### Known keywords

HISTORY
  Some “History” comments. Whether this information should be given with specific keywords or in an History record is still an open question. This information is not really needed for further data reduction, but it helps bookkeeping.
MAXIS
  Number of axes.
MAXISi
  Number of elements for axis i. i goes from 1 to MAXIS. The first axis is usually used to define effectively the spectrum. Thus MAXIS1 corresponds to the number of channels. The second and third one can be used to define the coordinates of the spectra when storing a full spectra cube.
CTYPEi
  The type of physical coordinate on axis i: frequency, velocity, right ascension, time... The unit of the values of CRVALi and CRPIXi are conventionnally defined by the value of the CTYPEi keyword. The authorized values and associated units are
  FREQ or FREQUENCY
    Frequency axis. Unit: [Hz]. Frequency for channel number n is defined by: F(n) = RESTFREQ + CRVALi + ( n - CRPIXi ) * CDELTi
  LAMBDA or WAVELENG
    wavelength axis. Unit: [m].
  VELO or FELO
    Velocity axis. Unit: [m.s]. VELO corresponds to the radio convention while FELO implies channels regularly gridded in frequency but expressed as velocity in the optical convention. CLASS will process VELO and FELO in the same way *** JP. The referential is defined by the second part of the keyword:
    - -LSR or CITY or empty second part: Local Standart Rest velocity.
    - -HEL Heliocentric (barycentric) velocity.
    - -OBS or -TOP The frame of rest of the observer/telescope (topocentric).
    - -EAR or -GEO Geocentric velocity
    The full keyword may thus looks like: FELO, VELOCITY, VELO-GEO...
  RA- or RA
    Right ascension axis. Unit: [degrees]. Right Ascension RA or galactic longitude GLON: The information as presented here is slightly incomplete, since it would be in general necessary to have an information about the kind of projection used. On most radio telescopes, it is simply assumed that the angular offset in RA is divided by the cosine of Declination to represent “true” angular offsets (valid only for a small field). Small telescopes may need more elaborate projection systems. In the current example, the position really observed is
    Dec = CRVAL3 + ( 1 - CRPIX3 ) * CDELT3
    Ra = CRVAL2 + ( 1 - CRPIX2 ) * CDELT2 / COS(Dec)
  DEC- or DEC
    Declination axis. Unit: [degrees]. Same caveats as for RA.
  GLAT
    Galactic latitude axis. Unit: [degrees]. Same caveat as for RA.
  GLON
    Galactic longitude axis. Unit: [degrees]. Same caveat as for RA.
  TIME or UT or UTC
    Time axis. Unit: [s]. The time since DATE-OBS.
  STOKES
    Stokes axis. The Stokes parameter of the data as defined in the basic paper of Wells et al.
CRPIXi
  The array location of the reference pixel along axis i. CRPIXi's value may be a fractional number of pixels and/or outside of the limits of the array. This descriptor is optional for degenerated axes (i.e. 0 or 1 element).
CRVALi
  The value of the physical coordinate on axis i at the reference pixel.
CDELTi
  The increment in physical coordinates along axis i. This descriptor is optional for degenerated axes. Warning: Although there is a difference between the frequency spacing between channels and the frequency resolution of each channel, CLASS currently handles only one value for both.
BSCALE
  A value of 1 is assumed when it is undefined. The following formula will be applied to the FITS data: `CLASS_spectrum = BZERO + BSCALE * FITS_spectrum`
BZERO
  A value of 0 is assumed when it is undefined. The following formula will be applied to the FITS data: `CLASS_spectrum = BZERO + BSCALE * FITS_spectrum`
DATAMIN
  Minimum value of your spectrum. Understood by CLASS but unused.
DATAMAX
  Maximum value of your spectrum. Understood by CLASS but unused.
BLANK
  Blanking value. If not specified, -1e38 assumed. BSCALE and BZERO are applied to the blanking value.
PCOUNT
  Usually 0. But if you provide your data as Variable Length arrays, then you will have to tweak this value which specifies the size of the variable length array heap, plus a gap (if any). See THEAP.
GCOUNT
  Always 1 in Binary tables.
THEAP
  Byte offset of heap area. The heap area is where the Variable Length Array data are stored. They are usually stored just after the binary table. The heap does not need to begin at the end of the main table data. The user may choose to provide a gap between the main table and the beginning of the heap. The size of this gap, in bytes, can be found using the value of the THEAP keyword. If there is no gap, the value of THEAP is NAXIS1 * NAXIS2. If there is, the size of the gap is the difference between the value of THEAP and NAXIS1 * NAXIS2. A value of NAXIS1 * NAXIS2 is assumed when this keyword is not defined.
BEAMEFF
  The main-beam efficiency.
FORWEFF or ETAFSS
  The forward spillover and scattering efficiency.
GAINIMAG
  Ratio Image/Signal.
MH2O
  Water vapor content. Unit: [mm].
PRESSURE
  The ambiant atmospheric pressure. Unit: [hPa].
TOUTSIDE
  The ambient temperature. Unit [K].
NPHASE
  Value: from 1 to 8. For multi-phased spectra (i.e. frequency switching) number of phases.
DELTAFj
  Frequency offset for phase j. Unit: [Hz].
PTIMEj
  Duration of phase j. Unit: [s].
WEIGHTj
  Weight of phase j.
TELESCOP
  A string value giving the telescope name. CLASS truncates this string to the first 12 characters. There is a CLASS convention about TELESCOP, which is not mandatory: TELESCOP contains the name of the instrument, the name of the backend and the name of the subband (if any). For example: “HIFI-HRV-02” for subband 2 of the High Resolution Spectrometer (Vertical polarization) of HIFI (one of the Herschel's instrument).
AZIMUTH
  The azimuth at TIME. Unit: [degrees]. If the TIME axis is non-degenerated, then this is the azimuth at the TIME of the first pixel on the TIME axis.
ELEVATIO
  The elevation at TIME. Unit: [degrees]. Same caveat as for AZIMUTH.
DATE-OBS
  or `DATE_OBS` A string giving the observation date and optionally the time at the observation start using the new FITS y2k convention. Since CLASS does not handle the TIMESYS keyword yet to indicate the time system, UTC is assumed. Accepted formats are
  - 'dd/mm/yy' (1900 is added to yy)
  - 'yyyy/mm/dd'
  - 'yyyy/mm/ddThh:mm:ss.s', where 'T' stands for the letter 'T' and has no meaning except as a separator between date and time. The trailing time is ignored, you have to provide it through UT, UTC or LST.
DATE-RED
  or `DATE_RED` or DATE Same as DATE-OBS, except it is the date at which the data have been reduced.
SCAN or SCAN-NUM
  or `SCAN_NUM` Scan number of the observation, according to the CLASS terminology.
SUBSCAN
  Subscan number of the observation, according to the CLASS terminology.
UT or UTC
  Universal time of the start of the observation provided as a string or a number. Then the format is
  - 'hh:mm:ss.s', when this keyword contains a string.
  - The number of seconds after midnight, when this keyword contains a number.
LST
  Sideral time of the start of the observation. See UT or UTC for information about the format.
OBSTIME or EXPOSURE
  The effective integration time. Unit: [s]. Necessary for some kind of weighting when adding several spectra.
TSYS
  The system temperature. Unit: [K]. Necessary for some kind of weighting when adding several spectra.
TAU-ATM
  or `TAU_ATM` The opacity at RESTFREQ. CLASS does not handle the opacity in the image band yet.
EQUINOX
  The equinox of coordinates. Possible values are 1950 or 2000.
OBJECT
  A string value giving an object name. CLASS truncates this string to the first 12 characters.
LINE
  Molecular line name, for bookkeeping. CLASS truncates this string to the first 12 characters.
RESTFREQ
  The observed frequency at the reference pixel of the frequency-like axis. Unit: [Hz].
IMAGFREQ
  The image sideband frequency corresponding to RESTFREQ for double sideband operation. Unit: [Hz].
VELOCITY
  The velocity at the reference channel. The referential frame must be defined by VELDEF.
VELO-LSR or VLSR
  The velocity at the reference channel, understood as a velocity in the Local Standard Rest frame. Thus, you don't need to use VELDEF to specify the frame. Unit: [m.s].
VELO-OBS or VELO-TOP
  The velocity at the reference channel, understood as a velocity in the frame of rest of the observer/telescope. Thus, you don't need to use VELDEF to specify the frame. Unit: [m.s].
VELO-HEL
  The velocity at the reference channel, understood as a velocity in the Heliocentric (barycentric) frame. Thus, you don't need to use VELDEF to specify the frame. Unit: [m.s].
VELO-EAR or VELO-GEO
  The velocity at the reference channel, understood as a velocity in the Geocentric frame. Thus, you don't need to use VELDEF to specify the frame. Unit: [m.s].
DELTAV or DELTAVEL
  Velocity spacings of the channels. This information is duplicate with the rest frequency and frequency spacing of channels, but convenient. The velocity of a given channel n is thus given by V(n) = VLSR + (n - CRPIXi) * DELTAV. If DELTAV is not provided, then the velocity resolution will be calculated by DELTAV = -c / RESTFREQ * CDELT1, where CDELT1 is the frequency spacing and c the celerity of light.
VELDEF
  The velocity definition and frame (8 characters). The first 4 characters describe the velocity definition. Accepted definitions are
  - RADI Radio convention.
  - OPTI Optical convention.
  - RELA Relativistic convention.
  CLASS only handles RADI and does not handle OPTI or RELA. If nothing specified, RADI is assumed. This is the list of the values CLASS understands
  - LSR or RADI-LSR Local Standart Rest frame.
  - HEL or RADI-HEL Heliocentric (barycentric) frame.
  - OBS or RADI-OBS Frame of rest of the observer/telescope. Note that TOP is not handled here.
  - EAR or RADI-EAR Geocentric frame.