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We describe in some detail the characteristics of a combined Fourier-Bessel transformation tech- nique to derive projected equatorial rotational velocities from spectral line proles. This technique shares with the Fourier-transformation method, developed by Gray, that it distinguishes rotational broadening of a spectral line from broadening by other mechanisms. The range of rotational velocity values that can be derived with this method is limited mainly by the spectral resolution (low velocities) and by line blending and the signal-to-noise ratio (high velocities). We discuss the uncertainty on the outcoming rotational velocity as a result of various eects, such as limb- darkening, spectral resolution, noise, data-preparation, and intrinsic broadening. We conclude that the Fourier-Bessel transformation method can provide rotational velocities , with a typical uncertainty down to a few percent. It does not include any modelling of individual stars with eects as anisotropic macroturbulence included and therefore is less suited for a detailed analysis of individual stars. It is suited for statistical investigation of a large sample of stars.

A combined Fourier-Bessel transformation method to derive accurate rotational velocities