This section is taken verbatim from AIPS++ Note 236
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.
OBJECT A string value giving an object name. TELESCOP A string value giving the telescope name. BANDWID The total bandwidth of the backend in units of Hertz. DATE-OBS 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. EXPOSURE The effective integration time in seconds. TSYS 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.
OBSERVER A string giving the observer's name. OBSID A string describing the observation. PROJID A string describing the project. SCAN 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. OBSMODE 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. MOLECULE A string used as a line identifier (with TRANSITI). TRANSITI A string used as a line identifier (with MOLECULE.) TEMPSCAL A string describing the scaling applied to reach the output intensity scale (``TB'',``TA'',``TA*'',``TR'',``TR*''). FRONTEND A string giving the name of the front end device. BACKEND A string giving the name of the back end device. TCAL The calibration temp (K). THOT The hot load temp (K). TCOLD The cold load temp (K). TRX The receiver temp (K). FREQRES The frequency resolution in Hz. This may differ from the channel spacing. TIMESYS The time system which applies to all time columns and keywords (see the y2k FITS DATE agreement). VELDEF 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 The radial velocity of the reference frame wrt the observer. Vframe - Vtelescope. RVSYS The radial velocity, Vsource - Vtelescope. OBSFREQ The observed frequency (Hz) at the reference pixel of the frequency-like axis. IMAGFREQ The image sideband freq (Hz) corresp. to OBSFREQ. LST The LST (seconds) at the start of scan. AZIMUTH 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. ELEVATIO The elevation at TIME (deg) (same caveat as for AZIMUTH) TAU The opacity at OBSFREQ. TAUIMAGE The opacity at IMAGFREQ. TAUZENIT The opacity per unit air mass. HUMIDITY The relative humidity (fraction, 0..1). TAMBIENT The ambient temp (K). PRESSURE The atmospheric pressure (mm Hg). DEWPOINT The dew point (K). WINDSPEE The wind speed (m/s). WINDDIRE The wind direction (deg. west of north). BEAMEFF The main-beam efficiency. APEREFF The antenna aperture efficiency. ETAL The rear spillover and scattering efficiency. ETAFSS The forward spillover and scattering efficiency. ANTGAIN K per Jy. BMAJ The major main-beam FWHM (deg). BMIN The minor main-beam FWHM (deg). BPA The beam position angle (degrees east of north). SITELONG The site longitude (deg). SITELAT The site latitude (deg). SITEELEV The site elevation (m). RESTFREQ The rest frequency (Hz).