Short Spacings in practice: command UV_SHORT
Keywords: imager

Next: Practical considerations Up: Short and Zero spacings Previous: The Zero spacing: an Contents Index

## Short Spacings in practice: command `UV_SHORT`

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

magenta`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:

  blue`SHORT_SD_FACTOR` The single-dish brightness unit to flux conversion factor. If set to zero, magenta`UV_SHORT` will attempt to derive it from the information available in the single-dish data
  blue`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.
  blue`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 magenta`UV_SHORT` in order to optimize the shape of the overall synthesized beam. blue`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).

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

  blue`SHORT_TOLE` The position tolerance in the single-dish map
  blue`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 magenta`UV_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 magenta`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 magenta`/RESAMPLING` option of command magenta`TABLE` in red CLASS.

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

magenta`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 magenta`UV_SHORT` when starting from a red CLASS Table is stored in the magenta`SHORT` buffer, and can be written by command magenta`WRITE`. This image can aslo be computed separately by command magenta`XY_SHORT`.

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

Next: Practical considerations Up: Short and Zero spacings Previous: The Zero spacing: an Contents Index