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UV_SINGLE

        UV_SINGLE

    This task prepares one or more (for mosaics) UV tables of short spacings
    from  a   single-dish  table  of  spectra  (as produced by CLASS command
    GRID).  This table may later be merged to an  interferometer  UV  table.
    Two major steps are performed for this:
      - Creation of a "well behaved" map from the spectra.
      - Extraction of UV spacings from this map.

    The "well behaved" map is obtained by linear  operations  (convolutions)
    from  the original spectra, in an optimum way from signal to noise point
    of view. The map is extrapolated smoothly towards zero at the  map  edge
    in  order  to avoid further aliasing in the Fourier transform operations
    required in step 2. This extrapolation has a scale length of  twice  the
    single-dish beam, in order to avoid spurious Fourier components.

    To obtain it, UV_SINGLE 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_SINGLE computes one or several 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$, in meters)
      -  Inverse Fourier transform to  the  image  plane
    and then for each pointing center
      -   Multiplication  of  the   image   by  the  primary  beam  of   the
        interferometer elements
      -  Fourier transform back to the UV plane.
      -  Creation of the UV table, with a given  weight  SD_WEIGTH$  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
1999-03-15