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. ⁹. 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 also 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 \(uv\) sampling may change).