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).