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Task: uvmodel Purpose: Add, subtract, etc, a model from a uv data set. Categories: uv analysis UVMODEL is a MIRIAD task that modifies a visibility dataset by combining or replacing it a model. Allowed operations are adding, subtracting, multiplying, dividing and replacing. The model is specified in the image domain, so that its Fourier transform is first computed before application to the visibilities. The model may be either an image (e.g., a CLEAN component image) or a point source. As an example, UVMODEL could be used to remove CLEAN components from a visibility data file. The residual data base could then be examined for anomalous points, which could in turn be clipped. UVMODEL could then be reapplied to add the CLEAN components back into the visibility data base for re-imaging. Key: vis Input visibility data file. No default Key: model Input model cube. The default is a point source model. This will generally be a deconvolved map formed from the visibility data being modified. It should be made with "channel" linetype. The model should have units of JY/PIXEL and be weighted by the primary beam. The task DEMOS can be used to extract primary beam weighted models from a mosaiced image. Key: select The standard uv selection subcommands. The default is all data. Key: options This gives extra processing options. Several values can be given though many are mutually exclusive, separated by commas. Option values can be abbreviated to uniqueness. Possible options are (no default): add Form: out = vis + model subtract Form: out = vis - model multiply Form: out = vis * model divide Form: out = vis / model replace Form: out = model flag Form: out = vis, but flag data where the difference between vis and model is greater than "sigma" sigmas. unflag Unflag any flagged data in the output. mosaic Select only those visibilities whose observing centre is within plus or minus three pixels of the model reference pixel. This is needed if there are multiple pointings or multiple sources in the input uv file. By default no observing centre selection is performed. mfs This is used if there is a single plane in the input model, which is assumed to represent the data at all frequencies. This should also be used if the model has been derived using MFCLEAN. zero Use zero as the value of the model if it cannot be calculated. This can be used to avoid flagging the data in the outer parts of the u-v-plane when subtracting a low resolution model. The operations add, subtract, multiply, divide, replace and flag are mutually exclusive. The operations flag and unflag are also mutually exclusive. The unflag option should be used with caution. Data in the output may still be flagged, if it was not possible to calculate the model. Key: clip Clip level. Pixels in the model below this level are set to zero. The default is not to perform any clipping. Key: flux If MODEL is blank, then the flux (Jy) of a point source model should be specified here. Also used as the default flux in the apriori option. The default is 1 (assuming the model parameter is not given). The flux can optionally be followed by i,q,u,v or the other polarisation mnemonics to indicate the polarisation type. The 3rd-6th parameter optionally specify the reference frequency in GHZ and the spectral index and higher order terms (as given by uvsfit or uvfmeas with options=mfflux). Key: offset The RA and DEC offsets (arcsec) of the point source from the observing centre. A point source to the north and east has positive offsets. Defaults are zero. Key: line The visibility linetype to use, in the standard form, viz: type,nchan,start,width,step Generally if there is an input model, this defaults to the linetype parameters used to construct the map. For a point source or planet model, the default is all channels. This parameter may be used if you wish to override these defaults, or if the relevant information is not present in the header, Key: sigma For options=flag, UVMODEL flags those points in the output that differ by more than "sigma" sigmas. The default is 100. Key: out Output visibility data file name. The output file will contain only as many channels as there are planes in the model cube. The various uv variables that describe the windows are adjusted accordingly. No default. Revision: 1.8, 2015/03/24 04:56:24 UTC