FTIR instrument design for the outer solar system atmospheric studies

We present the preliminary trade-off study and optical design of a Fourier Transform InfraRed interferometer dedicated to high spatial and spectral resolution measurements for outer solar system planets atmospheric studies. Based on the science objectives listed for the study of Saturn, Titan and other targets of interest, we identify our instrument requirements to be: (i) a wavelength range of 20 to 100 μm, (ii) a spectral resolution of 0.1 cm ^-1 , (iii) a spatial resolution of 1 mrad, and (iv) the capability to acquire spectra of an extended area within minutes to be able to add a temporal aspect to our measurements. A trade-off study comparing the relative merits of different instrument types allowed us to determine a Fourier Transform interferometer as our best design choice and set a few of the main instrument optical parameters. We later validated these parameters through a radiometric study yielding an SNR of 2995 for a scene simulating either Titan or Saturn, with an acquisition time for each group of spectra of 200 s. The optical design of the interferometer included several design choices balancing the need for throughput and compactness. The final choice corresponds to a full off-axis design to avoid any obscurations and allow a better separation between the telescope system and the interferometer relay system for both stray light and thermal considerations. The resulting optical design is quite compact with a total volume of 53×55×16 cm ³ which is 7.5 times smaller than the total size of the state of the art Cassini CIRS instrument.