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Task: uvlin Purpose: Separate continuum and lines in a spectral data-set. Categories: uv analysis UVLIN separates line and continuum in a spectral visibility data-set. It does this by fitting a low order polynomial to the real and imaginary parts of the line free channels of each spectrum. This polynomial function is taken to represent the continuum, and any deviation from this is taken to represent the line data. Optionally a constraint can be added that the line shape is constant, and its strength is proportional to the continuum level. This is a good approximation in many recombination line experiments. In this case, an iterative algorithm is used to estimate the line shape and to optimally disentangle the line and the continuum. See the related tasks CONTSEN and CONTERR, which give estimates of the noise amplification and the residual continuum. Key: vis The name of the input uv data set. No default. Key: select The normal uv selection commands. The default is to select everything. Key: line The normal uv linetype in the form: line,nchan,start,width,step The default is all channels. Note that if there are multiple spectral windows, the fitting process is performed on each window separately. Key: chans This specifies the channel ranges that contain only continuum (line free). It consists of a number of pairs, each pair giving a start and end channel. These are relative to the channels selected with the line parameter. The default is that all channels are line-free, which is quite a good approximation if the line is weak compared to the continuum. Key: out The name of the output uv data-set. There is no default name. The output can be either the line, continuum, or fitted line shape, as specified by the options keyword. The default is to write the line data. Key: order Order of polynomial used to fit the continuum. The default is 1 (i.e. a linear fit). Possible values are 0 to 5. Also see the `relax' option. Key: offset An offset (in arcsec) that would shift the data to the center of the dominant emission. Giving this shift allows UVLIN to reduce the error involved in approximating the continuum as a polynomial. The shift information is only used in the fitting process -- the output data is not shifted. The default is 0,0 (no shift). The following options determine what is written to the output data-set. Key: mode This determines what is written out. The default is to write out the line data. The following are possible values: line The output data-set is the line data. This is the default. fit The output data-set is the fitted line shape data. This is valid only when using the 'lpropc' option. chan0 The output data-set is an average of all the continuum data in a visibility record. continuum The output data-set is the continuum data. This has the same number of channels as the input data-set. ratio The output data-set is the input data divided by the fitted continuum data minus one. Key: options This gives extra processing options. Several options can be given, each separated by commas. They may be abbreivated to the minimum needed to avoid ambiguity. Possible options are: sun With this options, the OFFSET keyword is ignored, and a shift appropriate for the Sun is determined This can be useful to eliminate solar interference. twofit If either the OFFSET parameter is set, or the SUN option invoked, this options instructs UVLIN to perform a simultaneous polynomial fit for emission both at the phase centre and at the appropriate offset. nowindow Normally uvlin processes the different windows separately. This will usually give the best result when the different windows are at quite different frequencies. However when the windows are at similar frequencies, or perhaps even abutting, then it is better to perform a single fit over all windows simultaneously. The "nowindow" option causes uvlin to do a single fit over all windows. lpropc The line shape is constant throughout the data-set, and the intensity is proportional to the continuum strength. This cannot be used together with the 'twofit' option. relax Normally UVLIN attempts to avoid overfitting the data by reducing the order of the fit. It does this if a significant number of channels are flagged (more than 40% bad) or if it believes there is inadequate data to perform the fit (less than 5*(order+1) channels). You can overwrite this conservatism, if you know what you are doing, with the `relax' option. The following options can be used to turn off calibration corrections. The default is to apply any calibration present. nocal Do not apply the gains table. nopass Do not apply bandpass corrections. It is unwise to turn off bandpass correction, as the continuum estimation process will be confused by a bandpass which is not flat. nopol Do not apply polarization corrections.