Characterization of the Tunable Filter Imager Etalon on the JWST Fine Guidance Sensor

The Fine Guidance Sensor (FGS) on the James Webb Space Telescope (JWST) has a science observing capability provided by the Tunable Filter Imager (TFI). The TFI incorporates dielectric coated Fabry-Perot etalon plates with a small vacuum gap. The separation of the plates is controlled by the Etalon Control Electronics (ECE) board, using piezoelectric actuators (PZTs) and capacitive displacement sensors (CDS). The TFI measures over the wavelength range of 1.6 to 4.9 microns with a spectral resolution of R~100. We present the key components of the etalon system and the approach for characterizing and testing the system. Initial results from assembly-level testing are also presented.