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    Warning:    It   is   advised   to  use  this task  through  the  "Short
    Space Processing" widget available in the MAPPING main menu.

    This  task prepares  the UV  tables  of   the  missing   short  spacings
    from a single-dish table  of spectra  (the table format  is described in
    the GRID command of CLASS).  This table  should later be merged  to  the
    corresponding interferometer UV table.  Two major steps are performed:

      - Creation of a "well behaved" map from the spectra.
      - Extraction of UV visibilities from this map.

    It  is  advised  that  the  input  table is a collection of single-dish,
    Nyquist sampled  spectra  covering  twice   the   interferometric  field
    of  view  of interest. However, this  tasks does *not* make any  assump-
    tion. It thus try to compute  a "well behaved"  map by linear operations
    (convolutions)  from  the   original  spectra,  in an  optimum way  from
    signal  to noise  point of view. The  map is  extrapolated smoothly  to-
    wards  zero at  the map  edge in order  to  avoid  further  aliasing  in
    the  Fourier  transform  operations required in  step 2.  This  extrapo-
    lation   has a scale  length of  twice the single-dish beam, in order to
    avoid spurious Fourier components.

    In detail, UV_SHORT performs the following operations:

      - Resampling  (in space) of  the original  spectra on  a regular  grid by
        convolution  with  a small  (typically  1/4  of  the single-dish  beam)
        gaussian convolving kernel. In  this process, the weights of individual
        spectra is carried on a weight map.
      - Extrapolation by  zero outside  the convex hull  of the  mapped region.
      - Convolution  of  the  result  by  a  gaussian  twice  as  wide  as  the
      - single-dish beam.   Within the  convex hull of  the mapped  region, the
      - smoothed map is replaced by the original map.

    From  this map,  UV_SHORT  computes the  needed   UV  tables   (one  for
    each pointing center in case of a mosaic) in the following way:

      - Fourier transform of the single dish map;
      - Division by  the Fourier  transform of  the single dish  beam, up  to a
        maximum spacing (SD_DIAM$, in meters);
      - Inverse Fourier transform to the image plane and then for each pointing
      - Multiplication of the  image by the primary beam  of the interferometer
      - Fourier transform back to the UV plane;
      - Creation  of the  UV  table, with  a  given weight  SD_WEIGTH$ and  an
        appropriate calibration factor to Janskys SD_FACTOR$

    Both  the   single-dish and the  interferometer antennas are assumed  to
    have gaussian beams (SD_BEAM$ and IP_BEAM$, in radians).

Gildas manager 2014-07-01