## Short Spacings in practice: command UV_SHORT

Our algorithm to produce the short-spacing information is coded in the UV_SHORT command. UV_SHORT will add the short spacing information to the current uv table (read by command READ UV and optionally transformed by further UV_... processing commands).

UV_SHORT has a substantial number (17) of control variables, but with experience, they have been reduced to 5 significant ones, among which only 3 really matter in most cases but often can be used with their default values:

  SHORT_SD_FACTOR The single-dish brightness unit to flux conversion factor. If set to zero, UV_SHORT will attempt to derive it from the information available in the single-dish data
  SHORT_UV_TRUNC The longest baseline retained in the pseudo-visibilities. It defaults to the maximum theoretically possible, the single-dish diameter minus the interferometer diameter. Smaller values are allowed, and even recommended if the pointing quality of the single-dish data is moderate.
  SHORT_SD_WEIGHT The relative weight scaling factor between the pseudo-visibilities and the interferometer visibilities.

The relative weight of these visibilities is derived by UV_SHORT in order to optimize the shape of the overall synthesized beam. SHORT_SD_WEIGHT is a scale factor to this optimum weight, which may need to differ from 1 in case of poor uv coverage in the interferometer data or noisy single-dish data (it should be lower than 1 in this case).

UV_SHORT ? will list these 3 major ones, and UV_SHORT ?? the 2 remaining main ones:

  SHORT_TOLE The position tolerance in the single-dish map
  SHORT_MIN_WEIGHT The minimum (relative) weight for a spectrum in the single-dish map to be included.

as well as four optional ones needed only if the original single-dish and uv data lacks the proper information (antenna diameter and beam sizes)

The UV_SHORT command starts from data in a the format produced by the CLASS command TABLE command, and read in IMAGER through command READ SINGLE. Basically, this is a GDF table containing one line per spectrum, the columns representing the lambda offset, beta offset, weight, and the spectrum intensities. 6. This data must match spectrally the velocity sampling of the interferometric data. This can be obtained using the /RESAMPLING option of command TABLE in CLASS.

The READ SINGLE and UV_SHORT commands also support a 3-D data cube (as produced by e.g. command XY_MAP in CLASS) as input instead of a CLASS table. Again, the velocity axis must match that of the interferometric data.

UV_SHORT will automatically produce the Zero spacing from the single-dish data when the data does not allow other short spacings to be evaluated. The temporary image produced by UV_SHORT when starting from a CLASS Table is stored in the SHORT buffer, and can be written by command WRITE. This image can aslo be computed separately by command XY_SHORT.

Finally, as UV_SHORT adds the short spacing information, UV_SHORT /REMOVE allows to remove it (there is no direct “replace” possibility because the sampling may change).