Content-type: text/html Manpage of uvlin

uvlin

Section: User Commands (1)
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NAME

uvlin - Separate continuum and lines in a spectral data-set.  

PERSON RESPONSIBLE

rjs  

CATEGORIES

uv analysis  

DESCRIPTION

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.  

PARAMETERS

vis
The name of the input uv data set. No default.
select
The normal uv selection commands. The default is to select everything.
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.
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.
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.
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.
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.
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.
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 a 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.


 

Index

NAME
PERSON RESPONSIBLE
CATEGORIES
DESCRIPTION
PARAMETERS

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Time: 18:35:38 GMT, July 05, 2011