How to Measure Radial Velocity
All measurements are done with spectra normalized to the local continuum. Both methods described below give very similar results for nearly symmetric single-peaked lines.
Nearly symmetric single-peaked absorption or emission lines: Apply a heliocentric correction to the spectrum or spectral part that contains the line. Fit the line profile to a Gaussian and record the central wavelength or the central radial velocity. Usually the entire profile deviates from a Gaussian. In this case, use the lower part of the absorption line or the upper part of the emission line to fit to a Gaussian.
Double-peaked emission-line profiles or other non perfectly symmetric lines: Apply a heliocentric correction to the spectrum or spectral part that contains the line. Convert the wavelengths into radial velocities with a zero-point at the standard lab position of the line you are going to measure. Create a new profile for the same line, but with an inverse the wavelength scale (RVnew = -RVold). Plot both profiles on one graph and move the inverted profile toward the original one until they match in their middle parts. The resulting shift needs to be divided by two. Different observers may get different results using this procedure. Currently recommended intensity levels to do the inversed (mirrored) profile fitting for the H-alpha line in the spectrum of Delta Sco is between 1.5 and 1.9 of the local continuum.
Last updated: 11-Feb-2011