The IRAF Radial Velocity Analysis Package


The RV package presently consists of a Fourier Cross-correlation task, FXCOR, several psets to support it's functions, and two tasks for measuring velocities from spectral lines, RVIDLINES and RVREIDLINES. It may be used on one or two-dimensional data with or without a dispersion solution. Computed velocities, or simply shifts in the case of pixel data, are presented to the user graphically, in tabular form, and as a verbose log of the correlation.

The FXCOR Task

The FXCOR task is a one-dimensional Fourier Cross-correlation task based on the standard Tonry & Davis algorithm (Tonry&Davis, 1979, Astron. J, 84, 1511). Object spectra may be either one or two dimensional (in `onedspec' or `multispec' format), and may be correlated against a one or two dimensional template. When the task is started the screen comes up with a plot of the first correlation and fit to the peak. For example,

Input images may automatically be rebinned and/or continuum subtracted according to a user-specified parameter. Image header keyword translation is done using the KEYWPARS pset. A variety of fitting functions and output options are also supported.

The data may be viewed in a number of sub-modes of the task. The Spectrum Mode allows the user to view the raw, prepared or filtered spectra as well as interactively set the correlation sample regions. The Fourier Mode lets the user select filtering parameters and view various properties of the FFT. Finally, a Continuum Mode exists to permit interactive continuum fitting and subtraction.


RVIDLINES measures radial velocities from spectra by determining the wavelength shift in spectral lines relative to specified rest wavelengths. The basic usage consists of identifying one or more spectral lines (also called features), entering the rest wavelengths, and computing the average wavelength shift converted to a radial velocity. Additional lines can then be automatically added from a coordinate list of rest wavelengths.


RVREIDLINES takes spectral lines previously identified in a reference image and recorded in a database and identifies them in other spectra and determines a radial velocity. If the images are two or three dimensional or multiaperture format and a step greater than zero is specified then additional vectors (lines/columns/bands/apertures) in the reference image will be reidentified from the initial master reference vector (as defined by an image section or section parameter) provided they have not been reidentified previously or the override flag is set. For multiple aperture spectra images, called multiaperture, the step size is typically 1; i.e. reidentify features in all spectra. For two and three dimensional images, such as long slit and Fabry-Perot spectra, the step(s) should be large enough to minimize execution time and storage requirements but small enough to follow shifts in the features (see the discussion below on tracing). The set of reference identifications is applied to other images in the same lines, columns, bands, or apertures. In multiaperture images the same apertures are matched in the reference image regardless of actual line order; i.e. the apertures need not be in the same order or even have all apertures present.

Future Plans

Aside from mostly maintanence the package is fairly stable although there is always work that could be done and suggestions for improvements are welcomed. In the near term, work on a GUI for the FXCOR Task is currently underway.

See Also

rvcorrect, continpars, filtpars, observatory, keywpars, onedspec.specwcs, center1d, dispcor, stsdas.fourier