Keywords: map

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## header

A table header contains all the informations of a GDF header but some of these informations have a special meaning in this context. The `HEADER` is the standard way inside GILDAS to display in a human readable way the header of GDF file. For instance, `MAPPING> header 12cs21.uvt` would display

           1  W-GDF_RHSEC,  Absent section NOISE
           2  File : 12cs21-1.uvt                                                  REAL*4
           3  Size        Reference Pixel           Value                  Increment
           4     154   25.50000000000000       97980.02890200325     -6.5365856842222472E-02
           5    1320   0.000000000000000       0.000000000000000       1.000000000000000
           6       1   0.000000000000000       0.000000000000000       0.000000000000000
           7       1   0.000000000000000       0.000000000000000       0.000000000000000
           8  Blanking value and tolerance      1.23455997E+34   0.0000000
           9  Source name         HORSEHEAD
          10  Map unit            Jy
          11  Axis type           UV-RAW       RANDOM       UNKNOWN      UNKNOWN
          12  Coordinate system   EQUATORIAL
          13  Right Ascension     05:40:53.903        Declination        -02:28:22.00
          14  Lii        206.9592680349889            Bii      -16.80061668685155
          15  Epoch              2000.0000
          16  Projection type     AZIMUTHAL           Angle     0.000000000000000
          17  Axis 0     A0       05:40:54.270        Axis 0     D0      -02:28:00.00
          18  Minimum            0.0000000      at     0     0     0     0
          19  Maximum            0.0000000      at     0     0     0     0
          20  Axis 1 Line Name    12CS(2-1)           Rest Frequency   97980.95299999999
          21  Resolution in Velocity   0.20000000     in Frequency   -6.5365856842222472E-02
          22  Offset in Velocity        10.500000     Image Frequency  0.000000000000000
          23  Noise
          24  Beam                     2.494E-04   0.00       0.00

Comments:

Line 2
  Indicates the filename associated to the currently displayed header.
Lines 3-7
  Display the dimensions of the associated array. Here it is a rank 2 array of dimension 1320 columns times 154 lines, i.e. 1320 visibility spectra of 49 frequency channels (see previous section). Line 4 describes the frequency axis of the visibility spectra stored in the table. Be careful that this is a convention, i.e. it must be decoded using the particular form of the table. In our case, each spectra has 49 frequency channels of width -65.36 kHz, the frequency of the reference pixel 25.50 corresponding to 97980.02890200325 MHz. This last frequency is the frequency delivered by the correlator, i.e. seen by the observatory. In particular, this is the frequency that must be used to compute the primary beam of the interferometer.
Line 10
  Indicates the unit of the real and imaginary parts of the visibilities. *** Weights unit? JP ***
Line 11
  Indicates that this is table (`UV-RAW` and `RANDOM`).
Lines 12-15
  Describe the coordinate system.
Lines 16-17
  Describe the projection system. In the table format, `A0` and `D0` indicate the phase center while `Right Ascension` and `Declination` indicate where the antenna pointed when acquiring the signal. These information are in general identical for single field imaging and different for mosaicing.
Lines 18-19
  Unused for tables.
Lines 20-22
  Describe additional information about the frequency axis of the visibility spectra. In particular, the rest frequency (here 97980.953 MHz) corresponding to a velocity of 10.5 km/s in the velocity frame (in general LSR).
Line 23
  Unused for tables.
Line 24
  Indicates the primary beam size of the interferometer in radian (This is an indirect way to pass the size of the interferometer antennas).

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