Tunable Filter Imager for JWST: Etalon Opto-mechanical design and Test Results

The Tunable Filter Imager of the James Webb Space Telescope will be based on blocking filters and a tunable Fabry-Perot etalon with an average resolution of about 100. It will operate in two wavelength bands from 1.6 mu m to 2.5 mu m and from 3.1 mu m to 4.9 mu m at a cryogenic temperature of about 35K. It will respectively be used to study the First Light and re-ionization of the universe by surveying Lyman-alpha sources and to provide an in-depth study of proto-planetary systems as well as giant planets of nearby stars. The Tunable Filter Imager (TFI) is designed to image a sky field of view of 2.2 ' by 2.2 ' (magnified to 4.6 deg. x 4.6 deg. at the etalon). Its tunable etalon has an aperture of 56 mm. It operates at low orders 1 and 3 for the two wavelength bands which reduces the number of blocking filters to a number of eight. The etalon gap tuning between 2.5 mu m and 5.5 mu m is provided by piezoelectric actuators and will be servo controlled by using capacitive displacement sensors. In this paper, we present the etalon's opto-mechanical design that allows us to achieve the stringent requirements in terms of resolution over a wide infrared wavelength band, and operation at low gap at cryogenic temperature. Cryogenic test results will be shared as well.