XY_MAP TUNING
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XY_MAP TUNING

            MAP\XY_MAP: tuning parameters

        The  gridding process may be finely tuned through user defined values of
        the MAP structure.

        The header of an existing LMV cube may be used as  a  template  for  the
        grid definition. To do this, the user must set the variable
            MAP%LIKE       [string    ] Reference header for spatial parameters

        When MAP%LIKE is set to an empty string, the user may define the grid by
        hand through the following parameters
            MAP%CELL[2]    [arcsec    ] X and Y pixel size
            MAP%SIZE[2]    [pixel     ] X and Y map size
        Each MAP%CELL sign indicates the  axis  orientation:  East  is  left  if
        MAP%CELL[1]<0 and North is top if MAP%CELL[2]>0.

        The projection parameters can be customized with:
            MAP%SHIFT      [logical   ] Reproject the grid?
            MAP%PTYPE      [code      ] Grid projection type
            MAP%RA         [  HH:MM:SS] Grid reference position
            MAP%DEC        [+DDD:MM:SS] Grid reference position
            MAP%ANGLE      [degree    ] Grid position angle
        These  parameters are applied only if MAP%SHIFT is YES. MAP%PTYPE is one
        of the SIC%CODE%PROJ% codes (or H%PTYPE from the DEFINE HEADER command);
        null  code stands for automatic choice (projection taken from the table,
        except radio which is converted to azimuthal). Empty MAP%RA  or  MAP%DEC
        are  taken from the table header. MAP%ANGLE is the target angle (not the
        angle to be added).

        The natural beamwidth of the telescope is set in order of precedence by
            TAB%MAJOR      [radian    ] The resolution in the table header, if defin
            MAP%TELE       [string    ] The telescope name
            MAP%DIAM       [m         ] The telescope diameter (if MAP%DIAM = 0)
            MAP%BEAM       [arcsec    ] The beam size (if MAP%TELE = "" and MAP%DIAM
        Because of the convolution, the spatial resolution of the  gridded  data
        will by default be 5.4% wider than the telescope beamwidth at the obser-
        vation frequency. It is however possible to grid the data with  a  wider
        Gaussian  in  order  to smooth the data (e.g. to increase the signal-to-
        noise ratio of extended emission). On the other hand, it is by construc-
        tion impossible to request a final resolution smaller than the intrinsic
        resolution (beam size). The user can define the required  resolution  of
        the gridded data through
            MAP%RESO[2]    [arcsec    ] Final resolution of the gridded data
        The  resolution  can  not be smaller than the beam size (default 0 means
        automatic resolution). The command will deduce the size of the  gridding
        kernel from the telescope beamwidth and the required resolution.

        In  /NOGRID mode (see subtopic for details), XY_MAP will guess the X and
        Y increments from the spectra positions, but it  needs  some  tolerance.
        The  default  is  1/10  of the beam, which is enough for Nyquist sampled
        grids. However, if the beam is not known, or if the grid is oversampled,
        the tolerance can be customized through the variable
            MAP%TOLE       [arcsec    ] Tolerance for X and Y increments (/NOGRID mo

        It is advised to avoid changing the following more technical parameters.
        However those parameters are tunable through
            MAP%COL[2]     [integer   ] Range of columns to grid
            MAP%WCOL       [integer   ] Column of weights
            MAP%CTYPE      [integer   ] Convolution kernel type
            MAP%SUPPORT[2] [arcsec    ] Convolution kernel support size

        For ease of use, a sensible value of each gridding parameter will  auto-
        matically be computed from the context if the value of the corresponding
        MAP structure component is set to its default value, i.e. zero value and
        empty  string.  The only exception is the projection definition position
        (changing them requires in addition to set MAP%SHIFT to YES).