v_rotsini determination: line profile Fourier transforms. The Fourier transform of a photospheric line at 6709 Å (black line) and a telluric line at 9196 Å (blue line) are shown. The location of the first minimum of the photospheric line profile provides a measurement of v_rotsini. The method requires that the main line broadening mechanism is rotation velocity, that is, the first minimum of the photospheric line must be located at a lower frequency than telluric or arc lines, as shown in the figure. Arbitrary normalisation constants have been applied to the curves to improve readability.