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##

UV_SHORT

UV_SHORT
Warning: It is advised to use this task through the "Short
Space Processing" widget available in the MAPPING main menu.
This task prepares the UV tables of the missing short spacings
from a single-dish table of spectra (the table format is described in
the GRID command of CLASS). This table should later be merged to the
corresponding interferometer UV table. Two major steps are performed:
- Creation of a "well behaved" map from the spectra.
- Extraction of UV visibilities from this map.
It is advised that the input table is a collection of single-dish,
Nyquist sampled spectra covering twice the interferometric field
of view of interest. However, this tasks does *not* make any assump-
tion. It thus try to compute a "well behaved" map by linear operations
(convolutions) from the original spectra, in an optimum way from
signal to noise point of view. The map is extrapolated smoothly to-
wards zero at the map edge in order to avoid further aliasing in
the Fourier transform operations required in step 2. This extrapo-
lation has a scale length of twice the single-dish beam, in order to
avoid spurious Fourier components.
In detail, UV_SHORT performs the following operations:
- Resampling (in space) of the original spectra on a regular grid by
convolution with a small (typically 1/4 of the single-dish beam)
gaussian convolving kernel. In this process, the weights of individual
spectra is carried on a weight map.
- Extrapolation by zero outside the convex hull of the mapped region.
- Convolution of the result by a gaussian twice as wide as the
- single-dish beam. Within the convex hull of the mapped region, the
- smoothed map is replaced by the original map.
From this map, UV_SHORT computes the needed UV tables (one for
each pointing center in case of a mosaic) in the following way:
- Fourier transform of the single dish map;
- Division by the Fourier transform of the single dish beam, up to a
maximum spacing (SD_DIAM$, in meters);
- Inverse Fourier transform to the image plane and then for each pointing
center;
- Multiplication of the image by the primary beam of the interferometer
elements;
- Fourier transform back to the UV plane;
- Creation of the UV table, with a given weight SD_WEIGTH$ and an
appropriate calibration factor to Janskys SD_FACTOR$
Both the single-dish and the interferometer antennas are assumed to
have gaussian beams (SD_BEAM$ and IP_BEAM$, in radians).

Gildas manager
2014-07-01