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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.
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.
Gildas manager
2024-04-18