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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 2018-11-19