Task: uvsfit Purpose: Fit point sources to a given vis file. Categories: uv analysis UVSFIT is a Miriad task which fits model components to a visibility dataset. UVSFIT differs from UVFIT by fitting explicitly for the frequency dependence of source flux. Optionally the model or residual visibilities can be written out. UVSFIT can only handle a limited number of visibilities in fitting mode. You can reduce the data volume by averaging in frequency using the line parameter (apply bandpass first) or selecting a subset of the data. If you want to produce model or residual visibilities at full resolution for all the data, you can run UVSFIT again with all source parameters specified and fixed. Key: vis Name of the input visibility file or files. No default. Key: stokes Normal Stokes/polarisation parameter (e.g. i,q,u,v,ii etc). Only a single polarisation can be requested. The default is `ii' (i.e. Stokes-I for an unpolarised source). Key: line Normal line-type processing with normal defaults. Key: select Normal data selection. Default is all cross-correlation data. Key: object This gives the object type that uvsfit fits for. Several objects can be given (the objects can be of the same type, or different), and minimum match is supported. Possible objects are point A point source disk An elliptical or circular disk. gaussian An elliptical or circular gaussian. shell The 2D projection of a thin, spherical shell. ring A face-on, thin, elliptical or circular ring For example, to fit for a point source and gaussian, use: `object=point,gaussian'. Key: spar This gives initial estimates of source parameters. For each object given by the `object' keyword, either 3 (for point sources) or 6 (for disks and gaussians) values should be given. The values are as follows: Object Type SPAR values ----------- ----------- point flux,x,y gaussian flux,x,y,bmaj,bmin,pa disk flux,x,y,bmaj,bmin,pa shell flux,x,y,bmaj ring flux,x,y,bmaj,bmin,pa Here "flux" is the total flux density of the component, "x" and "y" are the offset positions (in arcsec) of the object relative to the observing center, "bmaj" and "bmin" are the major and minor axes FWHM (in arcsec), and "pa" is the position angle of an elliptical component (in degrees). The position angle is measured from north through east. You must give initial estimates for all parameters for each object (this includes parameters that are redundant or meaningless, such as "bmin" and "pa" for components that are constrained to be circular). The more complex the set of objects being fitted for, the more important it is to give a good estimate of the source parameters. Generally the estimates of the source position should be accurate to the fundamental resolution (for point sources) or the size of the component (for extended sources). Key: fix This gives a set a flag parameters, one parameter per source. Each parameter consists of a set of letters, which indicate which source parameters of a component are to be held fixed. These source parameters are fixed by the initial estimates given by the `spar' parameter. The letters corresponding to each source parameter are: f The flux is fixed. x The offset in RA is fixed. y The offset in DEC is fixed. a The major axis parameter is fixed. b The minor axis parameter is fixed. p The position angle parameter is fixed. c The gaussian, disk or ring is circular (not elliptical). For a source where all source parameters vary, a dash (-) can be used for this parameter. For example "fix=fx,fc" indicates that the flux and RA offset is to be fixed for the first source, whereas the second source, (which is presumably a gaussian, disk or ring) has a fixed flux, and is circular. Key: sspar This gives initial estimates of source spectral parameters. For each object given by the `object' keyword, 3 values should be given. The values, for all object types, are as follows: SSPAR values ----------- alpha0,alpha1,alpha2 Here "alpha0", "alpha1" and "alpha2" are defined in: lfr = ln(frq/frq0) alpha(frq) = alpha0 + lfr*(alpha1 + lfr*alpha2) giving the spectral index as a function of frequency so that flux(frq) = flux*(frq/frq0)**alpha(frq) gives the component flux at frequency frq. Here frq0 is the reference frequency. If alpha0, alpha1, alpha2 are not given, values of zero are used. If any sspar values are given, all (three per source) must be given. Key: sfix This gives a set a flag parameters for the spectral parameters, one parameter per source. Each parameter consists of a set of digits, indicating which of the spectral terms are to be held fixed. These source parameters are fixed by the initial estimates given by the `sspar' parameter. The digits corresponding to each source parameter are: 0 The spectral term alpha0 is fixed. 1 The spectral term alpha1 is fixed. 2 The spectral term alpha2 is fixed. For a source where all source parameters vary, a dash (-) can be used for this parameter. Key: freqref A frequency in GHz used as the reference frequency for the flux and spectral index results. Key: out Optionally, an output file will be written for each input file. If given, the number of output data-sets should equal the number of input visibility files, and either the model or residual visibilities can be saved. Key: options Extra processing options. Several can be given, separated by commas. Minimum match is used. Possible values are: residual The residual visibilities are written to the output. If an output is being created, the default is to make this the fitted model. Key: log Optionally, a log file will be written with the model parameters and their errors. For each fitted source, four lines are written to the log: line 1 has source number, object type and three or six parameter values, depending on the object type; line 2 has the formal errors in each (0.0 if not a free parameter); line 3 has the values of all three spectral terms; line 4 has the formal errors in those terms (again 0.0 if not included in the fit). Revision: 1.6, 2013/12/05 01:59:50 UTC

Generated by miriad@atnf.csiro.au on 21 Jun 2016