An undulator tuning curve calculated for the SOLEIL synchrotron (2.75 GeV, 500 mA stored current) and a 20 mm period in-vacuum undulator. The solid curves correspond to the range of energies traced out by each undulator harmonic as the gap is changed. Small gap values correspond to low energies. Only the strongest (odd) harmonic tuning curves are shown. The curve was calculated using the program SPECTRA (Kitamura et al., 2004).