Implementation ( READ UV, UV_MAP and UV_STAT)

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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 .
|  | If 0 (default value), only one beam is produced in total.
|  | If , 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 - .
|  | 0 means Natural weighting (as , 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 ([],
|  | 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 .
|  | and 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 ) MAP_SIZE.
MAP_TRUNCATE | 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 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 ]