UV_SHORT Zero_Spacing

          Zero  spacings  are computed when the single dish diameter is the same
    as the interferometer dish diameter. Zero spacing extraction  proceeds  dif-
    ferently for Class data tables and 3-D data cubes.

    In  the  data  cube  case,  the nearest pixel matching the direction of each
    field is taken as the Zero spacing for this field.  If  there  is  no  point
    close  enough,  according to the specified position tolerance SHORT_TOLE, an
    error occurs.

    In the Class data table case, all spectra within the specified position tol-
    erance of a field center are averaged together to produce the Zero  spacing.
    If none is found, an error occcurs.