Implementation ( READ UV, UV_MAP and UV_STAT)

In IMAGER, gridding in the uv plane and computation of the dirty beam and image are coded in the UV_MAP command. This command works on an internal buffer containing the uv table read from a file through the READ UV command.

The UV_MAP command is controlled by a set of SIC variables named with the prefix MAP_. Suitable defaults are provided, so that only specific cases should require customization by the user. A description of the all variables can be obtained through HELP UV_MAP. UV_MAP ? also gives their default and current values.

Basic usage - image characterization:

Name	Dim/Type	Description
BEAM_STEP	Int	Number of channels per common dirty beam, if \(> 0\) .
		If 0 (default value), only one beam is produced in total.
		If \(-1\) , an automatic guess is performed from the map size and
		requested precision ( MAP_PRECIS).
MAP_CELL	[2] Real	Pixel size in arcsecond. Enter 0,0 to let the task find the best values.
MAP_FIELD	[2] Real	Image size in arcsecond. MAP_FIELD has precedence over
		the number of pixels MAP_SIZE to define the actual map size
		when both variables are non-zero.
MAP_SIZE	[2] Int	Image size in pixels
MAP_CENTER	Char.	A character string to specify the new Map center and the new map
		orientation, see the next subsection related to the definition of the
		projection center of the image.

Weighting:

MAP_ROBUST	Real	Robust weighting factor, in range 0 - \(+ \infty\) .
		0 means Natural weighting (as \(+ \infty\) , actually).
		0.5 or 1 is usually a good choice for Robust Weighting.
		Default is 0,i.e. natural weighting. (Old name UV_CELL[1])
MAP_UVTAPER	[3] Real	Array of 3 values controlling the UV taper: major/minor axis
		at 1/e level [m,m] (first two values), and position angle ([ \({\deg}\) ],
		third value). By default (0,0,0). (Old name UV_TAPER[3]).
MAP_UVCELL	[2] Real	UV Cell size for Robust weighting. Default is 0, meaning that
		the cell size is derived from the antenna diameter.
		(Old name UV_CELL[2])
MAP_TAPEREXPO	Real	the taper exponent. Default 2, indicating Gaussian function.
		(Old name TAPER_EXPO).

Advanced:

MAP_PRECIS	Real	Position precision at the map edge, in fraction of pixel size,
		used (with the actual image size) to derive the actual number of
		channels which can share the same beam. Default value is 0.1.
MAP_POWER	Int	Rounding scheme for default image size, to numbers like \(2^{n}3^{p} 5^{q}\) .
		\(p\) and \(q\) are less than or equal to MAP_POWER.
		Default value is 2, for smallest image size. For MAP_POWER = 0
		MAP_SIZE is just a power of 2.
MAP_ROUNDING	Real	Maximum error between optimal size ( MAP_FIELD / MAP_CELL)
		and rounded (as a power of \(2^k 3^p 5^q\) ) MAP_SIZE.
MOSAIC_BEAM	Real	For a Mosaic, truncate the primary beam to the specified level
		(in fraction). Default value is 0.2.

Debug:

MAP_CONVOLUTION	Int	Gridding convolution mode in the \(uv\) plane.
		(default 5 for speroidal functions) (old name CONVOLUTION)
MAP_VERSION	Int	Version of code to be used. This is a temporary variable to allow
		comparison between the new and old codes without quitting
		IMAGER.

Parameters can be listed by the commands UV_MAP ? and CLEAN ?. More details are obtained using ?? or ??? as arguments instead of ?. A thorough description of each parameter can be obtained by typing: help UV_MAP  MAP_* or help CLEAN  CLEAN_*.

IMAGER> UV_MAP ?
 UV_MAP makes a dirty image and a dirty beam from the UV data

 Behaviour is controlled by a number of SIC Variables
 - BEAM_STEP and MAP_PRECIS control the dirty beam precision
 - MAP_CENTER controls shifting ang rotation
 - MAP_CELL[2], MAP_SIZE[2], MAP_FIELD[2] control the map sampling
 - MAP_UVTAPER[3], MAP_UVCELL and MAP_ROBUST
     control the beam shape and weighting scheme

---- Basic parameters                            Selected       Recommended
  Map Size (pixels)              MAP_SIZE        [ 0 0 ]   [ 0 0 ]
  Field of view (arcsec)         MAP_FIELD       [ 0 0 ]   [ 0 0 ]
  Pixel size (arcsec)            MAP_CELL        [ 0 0 ]   [ 0 0 ]
  Map center                     MAP_CENTER      [  ]
  Robust weighting parameter     MAP_ROBUST      [ 0 ]
  UV cell size (meter)           MAP_UVCELL      [ 0 ]          [ 0 ]
  UV Taper (m,m,°)               MAP_UVTAPER     [ 0 0 0 ]