UV_SHORT Step_1

          Step  (1) Creation of a "well behaved" map from the spectra.  This map
    is made available to the user as the SHORT datacube, and  can  be  saved  by
    WRITE SHORT.

      Step  (1)  only can be performed independently by command XY_SHORT, to use
    the SHORT image in command FEATHER for example, or for specific spectral re-
    sampling using command MAP_RESAMPLE SHORT.

    Step (1) only occurs if the input single-dish data set (read by READ SINGLE)
    is a table of spectra. The table format  is  described  in   the  CLASS\GRID
    command of CLASS.
      The identification of the input single-dish data set as a table of spectra
    is based on the Header.

    It  is  recommended  that  this input  table is a collection of single-dish,
    Nyquist sampled  spectra  covering the interferometric field  of   view   of
    interest  plus  at least two primary beams as a guard band on the periphery.
    However, UV_SHORT does *NOT* make any  assumption about the provided  cover-
    age.   It  thus  tries 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  towards zero at
    the map  edge in order  to avoid  further  aliasing  in the  Fourier  trans-
    form  operations required in  Step (2).  This  extrapolation   has  a  scale
    length  of   twice  the single-dish beam, in order to avoid spurious Fourier
    components.

    In detail, UV_SHORT (or XY_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 to  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.