20.22.6 Step_1

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, or for specific spectral resampling using command MAP_RESAMPLE SHORT.

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 the interferometric field of view of interest plus at least two primary beams as a guard band on the periphery. However, UV_SHORT does *NOT* make any assumption about the provided coverage. It thus tries 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 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 to  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.