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UV_SINGLE
UV_SINGLE
This task prepares one or more (for mosaics) UV tables of short spacings
from a single-dish table of spectra (as produced by CLASS command
GRID). This table may later be merged to an interferometer UV table.
Two major steps are performed for this:
- Creation of a "well behaved" map from the spectra.
- Extraction of UV spacings from this map.
The "well behaved" map is obtained by linear operations (convolutions)
from the original spectra, in an optimum way from signal to noise point
of view. The map is extrapolated smoothly towards zero at the map edge
in order to avoid further aliasing in the Fourier transform operations
required in step 2. This extrapolation has a scale length of twice the
single-dish beam, in order to avoid spurious Fourier components.
To obtain it, UV_SINGLE 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_SINGLE computes one or several 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
1999-03-15