Standard FITS Keywords

Last updated by Tom Kuiper on 2000 June 28.

General Keywords

The following keywords have been approved by the IAU.

Required Keywords

Keyword Type Standard Comments

SIMPLE logical IAU T for Basic FITS format
F otherwise

BITPIX integer IAU number of bits used for pixel value:
8, 16, 32 for integers
-32, -64 for IEEE floats

NAXIS integer IAU number of coordinate axes

NAXISn integer IAU number of points along coordinate axis n

Conditionally Required Keywords

Keyword Type Standard Comments

EXTEND logical IAU Only required in the main header if there might be extension records: T if they conforms to the IAU generalized extension standard.

XTENSION char IAU Replaces SIMPLE in extension headers to specify the extension type. IAU-approved extensions are 'TABLE', 'IMAGE', and 'BINTABLE'

PCOUNT integer IAU Parameters used to compute the size of an extension, together with BITPIX and NAXIS1...NAXISn. PCOUNT must be 0 and GCOUNT 1 if XTENSION = 'IMAGE'

GCOUNT integer IAU

TFIELDS integer IAU Number of fields in each row if XTENSION = 'TABLE'

TBCOLnnn integer IAU Starting character position of field nnn if XTENSION = 'TABLE'

TFORMnnn char IAU FORTRAN format specifier for field nnn if XTENSION = 'TABLE'

NMATRIX integer SDF number of data matrices when XTENSION = 'BINTABLE' and EXTNAME = 'SINGLE DISH'

Reserved Optional General Keywords

Keyword Type Standard Comments

BLANK as per BITPIX IAU tape value for undefined pixel

BSCALE real IAU real pixel value = stored_value*BSCALE + BZERO

BZERO real IAU

BUNIT char IAU Units of the pixel value; recommended units are 'K', 'JY/BEAM', 'JY/PIXEL', 'MAG/PIXEL', 'M/SEC', 'DEG'

CTYPEn char IAU Quantity bein measured along coordinate n. Recommended values are 'FREQ', 'RA', 'Dec', 'GLON', 'GLAT', 'MM', 'LL', 'ELON', 'ELAT', 'TIME', 'LAMBDA', 'VELO' 'VELO-LSR', 'VELO-HEL', 'VELO-OBS', 'PIXEL', ' ', 'STOKES', 'COMPLEX', 'DISTANCE', 'ANGLE', 'UU', 'VV', 'WW', 'HA'

CRVALx real IAU X = CRVALx + (i-CRPIXx)*CDELTx

CDELTx real IAU

CRPIXx integer IAU

CROTAx real IAU rotation angle, in degrees, of actual axis n of the array from the coordinate type given by CTYPEn. As there is no prescribed rule for describing such rotations, the nature of the rotation should be explained in detail using comments.

DATAMAX real IAU maximum amplitude of data in real, not storage, units

DATAMIN real IAU minimum amplitude of data in real, not storage, units

DATE-OBS char IAU date of data acquisition as 'CCYY-MM-DD' or 'CCYY-MM-DDThh:mm:ss[.sss...]', but see the Y2K Agreement

DATE char IAU date of file creation; same format as for DATE-OBS

ORIGIN char IAU installation where the FITS file is being written

TELESCOP char IAU data acquisition telescope

INSTRUME char IAU This seems ambiguous in radio astronomy as to frontend or backend. SEST defines it as the backend receiver. SDF has specific keywords FRONTEND and BACKEND.

OBSERVER char IAU observer

AUTHOR char IAU Name of the author(s) if this is a published data set instead of observation data

REFERENC char IAU The journal or report reference for a published data set

OBJECT char IAU object of observation

COMMENT char IAU

HISTORY char IAU another type of COMMENT

EPOCH real IAU epoch of coordinates

EQUINOX real IAU equinox of coordinates

GROUPS logical IAU Used for groups of small arrays of data

PTYPEn integer IAU

PSCALn integer IAU

PZEROn integer IAU

EXTNAME char IAU Gives the name and version of an extension protocol. EXTNAME is required with value 'SINGLE DISH' for the proposed standard "single dish" format

EXTVER char IAU

EXTlevel integer IAU Provides for a hierarchical structure of extension records.

TTYPEnnn char IAU Type (heading) of column nnn when XTENSION = 'TABLE' or 'BINTABLE'

TUNITnnn char IAU Units for column nnn when XTENSION = 'TABLE' or 'BINTABLE'

TSCALnnn real IAU Scale factor for column nnn when XTENSION = 'TABLE' or 'BINTABLE'

TZEROnnn real IAU Zero point for column nnn when XTENSION = 'TABLE' or 'BINTABLE'

TNULLnnn char IAU Null value for column nnn when XTENSION = 'TABLE' or 'BINTABLE'

TDISPnnn char IAU FORTRAN90 display format for column nnn when XTENSION = 'BINTABLE'

TDIMnnn char IAU dimension specifier, e.g. (50,40), if the field is an array (when XTENSION = 'BINTABLE'}

THEAP integer IAU byte offset of the heap area when a BINTABLE contains arrays as fields

Required Keywords
Keyword	Type	Standard	Comments
SIMPLE	logical	IAU	T for Basic FITS format F otherwise
BITPIX	integer	IAU	number of bits used for pixel value: 8, 16, 32 for integers -32, -64 for IEEE floats
NAXIS	integer	IAU	number of coordinate axes
NAXISn	integer	IAU	number of points along coordinate axis `n`
Conditionally Required Keywords
Keyword	Type	Standard	Comments
EXTEND	logical	IAU	Only required in the main header if there might be extension records: T if they conforms to the IAU generalized extension standard.
XTENSION	char	IAU	Replaces SIMPLE in extension headers to specify the extension type. IAU-approved extensions are 'TABLE', 'IMAGE', and 'BINTABLE'
PCOUNT	integer	IAU	Parameters used to compute the size of an extension, together with BITPIX and NAXIS1...NAXISn. PCOUNT must be 0 and GCOUNT 1 if XTENSION = 'IMAGE'
GCOUNT	integer	IAU
TFIELDS	integer	IAU	Number of fields in each row if XTENSION = 'TABLE'
TBCOLnnn	integer	IAU	Starting character position of field nnn if XTENSION = 'TABLE'
TFORMnnn	char	IAU	FORTRAN format specifier for field nnn if XTENSION = 'TABLE'
NMATRIX	integer	SDF	number of data matrices when XTENSION = 'BINTABLE' and EXTNAME = 'SINGLE DISH'
Reserved Optional General Keywords
Keyword	Type	Standard	Comments
BLANK	as per BITPIX	IAU	tape value for undefined pixel
BSCALE	real	IAU	real pixel value = stored_value*BSCALE + BZERO
BZERO	real	IAU
BUNIT	char	IAU	Units of the pixel value; recommended units are 'K', 'JY/BEAM', 'JY/PIXEL', 'MAG/PIXEL', 'M/SEC', 'DEG'
CTYPEn	char	IAU	Quantity bein measured along coordinate `n`. Recommended values are 'FREQ', 'RA', 'Dec', 'GLON', 'GLAT', 'MM', 'LL', 'ELON', 'ELAT', 'TIME', 'LAMBDA', 'VELO' 'VELO-LSR', 'VELO-HEL', 'VELO-OBS', 'PIXEL', ' ', 'STOKES', 'COMPLEX', 'DISTANCE', 'ANGLE', 'UU', 'VV', 'WW', 'HA'
CRVALx	real	IAU	X = CRVALx + (i-CRPIXx)*CDELTx
CDELTx	real	IAU
CRPIXx	integer	IAU
CROTAx	real	IAU	rotation angle, in degrees, of actual axis n of the array from the coordinate type given by CTYPEn. As there is no prescribed rule for describing such rotations, the nature of the rotation should be explained in detail using comments.
DATAMAX	real	IAU	maximum amplitude of data in real, not storage, units
DATAMIN	real	IAU	minimum amplitude of data in real, not storage, units
DATE-OBS	char	IAU	date of data acquisition as 'CCYY-MM-DD' or 'CCYY-MM-DDThh:mm:ss[.sss...]', but see the Y2K Agreement
DATE	char	IAU	date of file creation; same format as for DATE-OBS
ORIGIN	char	IAU	installation where the FITS file is being written
TELESCOP	char	IAU	data acquisition telescope
INSTRUME	char	IAU	This seems ambiguous in radio astronomy as to frontend or backend. SEST defines it as the backend receiver. SDF has specific keywords FRONTEND and BACKEND.
OBSERVER	char	IAU	observer
AUTHOR	char	IAU	Name of the author(s) if this is a published data set instead of observation data
REFERENC	char	IAU	The journal or report reference for a published data set
OBJECT	char	IAU	object of observation
COMMENT	char	IAU
HISTORY	char	IAU	another type of COMMENT
EPOCH	real	IAU	epoch of coordinates
EQUINOX	real	IAU	equinox of coordinates
GROUPS	logical	IAU	Used for groups of small arrays of data
PTYPEn	integer	IAU
PSCALn	integer	IAU
PZEROn	integer	IAU
EXTNAME	char	IAU	Gives the name and version of an extension protocol. EXTNAME is required with value 'SINGLE DISH' for the proposed standard "single dish" format
EXTVER	char	IAU
EXTlevel	integer	IAU	Provides for a hierarchical structure of extension records.
TTYPEnnn	char	IAU	Type (heading) of column nnn when XTENSION = 'TABLE' or 'BINTABLE'
TUNITnnn	char	IAU	Units for column nnn when XTENSION = 'TABLE' or 'BINTABLE'
TSCALnnn	real	IAU	Scale factor for column nnn when XTENSION = 'TABLE' or 'BINTABLE'
TZEROnnn	real	IAU	Zero point for column nnn when XTENSION = 'TABLE' or 'BINTABLE'
TNULLnnn	char	IAU	Null value for column nnn when XTENSION = 'TABLE' or 'BINTABLE'
TDISPnnn	char	IAU	FORTRAN90 display format for column nnn when XTENSION = 'BINTABLE'
TDIMnnn	char	IAU	dimension specifier, e.g. (50,40), if the field is an array (when XTENSION = 'BINTABLE'}
THEAP	integer	IAU	byte offset of the heap area when a BINTABLE contains arrays as fields

Spectral Line Keywords

The following keywords have been adopted for the Spectral-line Data Format (SDF).

Required Spectral Line Keywords

For the SDF format, the optional standard keywords OBJECT, TELESCOP, and DATE-OBS are required.

Keyword Type Standard Comments

FREQRES real SDF frequency resolution (Hz) of a spectrometer channel

BANDWID real SDF bandwidth (Hz) of the spectrometer

TSYS real SDF system (not receiver) temperature (K)

EXPOSURE real SDF effective integration time (sec), defined so that

TIME real SDF UT in seconds since 00^h UT

Optional Spectral Line Keywords

Keyword Type Standard Comments

OBSTIME real SEST True observed integration time (sec). For a raw spectrum, EXPOSURE = OBSTIME. For on/off switching processed in hardware, for example, EXPOSURE = OBSTIME/4

COMB

FREQRES real SDF The frequency resolution in Hz. This may differ from the channel spacing.

OBSID char SDF A string describing the observation.

PROJID char SDF A string describing the project.

SCAN real SDF scan number

SCAN-NUM integer SEST

real COMB

OBSMODE char SDF The type of data and observing mode (8 characters total). The type (LINE, CONT, PULS, etc) + the mode (PSSW, FQSW, BMSQ, PLSQ, LDSW, TLPW, etc). These rules do NOT define these observing modes. Writers are strongly encouraged to use the FITS comments to document these modes.

SEST Uses different mode identifiers

MOLECULE char SDF A string used as a line identifier (with TRANSITI).

TRANSITI char SDF A string used as a line identifier (with MOLECULE.)

LINE char SEST molecule formula and transition

TEMPSCAL char SDF A string describing the scaling applied to reach the output intensity scale (``TB'',``TA'',``TA*'',``TR'',``TR*'').

FRONTEND char SDF A string giving the name of the front end device.

BACKEND char SDF A string giving the name of the back end device.

TCAL real SDF The calibration temp (K).

THOT real SDF The hot load temp (K).

TCOLD real SDF The cold load temp (K).

TRX real SDF Receiver temperature (K)

TREC SEST

TIMESYS char SDF The time system which applies to all time columns and keywords (see the y2k FITS DATE agreement).

VELDEF char SDF The velocity definition and frame (8 characters). The first 4 characters describe the velicity definition. Possible definitions include:

RADI radio

OPTI optical

RELA relativistic

The second 4 characters describe the reference frame (e.g. ``-LSR'', ``-HEL'', ``-OBS''). If the frequency-like axis gives a frame, then the frame in VELDEF only applies to any velocities given as columns or keywords (virtual columns).

VFRAME real SDF The radial velocity of the reference frame wrt the observer. V_frame - V_telescope.

RVSYS real SDF The radial velocity, V_source - V_telescope.

OBSFREQ real SDF The observed frequency (Hz) at the reference pixel of the frequency-like axis.

CENTFREQ SEST sky frequency (Hz)

IMAGFREQ real SDF The image sideband freq (Hz) corresp. to OBSFREQ.

SEST

LST real SDF The LST (seconds) at the start of scan.

char SEST sidereal time at start of observation, e.g., '12:04:24.28'

AZIMUTH real SDF The azimuth at TIME (deg) (if the TIME axis is non-degenerate, then this is the azimuth at the TIME of the first pixel on the TIME axis.

SEST telescope azimuth readout (deg)

ELEVATIO real SDF The elevation at TIME (deg) (same caveat as for AZIMUTH)

SEST telescope elevation readout (deg)

TAU real SDF The opacity at OBSFREQ.

TAUIMAGE real SDF The opacity at IMAGFREQ.

TAUZENIT real SDF The opacity per unit air mass.

HUMIDITY real SDF The relative humidity (fraction, 0..1).

SEST

TAMBIENT real SDF The ambient temp (K).

TAMB Class

PRESSURE real SDF The atmospheric pressure (mm Hg).

SEST The atmospheric pressure (Pa).

DEWPOINT real SDF The dew point (K).

WINDSPEE real SDF The wind speed (m/s).

WINDDIRE real SDF The wind direction (deg. west of north).

BEAMEFF real SDF The main-beam efficiency.

SEST

APEREFF real SDF The antenna aperture efficiency.

SEST

ETAL real SDF The rear spillover and scattering efficiency.

ETAFSS real SDF The forward spillover and scattering efficiency.

ANTGAIN real SDF K per Jy.

BMAJ real SDF The major main-beam FWHM (deg).

BMIN real SDF The minor main-beam FWHM (deg).

BPA real SDF The beam position angle (degrees east of north).

SITELONG real SDF The site longitude (deg).

SITELAT real SDF The site latitude (deg).

SITEELEV real SDF The site elevation (m).

RESTFREQ real SDF The rest frequency (Hz).

SEST