World coordinate system (WCS) keywords say what each pixel means: a position on the
sky, a wavelength, a time. They are ordinary header
keywords. In the names below, i is a world axis, j a pixel axis, and a an optional
letter A to Z for an alternative description (blank for the
primary one). WCSNAMEa names each description.
Core keywords
| Keyword | Value | Default | Meaning |
|---|---|---|---|
WCSAXESa | Integer | NAXIS or the largest i or j used | Number of world axes. Must come before the other WCS keywords. |
CTYPEia | String | Blank: linear, undefined | Axis type. Non-linear axes use “4-3” form: a 4-character type, a hyphen, a 3-character algorithm code (RA---TAN). |
CUNITia | String | Blank | Units of CRVAL and CDELT; celestial axes are in degrees. |
CRPIXja | Real | 0.0 | Reference point on pixel axis j. The centre of the first pixel is 1.0. |
CRVALia | Real | 0.0 | World coordinate at the reference point. |
CDELTia | Real | 1.0 | World increment per pixel at the reference point; never 0. |
PCi_ja | Real | 1.0 when i = j, else 0.0 | Linear transformation matrix from pixel to intermediate axes. Must not be singular. |
CDi_ja | Real | 0.0 if any CD keyword is present | The matrix with the scale included (CDELT × PC). Never together with PCi_j. |
CROTAi | Real | 0.0 | Deprecated rotation angle; not with PCi_j, PVi_m or PSi_m. |
PVi_ma | Real | Depends on the algorithm | Numeric parameter m (0 to 99) of the axis algorithm. |
PSi_ma | String | Depends on the algorithm | Character parameter m of the axis algorithm. |
CNAMEia | String | Blank | Free name of the axis. |
CRDERia, CSYERia | Real | 0.0 | Random and systematic error of the coordinate. |
CZPHSia, CPERIia | Real | – | Phase zero point and period, for phase axes. |
A pixel position p goes to an intermediate coordinate
xi = CDELTi × Σj PCi_j × (pj − CRPIXj),
and then through the algorithm named in CTYPEi to the world coordinate.
Celestial axes
| CTYPE pair | Coordinates |
|---|---|
RA--, DEC- | Equatorial right ascension and declination |
GLON, GLAT | Galactic |
ELON, ELAT | Ecliptic |
HLON, HLAT | Helioecliptic |
SLON, SLAT | Supergalactic |
yzLN, yzLT | Planetary, lunar and solar systems (y and z chosen by the user) |
| Code | Projection | Code | Projection |
|---|---|---|---|
AZP | Zenithal perspective | PAR | Parabolic |
SZP | Slant zenithal perspective | MOL | Mollweide |
TAN | Gnomonic | AIT | Hammer-Aitoff |
STG | Stereographic | COP | Conic perspective |
SIN | Slant orthographic | COE | Conic equal-area |
ARC | Zenithal equidistant | COD | Conic equidistant |
ZPN | Zenithal polynomial | COO | Conic orthomorphic |
ZEA | Zenithal equal-area | BON | Bonne’s equal area |
AIR | Airy | PCO | Polyconic |
CYP | Cylindrical perspective | TSC | Tangential spherical cube |
CEA | Cylindrical equal area | CSC | COBE quadrilateralised spherical cube |
CAR | Plate carrée | QSC | Quadrilateralised spherical cube |
MER | Mercator | HPX | HEALPix grid |
SFL | Samson-Flamsteed |
| Keyword | Meaning |
|---|---|
RADESYSa | Reference frame, below |
EQUINOXa | Equinox of the frame, in years (EPOCH is the deprecated older name) |
LONPOLEa, LATPOLEa | Native longitude and latitude of the celestial pole |
DATE-OBS, MJD-OBS, DATE-AVG, MJD-AVG | Date of the observation, and the mean date, for frames that move |
RADESYS | Frame |
|---|---|
ICRS | International Celestial Reference System |
FK5 | Mean place, IAU 1984 system |
FK4 | Mean place, old Bessel-Newcomb system (avoid in new files) |
FK4-NO-E | FK4 without the eccentricity terms (avoid in new files) |
GAPPT | Geocentric apparent place, IAU 1984 system |
Spectral axes
A spectral CTYPEi has a type in characters 1 to 4 and, when the axis is
sampled linearly in a different quantity, an algorithm code in characters 6 to 8.
| Type | Quantity |
|---|---|
FREQ | Frequency |
ENER | Energy |
WAVN | Wavenumber |
VRAD | Radio velocity |
WAVE | Vacuum wavelength |
VOPT | Optical velocity |
ZOPT | Redshift |
AWAV | Air wavelength |
VELO | Apparent radial velocity |
BETA | Beta factor (v/c) |
| Algorithm codes | Axis sampled linearly in |
|---|---|
F2W, F2V, F2A | Frequency |
W2F, W2V, W2A | Vacuum wavelength |
V2F, V2W, V2A | Apparent radial velocity |
A2F, A2W, A2V | Air wavelength |
LOG | Logarithm of the quantity |
GRI, GRA | Grism dispersion (vacuum and air) |
TAB | A lookup table in a binary table extension |
| Keyword | Meaning |
|---|---|
RESTFRQa, RESTWAVa | Rest frequency and rest wavelength of the line |
SPECSYSa | Frame of the spectral axis |
SSYSOBSa | Frame in which the observer is at rest |
SSYSSRCa | Frame of ZSOURCEa |
VELOSYSa | Velocity of the observer relative to SPECSYS |
ZSOURCEa | Redshift of the source |
VELANGLa | Angle of the true velocity to the line of sight |
OBSGEO-X, -Y, -Z | Position of the observer, geocentric Cartesian, in metres |
Frames for SPECSYS, SSYSOBS and SSYSSRC:
TOPOCENT, GEOCENTR, BARYCENT,
HELIOCEN, LSRK, LSRD, GALACTOC,
LOCALGRP, CMBDIPOL and SOURCE (not allowed for
SSYSSRC).
Time
| Keyword | Meaning |
|---|---|
TIMESYS | Time scale, below; default UTC |
MJDREF, MJDREFI, MJDREFF | Reference time as a Modified Julian Date (whole, or integer and fraction parts) |
JDREF, JDREFI, JDREFF | Reference time as a Julian Date |
DATEREF | Reference time as an ISO date |
TIMEUNIT | Unit of times relative to the reference |
TIMEOFFS | Offset to add to all times |
TIMEDEL | Time resolution of the data |
TIMEPIXR | Position of the time stamp in its bin, 0.0 to 1.0 |
TIMSYER, TIMRDER | Systematic and random error of the times |
TREFPOS, TREFDIR | Where the times are measured, and the direction for that correction |
PLEPHEM | Solar system ephemeris used |
OBSORBIT | Orbit file of a space observatory |
OBSGEO-B, -L, -H | Observer’s geodetic latitude, longitude and height |
TSTART, TSTOP, TELAPSE | Start, stop and elapsed time |
XPOSURE | Effective exposure time |
DATE-BEG, DATE-END, MJD-BEG, MJD-END | Start and end of the data as dates |
JEPOCH, BEPOCH | Julian and Besselian epochs |
TRPOSn, TRDIRn | TREFPOS and TREFDIR for table column n |
Time scales for TIMESYS: TAI, TT,
TDT (deprecated name of TT), ET, IAT (deprecated
name of TAI), UT1, UTC, GMT, UT()
(with a qualifier), GPS, TCG, TCB,
TDB and LOCAL.
Places for TREFPOS: TOPOCENTER (default),
GEOCENTER, BARYCENTER, RELOCATABLE,
CUSTOM, and less often HELIOCENTER, GALACTIC,
EMBARYCENTER, MERCURY, VENUS,
MARS, JUPITER, SATURN, URANUS and
NEPTUNE.
A celestial header
CTYPE1 = 'RA---TAN'
CTYPE2 = 'DEC--TAN'
CRPIX1 = 512.5
CRPIX2 = 512.5
CRVAL1 = 10.68458
CRVAL2 = 41.26917
CDELT1 = -0.0002778
CDELT2 = 0.0002778
CUNIT1 = 'deg '
CUNIT2 = 'deg '
RADESYS = 'ICRS '