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Short Spacings in practice: command UV_SHORT

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

magentaUV_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:

blueSHORT_SD_FACTOR The single-dish brightness unit to flux conversion factor. If set to zero, magentaUV_SHORT will attempt to derive it from the information available in the single-dish data
blueSHORT_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.
blueSHORT_SD_WEIGHT The relative weight scaling factor between the pseudo-visibilities and the interferometer visibilities.
The relative weight of these visibilities is derived by magentaUV_SHORT in order to optimize the shape of the overall synthesized beam. blueSHORT_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).

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

blueSHORT_TOLE The position tolerance in the single-dish map
blueSHORT_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 magentaUV_SHORT command starts from data in a the format produced by the red CLASS command TABLE command, and read in [rgb]1,0,0IMAGER through command magentaREAD 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 magenta/RESAMPLING option of command magentaTABLE in red CLASS.

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

magentaUV_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 magentaUV_SHORT when starting from a red CLASS Table is stored in the magentaSHORT buffer, and can be written by command magentaWRITE. This image can aslo be computed separately by command magentaXY_SHORT.

Finally, as magentaUV_SHORT adds the short spacing information, magentaUV_SHORT /REMOVE allows to remove it (there is no direct ``replace'' possibility because the $uv$ sampling may change).


next up previous contents index
Next: Practical considerations Up: Short and Zero spacings Previous: The Zero spacing: an   Contents   Index
Gildas manager 2023-06-01