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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
RVSAO 2.0 ­ A Radial Velocity Package for IRAF
Douglas J. Mink and Michael J. Kurtz
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, Email:
dmink@cfa.harvard.edu
Abstract. RVSAO 2.0 is the latest release of a package for calculat­
ing apparent radial velocities of celestial objects from observed spectral
shifts. There are two main tasks in the package, XCSAO and EMSAO.
XCSAO cross­correlates the Fourier transform of an object's spectrum
against the transforms of a set of template spectra with known spectral
shifts to obtain a velocity and error. EMSAO finds emission lines in a
spectrum and computes the observed centers, getting individual shifts and
errors for each line as well as a single velocity combining all of the lines.
Three tasks which are new in this release are SUMSPEC, which combines
spectra after shifting them all to a specified redshift, LINESPEC, which
creates a spectrum at a specified redshift from a list of rest wavelengths,
and BCVCORR, which computes the correction needed to translate the
observed radial velocity to one relative to the solar system barycenter.
Full documentation of this software, including numerous examples of its
use, is on­line at http://tdc­www.harvard.edu/iraf/rvsao/
1. Introduction
The RVSAO IRAF external package was developed at the Smithsonian Astro­
physical Observatory to compute redshifts from spectra in as automatic a way
as possible. It has been used by several large redshift surveys and is also used for
stellar radial velocity work. An earlier version of the XCSAO task, which com­
putes radial velocities by cross­correlating spectra against templates of known
redshift, has been described by Kurtz et al. (1992). The EMSAO task, which
automatically identifies emission lines in a spectrum and computes their red­
shift has been described by Mink and Wyatt (1995). Mink and Wyatt (1992)
described how these IRAF tasks could be combined to reduce large amounts of
data in a pipeline. Both XCSAO and EMSAO have been improved over the
years, and new tasks have been added to prepare template spectra for cross­
correlation and to compute velocity corrections for data with di#erent header
keywords than are used by the Smithsonian's telescopes.
2. Changes in XCSAO
The cross­correlation algorithms in XCSAO have been changed very little over
the years, although the optional elimination of high­frequency filtering has been
93

94 Mink and Kurtz
added to enable the use of templates with narrow emission lines. Additional log
format options have been added at the request of users.
To totally eliminate the e#ects of bad night sky subtraction, or to remove
other features appearing at known positions in the observed wavelength space
of a spectrum, a new feature has been added to both XCSAO and EMSAO.
If the parameter fixbad is set to yes, XCSAO replaces sections of the spectrum
described in the file designated by the badlines parameter with values interpo­
lated from the ends of the sections. A line list is provided to remove the regions
around night sky emission lines.
To conform with IRAF conventions for multispec files, that is spectrum files
with multiple spectra from multiple apertures, new parameters specband and
tempband have been added to specify the band to be read from each aperture.
For example, in standard multispec files, band 1 is the object spectrum and
band 3 is the sky spectrum. The specnum and tempnum parameters, which
could specify either the band or the aperture, now specify only the aperture to
be used from the file.
3. EMSAO Line Fitting Improves
The major change in the EMSAO task has been to replace the old minimization
routine with one which has been used in a di#erent astronomical context to fit
multiple Gaussians. It is both faster and more robust than the old subroutine,
making it practical to routinely run EMSAO on every spectrum, whether it has
emission lines or not. There is always a danger of getting false emission lines, so
it is safest to run EMSAO on spectra against which an emission line template
has correlated well in XCSAO.
The sky spectrum, which is used to get the observed noise for better error
calculations, may come from a di#erent band skyband of a spectrum, such as the
subtracted sky of an apextracted multispec spectrum, as well as from a separate
aperture.
Many parameters which were built­in constants have been turned into task
parameters. mincont sets a minimum continuum level at which an equivalent
width is computed. There are many criteria for whether lines should be kept once
they have been fitted. The lwmin and lwmax parameters set the minimum and
maximum variation from the mean line width allowed for a line to be accepted.
lsmin is the minimum ratio of the equivalent width (or area) to its error for a
line to be accepted. A number is now appended to the line rejection flag in
tables of results to indicate why the line was dropped.
4. Creating Templates from Line Lists with LINESPEC
By cross­correlating both emission and absorption line objects with XCSAO, a
single output line can give both a reasonable redshift and a characterization of
the object. Since every emission line object is di#erent, a pure emission line
template, with idealized line profiles seemed optimum. LINESPEC was written
to use the line profile information provided by a reporting format added to
EMSAO to create a spectrum from mean profiles of various identified emission
lines.

RVSAO 2.0 ­ A Radial Velocity Package for IRAF 95
Figure 1. This is an artificial emission line spectrum created by
LINESPEC for SAO's FAST Spectrograph
For each line, the center wavelength in Angstroms, the half­width in Angstroms
if positive, in km/sec if negative, the height of line in arbitrary units, and the
name of line for labeling, are read from a file. For each line in the table, the
center is redshifted accordingly by a z (delta lambda / lambda) or apparent
Doppler shifting velocity. The linewidth, if it is tabulated in kilometers per sec­
ond, is converted to Angstroms at the shifted line center. The line width is also
broadened appropriately if the line is redshifted. For each line, a Gaussian at
the shifted center wavelength, half­width, and tabulated height is added to the
spectrum. After all of the lines are computed, a constant continuum level may
be added to the spectrum.
The computed spectrum is displayed, as shown in Figure 1, and may be
edited before it is written to a disk file. The header of the output spectrum image
includes one parameter per emission line with a vector of line characteristics in
the format used by EMSAO.
5. Adding Spectra with SUMSPEC
For a long time, SAO has been using composite absorption line spectra as tem­
plates for galaxy cross­correlation. To formalize the process of creating such
template spectra, SUMSPEC was written. It combines spectra, shifting them to
a common redshift. The VELOCITY header parameter of each of these spectra
is assumed to be a solar­system­barycenter­corrected velocity, and a barycentric
correction (computed by sumtemp or extracted from the BCV or HCV header
parameter) is subtracted to get the actual redshift of the spectrum. Each spec­
trum is shifted and rebinned to the desired wavelength range and bin size, which

96 Mink and Kurtz
may be linear in wavelength or in log­wavelength, then added to the summed
template. Input may be multispec or twodspec format, but output is always a
one­dimensional file. If the desired output velocity is set to INDEF, spectra are
redshifted to the solar system barycentric frame so spectra of the same object
observed at di#erent times throughout the year may be added to improve signal
to noise.
SUMSPEC can automatically find the wavelength range over which all of
the spectra to be added overlap. The output binsize may be specified explicitly
or computed from the desired number of pixels and wavelength range. The
continuum may be subtracted or divided from each spectrum before it is added
into the final composite spectrum.
The composite spectrum may include a list of all of the input spectra in its
header, so re­creation is possible. This can be turned o# if hundreds of spectra
are being added together.
6. More Options for Computing Heliocentric Velocity Correction
The XCSAO, EMSAO, and SUMSPEC tasks compute the velocity change needed
to correct the observed redshift to the redshift relative to the sun, or more ac­
curately, the solar system barycenter. They read the time of observation, object
position, and observatory position from the spectrum image header. Although
these tasks check several commonly­used alternative keywords for most of the
needed parameters, it is possible that it won't find all of them. A separate task,
BCVCORR, has been added to RVSAO to allow several alternate ways of spec­
ifying these three major pieces of information. BCVCORR can write its result
to the header of the image which it is processing; the other RVSAO tasks will
use this value when their svel corr parameter is set to ``file''.
7. RVSAO's Future
RVSAO will continue to change to meet the needs of astronomers for fast extrac­
tion of radial velocities from large numbers of spectra. While RVSAO is ready
for current multiaperture and multiple­order spectrographs, new instrumenta­
tion will surely require modifications to this software in the future.
References
Kurtz, M.J., Mink, D.J., Wyatt, W.F., Fabricant, D.G., Torres, G., Kriss, G,
and Tonry, J.L. 1992, in Astronomical Data Analysis Software and Sys­
tems I, ASP Conf. Ser., Vol. 25, eds. D.M. Worral, C. Biemesderfer, and
J. Barnes, 432.
Mink, D.J. and Wyatt, W.F. 1992, in Astronomical Data Analysis Software and
Systems I, ASP Conf. Ser., Vol. 25, eds. D.M. Worral, C. Biemesderfer,
and J. Barnes, 439.
Mink, D.J. and Wyatt, W.F. 1995, in Astronomical Data Analysis Software and
Systems IV, ASP Conf. Ser., Vol. 77, eds. R.A. Shaw, H.E. Payne, and
J.J.E. Hayes, 496.