Finesse as a function of wavelength for all channels and bands. The finesse of the first fringe component is modelled by a five coefficient spline along each slice (plain lines) and that of the second fringe component is a constant (dotted lines). Colours represent individual slices, ordered by increasing slice number from purple to yellow. Bands 1A and 1B are from FM ground test data, bands 1C to 4C are from NGC 7027 data. For band 3B, the finesses of the two components have similar values, creating the strongest beating.