RVSAO 2.0 - A Radial Velocity Package for IRAF
by
Douglas J. Mink and
Michael J. Kurtz
Smithsonian Astrophysical Observatory, Cambridge, MA 02138
presented at the Seventh Conference on Astronomical Data Analysis Software
and Systems in Sonthofen, German, September 1997
Abstract
RVSAO 2.0 is the latest release of a package for calculating apparent
radial velocities of celestial objects from observed spectral shifts.
There are two main tasks in the package, XCSAO and EMSAO. XCSAO
cross-correlates the Fourier transform of an object's spectrum against
the tranforms 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 reshift, 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 on-line at
http://tdc-www.harvard.edu/iraf/rvsao/
\end{abstract}
Keywords: redshift,IRAF,radial velocity,redshift survey
Introduction
The RVSAO IRAF external package was developed at the Smithsonian
Astrophysical 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 computes radial velocities
by cross-correlating 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 redshift 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 different header keywords than are
used by the Smithsonian's telescopes.
Changes in XCSAO
The cross-correlation algorithms in XCSAO have been changed very little
over the years, although the optional elimination of high-frequency
filtering has been 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 effects of bad night sky subtraction, or to remove
other features appearing at known positions in the observed wavlength space
of a spectrum, a new feature has been added to both XCSAO and EMSAO.
If the parameter {\it fixbad} is set to yes, XCSAO replaces
sections of the spectrum described in the file designated by the
{\it badlines} parameter with values interpolated 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
{\it specband} and {\it tempband} have been added to specify the band to
read from each aperture. For example, in standard multispec files, band
1 is the object spectrum and band 3 is the sky spectrum. The {\it specnum}
and {\it tempnum} parameters, which could specify either the band or the
aperture, now specify only the aperture to use from the file.
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 different 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 different band {\it 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 built-in constants have been turned into
task parameters. {\it 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 are fit.
The {\it lwmin} and {\it lwmax} parameters set the minimum and maximum
variation from the mean line width allowed for a line to be accepted.
{\it 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.
Creating Templates from Line Lists with
LINESPEC
By cross-correlating 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 different, 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.
For each line, the center wavelength in Angstroms, the half-width in
Angstroms if positive, in km/sec if negative, the height of line in
arbitrary units, and the name of line for labelling, are read from a
file. For each line in the table, the center is redshifted according
by a z (delta lambda / lambda) or apparent doppler shifting velocity.
The linewidth, if it is tabulated in kilometers per second, 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, half-width, 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.
Figure 1. This is an artificial emission line spectrum created by LINESPEC
for SAO's FAST Spectrograph.
Adding Spectra with
SUMSPEC
For a long time, SAO has been using composite absorption line spectra as
templates for galaxy cross-correlation. 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 solar-system-barycenter-corrected 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 spectrum is shifted and rebinned to the desired wavelength
range and bin size, which may be linear in wavelength or in log-wavelength,
then added to the summed template. Input may be multispec or twodspec format,
but output is always a one-dimensional 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 different 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 and/or from 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 off if hundreds
of spectra are being added together.
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 accurately, 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 commonly-used 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 specifying 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 {\it svel\_corr} parameter is set to "file".
RVSAO's Future
RVSAO will continue to change to meet the needs of astronomers for fast
extraction of radial velocities from large numbers of spectra.
While RVSAO is ready for current multiaperture and multiple-order
spectrographs, new instrumentation 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,
XCSAO: A Radial Velocity Package for the IRAF Environment,
in Astronomical Data Analysis Software and Systems I,
ASP Conf. Ser., Vol. 25, eds. D.M. Worral, C. Biemesderfer, and J. Barnes, 432.
[abstract]
[full text]
Mink, D.J. and Wyatt, W.F. 1992,
A Production System for Radial Velocity Measurements,
in Astronomical Data Analysis Software and Systems I,
ASP Conf. Ser., Vol. 25, eds. D.M. Worral, C. Biemesderfer, and J. Barnes, 439.
[abstract]
[full text]
Mink, D.J. and Wyatt, W.F. 1995,
EMSAO: Radial Velocities from Emission Lines in Spectra
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.
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