of the whole observing system expressed as a temperature — receiver, atmosphere, ground spillover, and sky combined. It is the \(T_{\text{sys}}\) in the radiometer equation and the single most important sensitivity number (Chapters 3–4).
SED (spectral energy distribution): A source's flux density (or luminosity) across many frequencies, from radio to X-ray, which reveals the emission mechanisms at work. Built by cross-matching catalogues (Chapter 14). (Not to be confused with Saturn Electrostatic Discharge, above.)
Sidelobe: A secondary maximum of a beam pattern, away from the main lobe, through which a telescope picks up unwanted off-axis signal. The dirty beam's sidelobes are exactly what CLEAN removes (Chapters 4, 9).
Supernova remnant (SNR): The expanding, synchrotron-bright shell of ejecta and shocked gas left by a stellar explosion (Cassiopeia A, the Crab). A prime non-thermal radio source — and not to be confused with signal-to-noise ratio (Chapter 43).
Surface brightness: Flux density per unit solid angle (e.g. Jy/beam or Jy/sr) — intensity that, unlike total flux, does not change with distance. In the radio it is equivalent to a brightness temperature (Chapter 2).

T¶

Training / validation / test split: Partitioning labelled data so a model learns on one subset, is tuned on a second, and is evaluated on a held-out third it never saw. Reporting performance on the test set guards against over-fitting and over-optimistic accuracy (Chapter 38).
Tapering / weighting: How visibilities are weighted before imaging, trading resolution against sensitivity. Natural weighting maximises sensitivity (favours short baselines); uniform maximises resolution; Briggs/robust weighting tunes smoothly between the two. Tapering further down-weights long baselines to emphasise extended structure (Chapters 8–9).
Technosignature: An observable sign of technology — for radio, an artificial narrowband transmission — sought by SETI (Chapter 21).
TOGA (Time of Group Arrival): The refined arrival-time measure used by the Blitzortung lightning network — derived from the dispersed sferic's group delay rather than a single onset sample — fed into multilateration to geolocate strokes (Chapter 40).
Tweek: A sferic that has propagated far in the night-time Earth–ionosphere waveguide; near the ~1.6–1.8 kHz waveguide cutoff its group velocity falls to zero, stretching the low-frequency tail into a brief descending tone whose cutoff measures the D-layer height (Chapter 40).
Type II burst: A slow-drifting solar radio burst produced by a shock (often a CME) climbing outward through the corona; its frequency drift gives the shock speed (Chapter 23).
Transit: The instant a source crosses the local meridian (due south or north), when it rides highest and its hour angle is zero. Transit telescopes observe only there, letting the sky drift past (Chapters 1, 34).

U¶

u-v plane / visibility: The plane of spatial frequencies sampled by an interferometer; each baseline lands at a point \((u, v)\). The visibility measured there is the Fourier transform of the sky brightness (the van Cittert–Zernike theorem), so imaging is an inverse Fourier transform (Chapter 8).

V¶

VIRGO: An open-source Python package for single-dish spectral-line and continuum radio astronomy; the engine behind the PICTOR online telescope (Chapters 16, 29).
VLBI (very long baseline interferometry): Interferometry with antennas thousands of kilometres apart — even on different continents — recording independently and correlating later. It yields the highest angular resolution in astronomy (micro-arcseconds), as in the Event Horizon Telescope.

W¶

Whistler: Lightning energy that leaked up a geomagnetic field line and dispersed in the magnetospheric plasma, returning as a descending tone with group delay \(t \propto f^{-1/2}\) — the close cousin of the interstellar \(t \propto \nu^{-2}\) pulsar/FRB sweep, and de-dispersed by the same machinery (Chapters 40, 13, 18).
Wouthuysen–Field effect: The coupling of the 21 cm spin temperature to the gas kinetic temperature by Lyman-α photons scattering off neutral hydrogen. Once the first stars provide those photons, it drives the cosmic-dawn absorption trough (Chapter 42).
White dwarf: The Earth-sized, electron-degenerate remnant of a low-mass star. A magnetic white dwarf — alone or in a binary — is a leading explanation for the minute-to-hour periods of long-period transients (Chapter 47).

Z¶

Zenith angle: The angle of a source from straight overhead (the zenith); \(0°\) at the zenith, \(90°\) at the horizon. Larger zenith angles mean more atmosphere along the line of sight, hence more absorption and noise.