Keywords: class

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### 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.
  ``

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