UV Table handling

Besides the READ UV and WRITE UV commands to read or write uv tables, IMAGER has a number of commands to manipulate the current uv table buffer. These commands have names starting by UV_. Most of them are in the CLEAN\ language, some in the ADVANCED\ one.

IMAGER works using UV buffers. Most commands only work on the current UV buffer, but some of them keep track of the previous buffer to allow the user to revert the operation.

Data inspection and editing:

• SHOW COVERAGE display the uv coverage
• SHOW UV display the uv data
• UV_FLAG allow flagging visibilities
• UV_PREVIEW provides a quick view of the visibilities as a function of frequencies, and attempts to automatically find the continuum level and parts of the bandwidth with spectral line emissions.

Data size reduction routines:

• UV_COMPRESS is a simple spectral smoothing, providing only channel averaging by integer number of channels.
• UV_RESAMPLE provides a more flexible spectral smoothing and resampling facility.
• UV_TIME can be used to time-average the UV data set, leading to faster processing. However, using UV_TIME too early may limit your ability to perform accurate phase self-calibration.

Continuum processing commands:

• UV_BASELINE allows to remove the continuum baseline, by 0th or 1st order baseline fitting of each visibility.
• Conversely, UV_FILTER will filter the spectral line range to leave only the channels with continuum emission. Both UV_BASELINE and UV_FILTER can use the results provided by UV_PREVIEW to specify where spectral lines may be found.
• UV_CONTINUUM converts a spectral line uv table into a bandwidth synthesis continuum uv table. UV_CONTINUUM is only useful for UV plane analysis: see Section 10 for details.
• UV_MERGE /FILE can merge several UV continuum tables with a specified spectral index to optimize the sensitivity.

Image preparation:

• UV_CHECK inspects the uv data to figure out how many different synthesized beams are needed.
• UV_SHORT adds the short (or zero) spacing information provided by an additional single dish data, read by READ SINGLE.
• UV_STAT evaluates the impact of robust weighting and tapering on the synthesized beam. It provides recommendations for the image and pixel sizes.
• UV_TRUNCATE restricts the uv baseline length range.

UV Plane analysis:

• UV_FIT fit simple source models to the visbilities.
• SHOW UV_FIT display the fit results, usually as a function of frequency, but also in other ways.

Miscellaneous:

• UV_DEPROJECT de-projects the \((u,v)\) coordinates given a specified phase center, orientation and inclination. This can be useful for inclined, flattened, nearly axi-symmetric structures such as proto-planetary disks or galaxies.
• UV_CIRCLE and UV_RADIAL compute the azimutal average of the visibilities. They are useful for rotationally symmetric structures such as proto-planetary disks or circumstellar envelopes, for example.
• UV_REWEIGHT changes the visibility weights.
• UV_SHIFT changes the phase center. This can be useful for UV plane analysis.
• UV_MERGE /FILE can merge several UV tables, Line or Continuum. It also allows stacking different spectral lines, by aligning them in velocity, as well as stacking emission from different sources.

The remaining UV_... commands are related to imaging and deconvolution: UV_MAP computes the dirty image, UV_RESTORE computes the Clean image from a Clean component list by removal of the Clean components in the uv plane, and imaging of the residuals. UV_RESIDUAL just computes the residuals by subtraction of the Clean components.

Finally, UV_SELF, in the CALIBRATE\ language, is a specific variant of UV_MAP used to compute the intermediate images required for self-calibration. It is not intended for direct use by normal users.