SHIFT_AND_ADD

     SHIFT_AND_ADD

     This task is recommended when there is a clear-cut separation of the ON
     and OFF images. This is the case of many filamentary structures, edge-on
     galaxies, and all object for which wobbling in-the-object can a priori
     be avoided. Contrary to RESTORE the single-beam maps are computed direc-
     tly from variable SIGNAL (the calibrated temporal sequence of dual-beam
     data) and NOT from the dual-beam maps obtained via MAKPLAN. The position
     variable SCAN_COORD is used to resample the data variable SIGNAL on an
     azimuth-elevation grid G. The signal is displaced by half the effective
     ON to OFF beam separation BEAMSEP once towards positive azimuths and
     once to negative azimuths, is gridded line by line (subscan by subscan),
     apodized in azimuth to the frequency cutoff of the telescope using the
     envelope of a sinc function tapered over a gliding window F and weighted
     according to the inverse density factor W required to obtain uniform data
     sampling.  The size of the gliding window is chosen to be 5x the beam-
     width at the effective observing frequency. The weighting factor W is
     introduced to control the form of the beam, and its value is related to
     the area density of the data at any point j in the new grid:

                           --
                      Wj = \    sin (uj)/(uj).Tj
                           /
                           -- F

                  uj = 2.A.pi.D/lambda.(j-SCAN_COORD)

     where T is an optional apodization function and A is the ratio of the
     halfpower width of the gridding function W to the beamwidth of the te-
     lescope. T can either be truncated-gaussian (by default), parabolic,
     linear or equal to unity (no taper). So far, the i-th point of a specific
     line in the single-beam map G can be written to:

                  --                          --
       Gi = 1/Wi.(\ SIGNAL.sin (uj)/(uj).Tj + \ SIGNAL.sin (uk)/(uk).Tk)
                  /                           /
                  -- F                        -- F'

     However, depending on variable SAA_MASK two different processes are
     executed:

        - SAA_MASK is set to NO: only data inside a window whose limits are
          defined by half the beam separation from either side of a line of
          position angle PA and whose azimuthal offset from the tracking
          center of the telescope is given in OFFSET, is retained.
        - SAA_MASK is set to YES: only data inside a polygonal support defined
          via SUPPORT is retained. Note, however, that the boundaries of the
          support region are equally clipped in each line (subscan) to the
          effective separation of the ON and OFF beams.

     Finally, the retained data is written to a file with the extension .saa.
     Apodization in elevation is done according to the setting of the logical
     SMOOTH_EL. The grid cells are by default quadratic and their size is set
     either to the sampling interval in azimuth or in elevation, whichever is
     larger. However, different sizes in both directions can be set using
     variable MAP_CELL. As for RESTORE compute UNBAL from the mean sign in-
     verted ratio of the ON to OFF beam amplitudes. Flagged channels are not
     reduced.