Step (1) Creation of a "well behaved" map from the spectra. This
map is made available to the user as the SHORT datacube, and can be
saved by WRITE SHORT.
Step (1) only can be performed independently by command XY_SHORT, to
use the SHORT image in command FEATHER for example.
Step (1) only occurs if the input single-dish data set (read by READ
SINGLE) is a table of spectra. The table format is described in the
CLASS\GRID command of CLASS.
The identification of the input single-dish data set as a table of
spectra is based on the Header.
It is recommended that this input table is a collection of single-dish,
Nyquist sampled spectra covering twice the interferometric field of
view of interest. However, UV_SHORT does *NOT* make any assumption. It
thus tries to compute a "well behaved" map by linear operations (con-
volutions) 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.
In detail, UV_SHORT (or XY_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.
Gildas manager
2023-06-01