A narrow gap, servo-controlled tunable Fabry-Perot filter for astronomy

We have constructed an 85 mm clear aperture servo-controlled Fabry–Pérot etalon with a nominal 4 µm gap. The etalon can be operated between orders 7 and 34. It behaves as a single period interference filter tunable over the wavelength range 4300–6400 Å with a bandwidth at 5000 Å of between 30 and 7.3 Å.