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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
- 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.
- The first axis is used to define effectively the spectrum. Thus NAXIS1 is the number of channels.
- 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.
- Could be Janskys.
- 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.
- 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).
- Stokes parameters as defined in the basic paper of Wells et al.
- Equinox of these coordinates.
- Molecular line name, for bookkeeping.
- Rest frequency.
- LSR Velocity of the reference channel. Heliocentric velocities can be
used also.
- 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
- Image frequency, for double sideband operation.
- System temperature, necessary for some kind of weighting when adding a number
of spectra.
- Integration time, used for the same reason as above.
- Scan number, for bookkeeping.
- Atmospheric opacity in the signal sideband.
- For multi-phased spectra (i.e. frequency switching) number of phases.
- For each phase, the frequency offset, the phase length and weight.
- The ratio of gains in the image and signal sidebands (in case of
double sideband operation).
- The telescope beam efficiency.
- The telescope forward efficiency.
The FITS interface for Continuum data is still experimental. Try it, and
send your comments...
Next: BINTABLE Mode
Up: CLASS FITS format
Previous: CLASS FITS format
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Gildas manager
2024-03-28