In [rgb]1,0,0IMAGER, gridding in the uv plane and computation of the dirty beam and image are coded in the magentaUV_MAP command. This command works on an internal buffer containing the uv table read from a file through the magentaREAD UV command.
The magentaUV_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. magentaUV_MAP ? also gives their default and current values.
Basic usage - image characterization:
Name | Dim/Type | Description |
blueBEAM_STEP | Int | Number of channels per common dirty beam, if ![]() |
If 0 (default value), only one beam is produced in total. | ||
If ![]() |
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requested precision (blueMAP_PRECIS). | ||
blueMAP_CELL | [2] Real | Pixel size in arcsecond. Enter 0,0 to let the task find the best values. |
blueMAP_FIELD | [2] Real | Image size in arcsecond. blueMAP_FIELD has precedence over |
the number of pixels blueMAP_SIZE to define the actual map size | ||
when both variables are non-zero. | ||
blueMAP_SIZE | [2] Int | Image size in pixels |
blueMAP_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:
blueMAP_ROBUST | Real | Robust weighting factor, in range 0 - ![]() |
0 means Natural weighting (as ![]() |
||
0.5 or 1 is usually a good choice for Robust Weighting. | ||
Default is 0,i.e. natural weighting. (Old name blueUV_CELL[1]) | ||
blueMAP_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 ([![]() |
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third value). By default (0,0,0). (Old name blueUV_TAPER[3]). | ||
blueMAP_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 blueUV_CELL[2]) | ||
blueMAP_TAPEREXPO | Real | the taper exponent. Default 2, indicating Gaussian function. |
(Old name blueTAPER_EXPO). |
Advanced:
blueMAP_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. | ||
blueMAP_POWER | Int | Rounding scheme for default image size, to numbers like
![]() |
![]() ![]() |
||
Default value is 2, for smallest image size. For blueMAP_POWER = 0 | ||
blueMAP_SIZE is just a power of 2. | ||
blueMAP_ROUNDING | Real | Maximum error between optimal size (blueMAP_FIELD / blueMAP_CELL) |
and rounded (as a power of ![]() |
||
blueMAP_TRUNCATE | Real | For a Mosaic, truncate the primary beam to the specified level |
(in fraction). Default value is 0.2. |
Debug:
blueMAP_CONVOLUTION | Int | Gridding convolution mode in the ![]() |
(default 5 for speroidal functions) (old name blueCONVOLUTION) | ||
blueMAP_VERSION | Int | Version of code to be used. This is a temporary variable to allow |
comparison between the new and old codes without quitting | ||
[rgb]1,0,0IMAGER. |
Parameters can be listed by the commands magentaUV_MAP ? and magentaCLEAN ?. More details are obtained using ?? or ??? as arguments instead of ?. A thorough description of each parameter can be obtained by typing: magentahelp UV_MAP blueMAP_* or magentahelp CLEAN blueCLEAN_*.
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 ]
Next: Defining the Projection Center
Up: Imaging
Previous: Weighting and Tapering
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Gildas manager
2023-06-01