METHOD

##
METHOD

            FIT\METHOD Arg [Parameters ...]

        Select the line profile for fits. Five type of profiles are available.

          - GAUSS     Select gaussian profiles.
            Up to 10 Gaussian, dependent or independent can be fitted in a spec-
            trum, according to command lines. Fitted parameters are:
                1) Area
                2) Velocity, and
                3) Width (FWHM)

          - SHELL     Horn-type profiles for Circumstellar Shells.
            Fitted parameters are:
                1) Area [K.MHz]
                2) Center offset [in MHz]
                3) Full Width at zero level, and
                4) Horn-to-center ratio (-1 for parabolic optically thick lines,
                   0 for flat-topped lines, Infinity for perfectly double peaked
                   lines).
            Profiles  are symmetric. Up to 10 dependent or independent lines may
            be fitted. X axis must be frequency.

          - NH3(1,1) or NH3(2,2) or NH3(3,3)
            Compute ammonia line profiles, with the assumptions of gaussian  ve-
            locity distribution and equal excitation temperatures. Fitted param-
            eters are:
                1) Main group opacity times ( Excitation temperature minus Back-
                   ground temperature),
                2) Velocity,
                3) Width (FWHM = sqrt(8.0*alog(2.0)) * line 2nd order moment),
                4) Main group opacity.
            Up  to  10  independent lines may be fitted. The hyperfine structure
            description can be found in the structure HFS% right after the  com-
            mand METHOD is invoked.

          - HFS FileName    for HyperFine Structure
            This  is  very similar to the NH3 method, but the coefficients which
            describe the hyperfine structure are read in the file  FileName.  It
            must contain:
                - the number of hyperfine components (< 40, first line),
                - the  velocity offset and relative intensity (repeated for each
                  component, one per line),
            The hyperfine structure description read from the this file  can  be
            found  in  the  structure HFS% right after the command METHOD is in-
            voked.  The fitted parameters are the same as for  the  NH3  method,
            except for the main group opacity.

            In the HFS method the main group opacity refers to the total opacity
            of the combined hyperfine components present in the description file
            divided  by the sum of their relative intensities. i.e. if the given
            relative intensities are normalised (their sum is 1)  the  resulting
            main group opacity will be the sum of the opacities of all hyperfine
            components present in the description file. This behaviour is useful
            for  example  in the case where we have only a part of the hyperfine
            components of a transition available in a spectrum. In this case one
            may  fit  only  the hyperfine components present on the spectrum and
            still recover the total opacity of the main transition.  The  safest
            way to avoid mistakes is to always use the statistical weight of the
            upper hyperfine level divided by the total statistical weight of the
            upper  level  of the main transition. In this manner one will always
            recover the total opacity of the main transition over all  hyperfine
            components.

          - ABSORPTION Filename   for hyperfine structures in absorption
            Fitted parameters are:
                1) One continuum level
            and for each line:
                2) Opacity
                3) Velocity, and
                4) Width (FWHM)

          - CONTINUUM [Width [Area_Ratio [Width_Ratio]]]  for Continuum drifts
            Fit  a  single gaussian and a linear baseline at the same time, with
            automatic determination of the parameters. Uses beam-switched infor-
            mation  for  two-component  fitting  if required. The command METHOD
            CONTINUUM accept additional arguments, which can be used to  custom-
            ize the fit:
                -  Width  is  the  width of the beam in arc seconds. By default,
                  Width is a free parameter.
                - Area_Ratio is the reference to main beam area ratio, used  for
                  beam-switch observing. By default, Area_Ratio is 1.
                -  Width_Ratio  is  the reference to main beam width ratio, used
                  for beam-switch observing. By default, Width_Ratio is 1.
            A * can be used instead of a numerical value for each parameter, and
            indicates that the parameter is free.

          - BASECONT
            Same  as  CONTINUUM,  except  that VISUALIZE will plot the baselined
            fitted model. This makes sense if the plotted data is the  baselined
            drift  while  the  model parameters still provide the baseline (off-
            set+slope) information.

        Note that in command LINES, one enters the peak line antenna temperature
        instead  of  the first variable in the guesses, except for SHELL method.
        Command LINES has no effect for CONTINUUM.

        CONTINUUM and GAUSS methods only are relevant for continuum data,  while
        GAUSS,  SHELL,  NH3, HFS and ABSORPTION methods can be used for spectro-
        scopic data.