UV_SHORT Step_1
Keywords: imager

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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.

        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  twice  the interferometric field  of
        view  of interest. However, UV_SHORT does *NOT* make any  assumption. It
        thus  tries to compute  a "well behaved" map by linear operations  (con-
        volutions) 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.

        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 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.