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P. L. Shopbell
Rice University, Department of Space Physics, P.O. Box 1892, Houston, TX 77251
J. Bland-Hawthorn
AAO, P.O. Box 296, Epping, NSW 2121, AUSTRALIA
is under development as a
joint effort of ourselves and the IRAF group. In this paper, we describe an
important component of the Fabry-Perot package, the fpplot task for
spectral plotting and fitting. While this task has many similarities with
the familiar splot and specplot tasks in the onedspec
package, fpplot has been optimized and extended specifically for use
with imaging Fabry-Perot data. The task provides for the display and analysis
of grids of spectra, including functions for binning, scaling, masking, and
overplotting spectra. The most important features of fpplot use the
IRAF nlfit and inlfit nonlinear fitting libraries
to perform both interactive and background fitting of Fabry-Perot spectra.
Automated techniques are essential for quantifying the thousands of
spectra in a Fabry-Perot data cube for velocity and photometric studies.
An example is given from current work involving the starburst galaxy M82.
Since their inception, the complexity of imaging Fabry-Perot interferometers has made the reduction of their data a daunting task. Large data sets and complex instrumental profiles have caused the majority of Fabry-Perot observations to be interpreted simply as velocity maps, ignoring the enormous amount of photometric information also present. A primary goal of the IRAF Fabry-Perot package is to enable the photometric reduction of imaging Fabry-Perot data.
Once the characteristic Airy instrumental profile of the Fabry-Perot etalon is removed from the data, by a process we call ``phase calibration'' (Shopbell, Bland-Hawthorn, & Cecil 1992), data visualization can take two forms:
Figure: Two forms of imaging Fabry-Perot data visualization. The left cube
depicts a series of monochromatic spatial frames sampled regularly in
wavelength. The right cube depicts a grid of spectra sampled regularly
in the spatial dimensions.
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Early stages of the reduction of Fabry-Perot data, including cosmetic cleaning, flat-fielding, and alignment, employ the first visualization model. The later stages of reduction, such as phase correction, sky subtraction, and spectral fitting, employ primarily the second visualization model. While there are clearly tools available for the manipulation and display of images and image mosaics, there is currently a lack of useful visualization tools for application to the spectral domain of Fabry-Perot data. fpplot has been designed to assist the user in the advanced analysis stages of imaging Fabry-Perot data.
Figure: A spectral grid from Fabry-Perot data of the starburst galaxy M82
(H
/[N II]), illustrating an overlaid contour map, two
irregular areas of masked spectra, and a large area of spectra fitted
automatically with a four-component Gaussian model.
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The fpplot task in the IRAF Fabry-Perot package has many features found
in the splot and specplot tasks in the onedspec and
twodspec packages, as well as many options added specifically for
Fabry-Perot data analysis. First there is the display of spectra.
The displayed spectral grid merely represents a ``window'' onto the full
spectral cube. The limits of the view may therefore be shifted and zoomed
to view large-scale trends or details of individual spectra. Next, the spectra
may be arbitrarily binned in the spatial dimensions, allowing the user to view
large spatial variations. Binning in the spectral dimension is also possible.
Additionally, one can scale the spectra. The spectra may be scaled to a variety
of limits, including the extremes of the entire data set or each spectrum's
extremes (i.e., autoscaling). In addition to the above, many overplotting
options are provided for the comparison of data with other data or models.
Additional Fabry-Perot spectral cubes, spectral fits, and image contour maps
may be overplotted. Also included is the sophisticated spectral fitting
module fpplot which allows for both
interactive and background fitting of spectra with multiple-component Gaussian
functions. The interactive form is very similar to that provided by the
splot task, including point rejection, residual plotting, etc. The
automatic form uses spectra that have already been fitted to determine initial
conditions for fitting additional spectra. Currently Gaussian functions and
a linear continuum are used; Lorentzian and Voigt line fitting, as well as
non-linear continuum fitting are under development.
Lastly, to enable efficient automated fitting, full support is provided for spatial
masking, via the IRAF PLIO routines (Tody 1988). Using masks, the user can
remove bad pixels, sky regions, etc. from the fitting process.
Figure demonstrates several of these features using Fabry-Perot
data from a central region of the starburst galaxy M82 (Shopbell 1995).
The most significant capability of the fpplot task is that of fitting
spectra. A small (512 
512) CCD, assuming a useful data
coverage of 50%, yields over 125,000 distinct Fabry-Perot spectra. If the
instrument characteristics are well understood, these spectra
can be fit to yield not only radial velocities, but emission line fluxes and
dispersions as well. However, such a study clearly requires an automated
means of fitting the emission line spectra.
The fitting portions of fpplot are modeled after those found in the splot task (in particular, those activated by the `d' and `k' keys). Single Gaussian functions can be fit; multiple Gaussians can be deblended. As with splot, fpplot employs IRAF's nlfit and inlfit nonlinear least squares fitting routines (Davis 1991) to fit the Gaussian profiles and, optionally, a linear continuum.
Major additions to fpplot enable it to fit large numbers of spectra in an almost entirely automated fashion. The user need only fit a few ``characteristic'' spectra interactively. fpplot will then propagate these fits across the desired region, using the fits of spatially nearby spectra as initial guesses for the current spectrum's fit. This propagation method is similar to that used in the FIGARO longslit package (Wilkins & Axon 1992). Because Fabry-Perot spectra are typically of low spectral resolution and encompass small wavelength ranges, this method of fitting works especially well. The typical problems of incomplete spectral coverage and inadequate continuum are still present however, as well as difficulties arising from multiple spectral components and rapidly varying spatial features. The use of an iterative procedure involving interactive verification of selective fits appears to solve these problems adequately.
The authors would like to thank the National Optical Astronomy Observatories, and the IRAF group in particular, for their continued support of this project. Partial support of P.L.S. has been provided by the Sigma Xi Grants-In-Aid of Research program and the Texas Space Grant Consortium.
Davis, L. 1991, NLFIT/INLFIT README files, IRAF v2.10 distribution (Tucson, NOAO)
Shopbell, P. L., Bland-Hawthorn, J., & Cecil, G. 1992, in Astronomical Data Analysis Software and Systems I, ASP Conf. Ser., Vol. 25, eds. D.M. Worrall, C. Biemesderfer, & J. Barnes (San Francisco, ASP), p. 442
Shopbell, P. L. 1995, Ph.D. Thesis, Rice University
Tody, D. 1988, PLIO README files, IRAF v2.10 distribution (Tucson, NOAO)
Wilkins, T. N., & Axon, D. J. 1992, in Astronomical Data Analysis Software and Systems I, ASP Conf. Ser., Vol. 25, eds. D.M. Worrall, C. Biemesderfer, & J. Barnes (San Francisco, ASP), p. 427
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