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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
    MAP%LIKE  variable  to the template filename. When MAP%LIKE is set to an
    empty string, the user may define the grid by hand through the following
    parameters

    MAP%CELL     [arcsec    ]   X and Y pixel size
    MAP%SIZE     [pixel     ]   X and Y map   size
    MAP%ANGLE    [degree    ]   Grid position angle
    MAP%RA       [  HH:MM:SS]   Grid reference position
    MAP%DEC      [+DDD:MM:SS]   Grid reference position
    MAP%SHIFT    [logical   ]   Shift and/or Rotate grid?

    The natural beamwidth of the telescope is set in order of precedence by

    TAB%MAJOR    [radian    ]   The resolution in the table header, if defined
    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 = 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). The user have to define the required
    resolution of the gridded data through

    MAP%RESO     [arcsec    ]   Final resolution of the gridded data (default is

    the command will deduce the size of the gridding kernel from  the  tele-
    scope 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 mode)

    It is advised to avoid changing the following more technical parameters.
    However those parameters are tunable through

    MAP%COL      [integer   ]   Range of columns to grid
    MAP%WCOL     [integer   ]   Column of weights
    MAP%CTYPE    [integer   ]   Convolution kernel type
    MAP%SUPPORT  [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 position reference and/or posi-
    tion angle of the grid:  Changing  them  requires  in  addition  to  set
    MAP%SHIFT to YES.


Gildas manager 2018-02-17