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  derived  from  (in  order  of
    precedence):
        TAB%MAJOR      [radian    ] The resolution in the table header
        MAP%TELE       [string    ] The telescope name (if no resolution in tabl
        MAP%DIAM       [m         ] The telescope diameter (if MAP%TELE = "")
        MAP%BEAM       [arcsec    ] The beam size (if MAP%DIAM = 0)
    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).