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Implementation (READ UV, UV_MAP and UV_STAT)

In MAPPING, gridding in the $uv$ plane and computation of the dirty beam and image are coded in the UV_MAP command. This commands works on an internal buffer containing the $uv$ table read from a file through the READ UV command. The UV_MAP behavior is controlled through the following sic variables

Gridding kernel
CONVOLUTION
Gridding in the $uv$ plane is done through convolution and this variable is the kind of convolution kernel. Default is 5, meaning spheroidal functions. Unless you are a real expert, you should never change the value of this variable: Spheroidal functions are the right answer.
Data cube setup
WCOL
Number of the used weight column. The default is to compute only one dirty beam for the whole data cube. This default may be incorrect when adding data obtain at different rest frequencies.
MCOL
First and last velocity/frequency channels to map (default value is 0 meaning all the channels).
MAP_CELL
Pixel size in arcsec. Default is 0, meaning between 1/3 and 1/4 of the synthesized beam.
MAP_SIZE
Map size in pixel. Default is 0, meaning a power of two which gives about twice the size of the primary beam size. For efficiency of FFT, it is better to pick a power of two.
MAP_RA
Right Ascension of the phase center. Default is an empty string, meaning do not change the right ascension.
MAP_DEC
Declination of the phase center. Default is an empty string, meaning do not change the declination.
MAP_ANGLE
Map position angle in degree. Default is 0, meaning do not rotate the map.
UV_SHIFT
Logical which toggle the change of phase center and position angle. This may be used to align the map axes with a feature of interest of the source, by rotation of the $uv$ coordinates.
Weighting
WEIGHT_MODE
Kind of weighting. Possibilities are NATURAL or UN (which stands for ROBUST...). Default is NATURAL weighting to maximize the point source sensitivity.
UV_CELL
Size of the $uv$ plane cell and threshold of the robust weighting. Default are 7.5 m (half the diameter of the PdBI antenna) and 1.
Tapering
UV_TAPER
Properties of the Gaussian used to apodize the weights of the $uv$ visibilities (minor and major axes in meters and position angle in degree). Default is 0, meaning no tapering.
TAPER_EXPO
*** ? JP *** Default is 0, meaning the use of a Gaussian as tapering function.
The current values of all those parameters may be found through the INPUT UV_MAP command. The main difficulty in the use of the robust weighting and tapering is the choice of the right parameters for respectively UV_CELL[2] and UV_TAPER. MAPPING offers the UV_STAT command to guide your choice.
       1 MAPPING> read uv demo.uvt
       2 MAPPING> uv_stat weight
       3 Robust      Major    Minor     PA            Noise      Sidelobe    
       4               (")      (")     (deg)    (mJy)     (K)       %      
       5    0.10      0.780    0.479     19.8     0.988    0.066  -21.1  18.3
       6    0.18      0.784    0.485     19.5     0.889    0.058  -20.9  19.2
       7    0.32      0.785    0.493     19.3     0.798    0.051  -20.4  20.2
       8    0.56      0.786    0.503     19.3     0.723    0.045  -19.7  21.5
       9    1.00      0.784    0.509     19.4     0.672    0.042  -19.3  23.8
      10    1.78      0.789    0.507     18.3     0.632    0.039  -19.2  26.3
      11    3.16      0.820    0.505     14.8     0.601    0.036  -16.0  30.2
      12    5.62      0.828    0.525     13.1     0.586    0.033  -13.0  32.3
      13   10.00      0.851    0.539     13.2     0.577    0.031  -11.9  34.2
      14   None       0.882    0.559     13.2     0.568    0.029  -10.4  38.2
      15 MAPPING> uv_stat taper
      16 Taper       Major    Minor     PA            Noise      Sidelobe    
      17  (m)          (")      (")     (deg)    (mJy)     (K)       %      
      18   30.00      6.586    3.533     10.6     1.727    0.002  -58.8  26.5
      19   42.43      5.724    2.896     11.7     1.385    0.002  -36.8  25.8
      20   60.00      5.100    2.485     12.8     1.178    0.002  -31.4  24.5
      21   84.85      4.623    2.279     13.2     1.035    0.002  -28.8  22.6
      22  120.00      3.956    2.058     13.2     0.895    0.003  -26.5  23.4
      23  169.71      2.173    1.565     21.8     0.752    0.005  -23.1  27.8
      24  240.00      1.161    1.072    161.1     0.646    0.013  -19.8  32.4
      25  339.41      0.962    0.756     11.9     0.594    0.020  -17.5  35.1
      26  480.00      0.910    0.645     13.2     0.576    0.024  -16.1  36.8
Comments:
Line 1
Read a $uv$ table in an internal buffer as the UV_STAT works only on this buffer.
Lines 2-14
Displays the synthesized beam (major, minor, position angle), the noise levels (point source in mJy and brightness in K) and the level of the sidelobes as a function of the threshold (1st column) to be used in the robust weighting method. The usual way to choose the right threshold is to select the one that gives a substantially better spatial resolution without increasing the noise by more than 10%.
Lines 15-26
Displays the same parameters as above as a function of the tapering distance (1st column) in meter.


next up previous contents index
Next: Typical imaging session Up: Imaging Previous: Weighting   Contents   Index
Gildas manager 2018-12-13