Effect on the measured intrinsic global GC velocity dispersions due to the finite FLAMES fibre size (1.2″diameter) and fibre positioning error (±0.1″) for a range of values for distance, seeing and intrinsic GC size. Positive values indicate an overestimation of the true dispersion due to finite aperture (black lines), while negative values indicate an underestimation of the velocity dispersion due to fibre centring (orange lines).