## Simple SPECTRUM Mode

FITS headers written by CLASS depend on the informations present in the corresponding CLASS headers. Any missing information will also be omitted in FITS (and vice versa). A typical FITS header written by CLASS looks like this :

    SIMPLE  =                    T         /
    BITPIX  =                   32         /
    NAXIS   =                    4         /                                (1)
    NAXIS1  =                  921         /                                (2)
    NAXIS2  =                    1         /                                (3)
    NAXIS3  =                    1         /                                (3)
    NAXIS4  =                    1         /                                (3)
    BLANK   =           2147483647         / Blanking value
    BSCALE  =  0.237091768440E-008         /
    BZERO   =  0.192957639694E+001         /
    DATAMIN = -0.316193056107E+001         /
    DATAMAX =  0.702108430862E+001         /
    BUNIT   = 'K (Ta*)'                    /                                (4)
    CTYPE1  = 'FREQ    '                   /                                (5)
    CRVAL1  =  0.000000000000E+000         / Offset frequency
    CDELT1  =  0.390625000000E+005         / Frequency resolution
    CRPIX1  =  0.578812988281E+003         /
    CTYPE2  = 'RA---GLS'                   /                                (6)
    CRVAL2  =  0.315384333333E+003         /
    CDELT2  =  0.835555750817E-002         /
    CRPIX2  =  0.000000000000E+000         /
    CTYPE3  = 'DEC--GLS'                   /                                (6)
    CRVAL3  =  0.681748750000E+002         /
    CDELT3  =  0.505729609368E-002         /
    CRPIX3  =  0.000000000000E+000         /
    CTYPE4  = 'STOKES  '                   /                                (7)
    CRVAL4  =      1.0000000000000         /
    CDELT4  =      1.0000000000000         /
    CRPIX4  =      0.0000000000000         /
    TELESCOP= '30M'                        /
    INSTRUME= '30ME0HUI-V01'               /
    OBJECT  = 'NGC7023'                    /
    RADESYS = 'FK5     '                   /
    EQUINOX =  0.200000000000E+004         /                                (8)
    LINE    = '13co10'                     / Line name                      (9)
    RESTFREQ=  0.110201354300E+012         / Rest frequency                 (10)
    VELO-LSR=  0.250000000000E+004         / [m/s] Velocity of referance ch (11)
    SPECSYS = 'LSRK'                       / Reference frame
    DELTAV  = -0.106265872717E+003         / Velocity spacing of channels   (12)
    IMAGFREQ=  0.972814604954E+011         / Image frequency                (13)
    TSYS    =  0.158461090088E+003         / System temperature             (14)
    OBSTIME =  0.492044001818E+000         / Integration time               (15)
    SCAN-NUM=  0.100000000000E+004         / Scan number                    (16)
    TAU-ATM =  0.189633205533E+000         / Atmospheric opacity            (17)
    NPHASE  =                    1         / Number of frequency phases     (18)
    DELTAF1 =  0.000000000000E+000         / Frequency offset Phase 1       (19)
    PTIME1  =  0.493851512671E+000         / Duration of Phase 1            (19)
    WEIGHT1 =  0.000000000000E+000         / Weight of Phase 1              (19)
    GAINIMAG=  0.501190014184E-001         / Image sideband gain ratio      (20)
    BEAMEFF =  0.949999988079E+000         / Beam efficiency                (21)
    FORWEFF =  0.949999988079E+000         / Image sideband gain ratio      (22)
    ORIGIN  = 'CLASS-Grenoble  '           /
    DATE    = '2023-11-07T00:00:00.000'    / Date written
    TIMESYS = 'UTC             '           /
    DATE-OBS= '2009-09-07T18:47:46.352'    / Date observed
    DATE-RED= '2009-09-15T00:00:00.000'    / Date reduced
    ELEVATIO=  0.486971109330E+002         /  Telescope elevation
    AZIMUTH =  0.385419378637E+003         /  Telescope azimuth
    UT      = ' 18:47:46.352'              /  Universal time at start
    LST     = ' 17:42:05.449'              /  Sideral time at start
    END

1. Although only one axis is really necessary, it is very convenient to define four, use the first one for the channels, and the three last ones to code the positions and stokes parameters.
2. The first axis is used to define effectively the spectrum. Thus NAXIS1 is the number of channels.
3. NAXIS2, NAXIS3, and NAXIS4 are all one for a single spectrum. Note however that it is possible to store a raster map with a similar header as this one.
4. Could be Janskys.
5. First axis defined in terms of frequency (in the signal sideband in case of double sideband operations). The frequency of a specific channel is given by
   F(i) = RESTFREQ + CRVAL1 + ( i - CRPIX1 ) * CDELT1
   in which the Rest frequency RESTFREQ is defined later in the header.
6. Second axis, Right Ascension RA (as in this case) 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)
   That is, CDELT2 and CDELT3 represents angular offsets from the reference position (CRVAL2,CRVAL3) in a Global Sinusoidal projection (RADIO projection).
7. Stokes parameters as defined in the basic paper of Wells et al.
8. Equinox of these coordinates.
9. Molecular line name, for bookkeeping.
10. Rest frequency.
11. LSR Velocity of the reference channel. Heliocentric velocities can be used also.
12. 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 is thus given by
    V(i) = VLSR + ( i - CRPIX1 ) * DELTAV
13. Image frequency, for double sideband operation.
14. System temperature, necessary for some kind of weighting when adding a number of spectra.
15. Integration time, used for the same reason as above.
16. Scan number, for bookkeeping.
17. Atmospheric opacity in the signal sideband.
18. For multi-phased spectra (i.e. frequency switching) number of phases.
19. For each phase, the frequency offset, the phase length and weight.
20. The ratio of gains in the image and signal sidebands (in case of double sideband operation).
21. The telescope beam efficiency.
22. The telescope forward efficiency.

The FITS interface for Continuum data is still experimental. Try it, and send your comments...