Glossary¶

Quick reference for terms used across the app and this documentation. Astronomy and tooling vocabulary mixed; see also the cited pages for deeper explanations.

Beam (synthesised beam)¶: The angular resolution element of a radio interferometer, described by the FITS header keywords BMAJ (major axis), BMIN (minor axis), and BPA (position angle), all in degrees. VisIVO draws a filled semi-transparent ellipse in the bottom-left corner of the 2-D slice view when these keywords are present. See Cube viewer — Beam indicator.
Blink / Compare¶: Rapid alternation of visibility between layers 0 and 1 in the image viewer, controlled by a configurable timer (50–1000 ms). Standard technique for detecting transients, proper motion, or artefact differences. See Image viewer — Blink / Compare.
BUNIT¶: FITS header keyword carrying the brightness / pixel-value unit (e.g. Jy/beam, K, counts). All region statistics and moment maps report values in BUNIT when it is set on the source cube / image.
CTYPE¶: FITS header keyword that names each WCS axis (e.g. RA---SIN, DEC--SIN, VELO-LSR). VisIVO inspects CTYPE to decide axis order, whether axis 3 is spectral, and which projection to use.
Channel¶: One pixel along the spectral axis of a cube. The cube has nchan channels; navigating with the slice slider moves you channel-by-channel.
Cutting plane¶: The plane indicator in the 3-D cube view that shows where the current 2-D slice sits along the spectral axis. The plane is textured with the slice contents and is interactive (see Cube viewer).
dataset_id¶: Backend identifier assigned to a FITS file the first time you open it via /v1/datasets/open. The client carries it as X-Visivo-Dataset in subsequent requests. Different opens of the same file inside the same session reuse the same ID.
EW¶: Equivalent width — a model-free width estimate equal to ∫ I dv / max(I). Use it when the line profile is non-Gaussian. See Line-width maps.
FWHM¶: Full width at half maximum — the spectral width at which the intensity drops to half of the peak. For a Gaussian profile, FWHM = 2.3548 σ.
FITS¶: Flexible Image Transport System — the standard astronomy file format for images, cubes, tables, and headers.
FK5 / J2000¶: Equatorial coordinate system (RA, Dec) defined by the FK5 catalogue at epoch J2000.0. Default sky frame in VisIVO.
Galactic¶: Coordinate system aligned with the plane of the Milky Way (longitude l, latitude b). VisIVO converts to/from FK5 with wcscon().
HiPS¶: Hierarchical Progressive Survey — multi-resolution tiled image format used by the IVOA to serve all-sky surveys. See HiPS viewer.
Isosurface¶: A closed 3-D surface where the cube’s intensity equals a given threshold. Useful to highlight dense clumps, jets, shells. Extracted server-side with vtkFlyingEdges3D.
Line identification (spectral-line overlay)¶: Overlay of expected spectral-line positions on the spectrum plot. Loaded from a two-column CSV file (frequency,label); lines are drawn as vertical dashed amber markers with rotated labels. See Cube viewer — Line identification overlay.
LRU cache¶: Least Recently Used — eviction policy of the backend’s PRODUCT_CACHE. Compute results (moments, isosurfaces, line-width maps, PV diagrams) are memoised so identical re-requests are free.
LSRK / LSR¶: Local Standard of Rest (Kinematic) — reference frame for radial velocities of Galactic objects. Carried via the SPECSYS FITS keyword; VisIVO reports it in the cube’s spectral axis tooltip.
MAD¶: Median Absolute Deviation — robust estimator of dispersion. Multiplied by 1.4826 it gives a robust 1σ estimate for Gaussian data, insensitive to outliers. Used in noise estimation and region stats.
Measurement tools (Ruler / Angle)¶: Interactive distance and angle measurement on the image viewer. Ruler shows pixel and angular distance (Haversine); Angle shows the angle at the middle vertex of three clicked points. See Image viewer — Measurement tools.
Moment (M0, M1, M2, …)¶: Statistical summary of a spectral line projected on the spatial axes. See Moment maps for the formulas and use cases.
PV¶: Position-Velocity diagram — 2-D plot of intensity along a path on the sky (X) vs. velocity (Y). Standard tool for kinematic inspection. See Regions, PV, noise.
Probe¶: Click-to-extract spectral profile at a single pixel of the cube’s 2-D slice (or via 3-D plane click). See Cube viewer.
ROI¶: Region of Interest. In the cube viewer, the Camera ROI is the spatial sub-volume corresponding to the current 3-D camera viewport; the backend serves only that ROI at full resolution.
SAMP¶: Simple Application Messaging Protocol — IVOA standard for inter-VO application messaging. Lets VisIVO exchange FITS / catalogues with TOPCAT / Aladin / DS9 at runtime. See SAMP.
Session¶: Backend-side container of all the datasets opened in one workflow. Identified by X-Visivo-Session, returned by /v1/datasets/open. Multi-cube tools (Spectral Stacking) operate within a single session.
SNR¶: Signal-to-Noise Ratio. Computed per pixel as peak / σ (with σ from MAD). VisIVO uses an SNR cutoff to skip background pixels in the line-width Gauss fit.
Spectral smoothing¶: Display-only 1-D convolution applied to the spectrum plot in the Spectral Profile window. Available kernels: Hanning, Boxcar (3/5/7 channels), Gaussian (σ=1/2 channels). The smoothing is NaN-safe and affects the stats bar but not CSV export. See Cube viewer — Spectral smoothing.
Spectral axis¶: Third FITS axis carrying frequency / velocity / wavelength information. CTYPE3 examples: VELO-LSR, VRAD, VOPT, FREQ. VisIVO reports the active axis kind, unit, and trustworthiness in the cube viewer status bar.
SPECVAL (SPECTYPE, SPECUNIT)¶: Non-standard FITS keywords written by VisIVO when Tools → Export Current Channel as 2-D FITS… extracts a single channel of a spectral cube. Since the output drops the spectral axis (NAXIS=2), VisIVO preserves the channel’s spectral coordinate as a scalar: SPECVAL stores the value (e.g. 1.284e+09 for 1.284 GHz), SPECTYPE mirrors the source cube’s CTYPE3 (e.g. FREQ, VRAD), and SPECUNIT mirrors CUNIT3 (e.g. Hz, km/s). The Stokes / spectral-index / Faraday-RM dialogs in the image viewer use these to prompt the user with the right frequency at recall time.
VBT¶: VisIVO Binary Table — efficient column-oriented binary format used by some VisIVO datasets for million-row point clouds.
VLKB¶: Via Lactea Knowledge Base — VisIVO’s data catalogue / cutout service. Reachable via the OIDC PKCE flow from the Startup Dialog.
VTK¶: Visualization Toolkit — the open-source 3-D rendering library underpinning the viewer windows.
WCS¶: World Coordinate System — the mapping from pixel indices to physical sky / spectral coordinates, defined by FITS keywords CTYPE, CRVAL, CRPIX, CDELT, CD, …. VisIVO reads it via libwcs.
worker pool¶: Backend process pool that runs the CPU-bound work. Default size 4 (controlled by VISIVO_WORKERS). Heavy tasks are gated by a semaphore (VISIVO_HEAVY_SLOTS) so interactive requests always have a free worker; see heavy-task throttle.