The Optical Design of a Far Infrared Imaging FTS for SPICA Carmen Pastor 1 , Pablo Zuluaga 1 , Willem Jellema 2 , Luis Miguel González Fernández 1 , Tomas Belenguer 1 , Josefina Torres Redondo 3 , Peter Paul Kooijman 2 , Francisco Najarro 3 , Martin Eggens 2 , Peter Roelfsema 2 , Takao Nakagawa 4 1 Instituto Nacional de Técnica Aeroespacial (INTA), Carretera de Ajalvir Km 4.5, 28850 Torrejón de Ardoz, Madrid, Spain; 2 SRON Netherlands Institute for Space Research, Department of Low Energy Astrophysics, P.O. Box 800, 9700 AV Groningen, The Netherlands; 3 Centro de Astrobiología (INTA-CSIC), Carretera de Ajalvir Km 4.5, 28850 Torrejón de Ardoz, Madrid, Spain; 4 Japan Aerospace Exploration Agency (JAXA), Institute of Space and Astronautical Science (ISAS), Department of Infrared Astrophysics, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210 Japan ABSTRACT This paper describes the optical design of the far infrared imaging spectrometer for the JAXA’s SPICA mission. The SAFARI instrument, is a cryogenic imaging Fourier transform spectrometer (iFTS), designed to perform background- limited spectroscopic and photometric imaging in the band 34-210 µm. The all-reflective optical system is highly modular and consists of three main modules; input optics module, interferometer module (FTS) and camera bay optics. A special study has been dedicated to the spectroscopic performance of the instrument, in which the spectral response and interference of the instrument have been modeled, as the FTS mechanism scans over the total desired OPD range. Keywords: far infrared, optical design, iFTS, interferometry, diffraction analysis 1. INTRODUCTION SpicA FAR infrared Instrument, SAFARI, is an imaging spectrometer which is being designed to map large areas of the sky in the far infrared. The SPICA mission, having a large cold telescope cooled to 6K above absolute zero, will provide an optimum environment where instruments are limited only by the cosmic background itself. [2] The optical design of an instrument working for the far infrared and involving interferometry at the same time, as in our case for SAFARI, is quite challenging and must pay great and dedicated attention to at least three main design working areas on top of the base geometrical optical design; areas such as the propagation & diffraction analysis, the interference modeling of the FTS scanning, and the baffling strategy. The first area of consideration is dedicated to the beam diffraction effects, which must be unavoidably taken into account, as the working wavelength is considered to be large compared with the size of the optical components. On the other hand, the interference modeling of the FTS while scanning over the total desired OPD range, will give us important information about the spectral response of the instrument as built, even taken into account the manufacturing and alignment tolerances. The third area of study would be the stray light control, as all the optical components and mechanical parts that are warmer than a few kelvin are highly self-luminous at these wavelengths, and they are therefore potential spurious signal contributors to the sometimes faint astronomical sources of interest. We will not cover the SAFARI baffling strategy in this paper, but we are keeping in mind the future need of providing sufficient room around the beam and mounts for stray-light baffles and oversized apertures. The general scope of this paper is to present the conceptual or reference optical design and its current status. CODEV and MatLab have been the main software packages we have used for the design and analysis of this optical system. Space Telescopes and Instrumentation 2014: Optical, Infrared, and Millimeter Wave, edited by Jacobus M. Oschmann, Jr., Mark Clampin, Giovanni G. Fazio, Howard A. MacEwen, Proc. of SPIE Vol. 9143, 91434B · © 2014 SPIE CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2056791 Proc. of SPIE Vol. 9143 91434B-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 12/19/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx