The 53 keywords currently defined in the FITS standard are as follows:
(blank) AUTHOR BITPIX BLANK BLOCKED BSCALE BUNIT BZERO CDELTn COMMENT CROTAn CRPIXn CRVALn CTYPEn DATAMAX DATAMIN DATE DATE-OBS END EPOCH EQUINOX EXTEND EXTLEVEL EXTNAME EXTVER GCOUNT GROUPS HISTORY INSTRUME NAXIS NAXISn OBJECT OBSERVER ORIGIN PCOUNT PSCALn PTYPEn PZEROn REFERENC SIMPLE TBCOLn TDIMn TDISPn TELESCOP TFIELDS TFORMn THEAP TNULLn TSCALn TTYPEn TUNITn TZEROn XTENSION
A concise definition of each of these is available from the FWG html page:
53 Standard FITS keywords or local file here
A summary of the standard keywords follows
BLANK BSCALE BUNIT BZERO CDELT CROTAn CRPIXn CRVALn CTYPEn CTYPEn DATAMAX DATAMIN
author of the data
-64,-32,8,16,32 - bits per data value
is physical blocksize a multiple of 2880?
descriptive comment
date of file creation 'yyyy-mm-ddTHH:MM:SS[.sss]'.
date of the observation The new Y2K compliant date format is 'yyyy-mm-dd' or 'yyyy-mm-ddTHH:MM:SS[.sss]'.
marks the end of the header keywords
deprecated
equinox of celestial coordinate system
(T/F)- may the FITS file contain extensions?
[1:] (default 1) hierarchical level of the extension. The value shall be 1 for the highest level; levels with a higher value of this keyword shall be subordinate to levels with a lower value.
The text should contain a history of steps and procedures associated with the processing of the associated data.
name of instrument
number of axes. In FITS 'TABLE' or 'BINTABLE' extensions, the value of NAXIS is always 2.
size of the axis
name of observed object
observer who acquired the data
organization responsible for the data
bibliographic reference
does file conform to the Standard?
name of telescope
marks beginning of new HDU. The value field shall contain a character string giving the name of the extension type.
[1:] (default 1) hierarchical level of the extension. The value shall be 1 for the highest level; levels with a higher value of this keyword shall be subordinate to levels with a lower value.
name of the extension used to distinguish among different extensions of the same type, i.e., with the same value of XTENSION, in a FITS file.
[1:] (default 1) version of the extension. The value field shall contain an integer, to be used to distinguish among different extensions in a FITS file with the same type and name, i.e., the same values for XTENSION and EXTNAME. The values need not start with 1 for the first extension with a particular value of EXTNAME and need not be in sequence for subsequent values.
number of axes. In FITS 'TABLE' or 'BINTABLE' extensions, the value of NAXIS is always 2.
size of the axis
begining column number. The value field of this indexed keyword shall contain an integer specifying the column in which field n starts in an ASCII TABLE extension.
dimensionality of the array. The value field of this indexed keyword shall contain a character string describing how to interpret the contents of field n as a multidimensional array, providing the number of dimensions and the length along each axis.
display format
number of columns in the table
column data format
offset to starting data heap address. The value field of this keyword shall contain an integer providing the separation, in bytes, between the start of the main data table and the start of a supplemental data area called the heap. The default value shall be the product of the values of NAXIS1 and NAXIS2. This keyword shall not be used if the value of PCOUNT is zero. A proposed application of this keyword is presented in Appendix B.1 of the FITS Standard.
value used to indicate undefined table element
linear data scaling factor. physical_value = TZEROn + TSCALn * field_value
column name. The use of identical names for different fields should be avoided.
column units
column scaling zero point. The default value for this keyword is 0.0. This keyword may not be used if the format of field n is A, L, or X.
marks beginning of new HDU. The value field shall contain a character string giving the name of the extension type.
group count. In most other cases this keyword will have the value 1.
indicates random groups structure The value field shall contain the logical constant T. The value T associated with this keyword implies that random groups records are present.
Reserved for use within the FITS Random Groups structure.
name of random groups parameter
parameter scaling zero point
parameter count. This keyword was originated for use with FITS Random Groups and represented the number of parameters preceding each group. It has since been used in 'BINTABLE' extensions to represent the size of the data heap following the main data table. In most other cases its value will be zero.
The additional GBT specific standard keywords
BACKEND BANDWDTH BANK DATEBLD DEVICE DELTA FEED FITSVER GBTMCVER MANAGER PORT RECEIVER RECEPTOR SAMPLER SIMULATE UTCSTART UTDSTART
From Single dish FITS
These must be provided in all SDFITS tables. They are essential and common to all observations and telescopes. All single dish FITS readers and writers must acknowledge (write and properly interpret) all CORE keywords.
A string value giving an object name.
A string value giving the telescope name.
The total bandwidth of the backend in units of Hertz.
A string giving the observation date and optionally the time at the start using the new FITS y2k convention. The TIMESYS keyword may be used to indicate the time system. UTC is assumed if TIMESYS is absent.
The effective integration time in seconds.
The system temperature in Kelvin.
These have agreeded definitions and interpretations however their presense is optional. These are largely common to all observations and telescopes but not essential. These may be ignored by a single dish FITS reader.
A string giving the observer's name.
A string describing the observation.
A string describing the project
A scan ID number. Typically this is an identification number given to a chunk of data when the data is taken. Not all telescopes provide a scan ID number.
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.
A string used as a line identifier (with TRANSITI).
A string used as a line identifier (with MOLECULE.)
A string describing the scaling applied to reach the output intensity scale (``TB'',``TA'',``TA*'',``TR'',``TR*'').
A string giving the name of the front end device.
A string giving the name of the back end device.
The calibration temp (K).
The hot load temp (K).
The cold load temp (K).
The receiver temp (K).
The frequency resolution in Hz. This may differ from the channel spacing.
The time system which applies to all time columns and keywords (see the y2k FITS DATE agreement).
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).
The radial velocity of the reference frame wrt the observer. Vframe -Vtelescope.
The radial velocity, Vsource - Vtelescope.
The observed frequency (Hz) at the reference pixel of the frequency-like axis.
The image sideband freq (Hz) corresp. to OBSFREQ.
The LST (seconds) at the start of scan.
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.
The elevation at TIME (deg) (same caveat as for AZIMUTH)
The opacity at OBSFREQ.
The opacity at IMAGFREQ.
The opacity per unit air mass.
The relative humidity (fraction, 0..1).
The ambient temp (K).
The atmospheric pressure (mm Hg).
The dew point (K).
The wind speed (m/s).
The wind direction (deg. west of north).
The main-beam efficiency.
The antenna aperture efficiency.
The rear spillover and scattering efficiency.
The forward spillover and scattering efficiency.
K per Jy.
The major main-beam FWHM (deg).
The minor main-beam FWHM (deg).
The beam position angle (degrees east of north).
The site longitude (deg).
The site latitude (deg).
The site elevation (m).
The rest frequency (Hz).
This data dictionary contains FITS keywords that have been widely used within the astronomical community. It is recommended that these keywords only be used as defined here. These keywords may be grouped within the following 7 broad categories:
TITLE FILENAME FILETYPE ROOTNAME PROGRAM CREATOR CONFIGUR NEXTEND HDUNAME HDUVER HDULEVEL TLMINn TLMAXn TDMINn TDMAXn TDBINn TSORTKEY PROGRAM CREATOR CONFIGUR HDUCLASS HDUDOC HDUVERS HDUCLASn
SUNANGLE MOONANGL RA DEC RA_NOM DEC_NOM RA_OBJ DEC_OBJ RA_PNT DEC_PNT PA_PNT RA_SCX DEC_SCX RA_SCY DEC_SXY RA_SCZ DEC_SCZ ORIENTAT AIRMASS LATITUDE OBJNAME OBS_ID
OBS_MODE DATAMODE APERTURE DETNAM FILTER FILTERn GRATING GRATINGn SATURATE
TIME-OBS TIME-END DATE-END EXPOSURE EXPTIME TELAPSE ELAPTIME ONTIME LIVETIME
HIERARCH INHERIT CONTINUE
CHECKSUM DATASUM CHECKVER
GRPNAME GRPIDn GRPLCn
Or local file Local version