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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[2] [arcsec ] X and Y pixel size. Each size sign indicates
the axis orientation: East is left if
MAP%CELL[1]<0 and North is top if MAP%CELL[2]>0
MAP%SIZE[2] [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). 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 (default
is 0). Can not be smaller than the beam size.
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[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 position reference and/or posi-
tion angle of the grid: Changing them requires in addition to set
MAP%SHIFT to YES.
Gildas manager
2023-06-01