NAME

vuespec -- Interactively fit model components to input spectra

USAGE

vuespec

DESCRIPTION

This task fits a model to one or more input spectra; this is the interactive version of the "autospec" task. The model is constructed from a set of one or more components which are stored in an STSDAS binary table, called a component DataBase. The DataBase may be constructed using the non-interactive "populate" task.

The spectra can be imported from a variety of common formats, including ST tables, FITS tables, and IRAF images. If fitting to more than one spectrum, they need not have the same dispersion scale. The flux and dispersion arrays for the input spectra are automatically converted to a set of internal units (PHOTLAM: see the description of the "flux_unit" parameter below) for fitting. The results of the fit can be written to an output DataBase; the physical units are specified interactively.

The various component parameters are fit using a choice of algorithms to one or more "domains" or portions of the input spectra. Domains are specified interactively with mouse actions, although initial domains may be specified with the "fit_domains" task parameter: the syntax is much like that described in the help for "ranges" except that no "step size" is allowed, and sexigesimal numbers are not supported. Specifying domains is a good way to exclude portions of spectra that would otherwise difficult or inconvenient to fit.

More detailed help may be accessed through the "Help" button once this task has been invoked. Alternatively, you can direct your Web browser to URL http://www.stsci.edu/ASpec.html.

PARAMETERS

input = "" [string]

List of spectra to which the fit will be performed.

db_intab = "" [string]

List of DataBase tables from which the component definitions and initial values will be drawn.

(db_outtab = "") [string]

Output DataBase table to contain the fitted components. If blank, the components will be appended to the (first) input DataBase table.

(flux_unit = "FLAMDA") [string]

System of flux units for the output DataBase table. The possible choices are: "PHOTLAM|PHOTNU|FLAMDA|FNU|JY|MJY" and are adopted from the STSDAS "synphot" package. Briefly, PHOTLAM and PHOTNU both express brightness in units of equivalent photons/cm**2/s: the former per unit wavelength (Angstroms), and the latter per unit frequency (Hz). FLAMDA and FNU both express brightness in units of ergs/cm**2/s: again per unit Ang or per unit Hz, respectively. JY and MJY are Janskys and milli-Janskys, respectively.

(dsprsn_unit = "ANG") [string]

Dispersion units for the output DataBase table. The possible choices are: "ANG|NM|MICRON|CM|HZ|MHZ|GHZ|EV|KEV|MEV". The latter three are expressed as the frequency of a photon with the equivalent energy in electron volts (eV), or a multiple thereof.

(fitpars = "") [pset]

Parameter set for specifying the fit(s) to be performed.

(algorithm = "simplex") [string]

Choice of algorithm for the fit. The possible choices are "simplex" and "marquadt".

(fit_toler = 1.e-4) [real]

Global tolerance to define convergence for fits. The fit is said to have converged when the merit function changes by less than this fractional amount.

(max_iter = 300) [int]

Maximum number of iterations of the fit algorithm before stopping; if this limit is reached, a warning message is issued; the components are written to the output DataBase in any case.

(fit_domains = "*") [string]

String specifying the dispersion domain(s) over which the fit will be performed; the default is the entire extent of all input spectra. The domains may be read from an input text file if the name of the file is specified here, preceeded by an "@" sign. Separate domains may be specified in this case, one per line of text, which will result in independent fits to each domain. The units for the dispersion are taken from the "dsprsn_unit" parameter.

(spec_io = "") [pset]

Parameter set for specifying how to find or construct all of the attributes of a spectrum: the flux, wavelength, uncertainty, and mask arrays. The parameters include default column names for the input spectra if they are tables, or file extension names if the various arrays are stored in separate files. These parameters can also be set interactively; type "help spiopar" for a description of these parameters.

EXAMPLES

1. Fit a model to a spectrum called "NGC6543.tab" with components stored in the table "NGC6543_db.tab". Specify the initial displayed flux units to "FLAMDA" and the dispersion coordinates to angstroms. Specify one initial fit domain between 4600 and 4800 Ang.

   cl> vuespec NGC6543.tab NGC6543_db.tab flux_unit="FLAMDA" \
   >>> dsprsn_unit= "ANG" fit_domains="4600:4800"

REFERENCES

This package was written by J.D. Eisenhammer, S.J. Hulbert, Z.G. Levay, and R.A. Shaw. Support for this software development was provided by the Astrophysics Data Program through NASA grant NAG5-31859 to STScI.

SEE ALSO

autospec, populate, ranges, spiopars