Computation

For a relatively simple program to drive, uvlin has a rather large number of input parameters. Task uvlin behaves somewhat like a copying program, taking an input dataset (selecting, calibrating and performing Stokes conversion, as appropriate), performing the continuum subtraction, and writing out a resultant dataset. Input parameters are:

• vis: The name of the input dataset.
• select: The subset of data to select.
• line: The normal line parameter.
• chans: This gives the set of channels to be used in the fitting process. It consists of a number of pairs giving the start and end range of the channels to include in the fitting process. Note that there channel numbers are relative to the channels selected with the line parameter.
• out: The output dataset.
• order: The fit order. The default is 1.
• offset: An offset to be applied to the data before the fit, and ``deapplied'' after the fit. The net result is that the phase centre of the data is unchanged. However the fitting process is better behaved if the phase centre during the fitting process is located at (or near) the dominant continuum emission. The default is to do no shifting.
• mode: This determines what form of data are written to the output dataset. Normally you will want to write the line data (the data after continuum subtraction). However you can also write out the fitted continuum ( mode=continuum) or a fitted channel-0 dataset ( mode=chan0). Task uvlin also has a option where it can solve for line shape (see options=lpropc below). In this case, you can also write out the fitted line ( mode=fit).

• options: The options parameter gives the normal ability to turn off calibration ( options=nocal,nopass,nopol). Note that you will generally want calibration to be applied, as a copy of the dataset is being formed. It is particularly important to apply bandpass correction. There are a few extra switches in addition to the calibration switches:

  sun: With this option, the offset keyword is ignored, and a shift appropriate for the Sun is determined instead. Thus, if your continuum is dominated by the Sun, this will help eliminate it. You will normally want to use the ``twofit'' option with this (see below). Also see Bob Sault's paper for information and an example.

  twofit: If an ``offset'' is given, or if the ``sun'' option is used, uvlin normally fits for continuum just at the offset or solar position. By using the ``twofit'' option, uvlin

  will fit for continuum at both the phase centre and the offset/solar position.

  relax: Normally uvlin attempts to avoid overfitting the data if it finds a significant number of flagged continuum channels (more than 40% bad). If it does find a large number of flagged channels, it starts reducing its fit order. The relax option tells uvlin to fit the order you ask for regardless of large numbers of flagged channels.

  lpropc: In recombination line experiments, it might be a good approximation that the line strength is proportional to the continuum. Using this extra constraint, uvlin can simultaneously fit for both a continuum spectrum (varies from visibility to visibility) and a line spectrum (which is a function of the continuum strength). This results in an iterative algorithm, which can take an appreciable time, with unfortunately only modest improvement in results. You probably do not want to use this option.

Typical inputs to uvlin are: