Optical design of the SCUBA-2 FTS

FTS-2, an imaging Fourier Transform Spectrometer (IFTS) for use with the SCUBA-2 sub-millimetre bolometer array camera on the James Clerk Maxwell Telescope (JCMT), is currently under development by the University of Lethbridge and will be operational in 2008. SCUBA-2 features two DC- coupled, monolithic TES filled arrays operating at 450 and 850 mum with a total of ~10,000 bolometers. With its larger format and increased sensitivity, SCUBA-2 promises a factor of 1000 increase in mapping speed compared to its predecessors. When coupled with SCUBA-2, FTS-2 will fill a niche between the dual band SCUBA-2 continuum images and the higher spectral resolution, but smaller images produced by the JCMT heterodyne facility instrument HARP-B. The preliminary design of the FTS-2 system has been presented elsewhere; here we review the final optical design of the instrument as the project enters the construction phase. Since the FTS-2 project began after the SCUBA-2 feed optics design was already well underway, the optical design of FTS-2 was tightly constrained and presented unique challenges. In order to maximize the use of the available space, FTS-2 incorporates a folded Mach-Zehnder design including corner cubes in the moving mirror assembly and powered optics with polynomial surfaces in the interferometer arms. A retractable pickoff mirror system allows SCUBA-2 to be used with or without the spectrometer. Both input ports are placed on the sky for atmospheric cancellation, and both output ports are imaged on separate quadrants of the SCUBA-2 arrays. INO (Institut National d´Optique), with guidance from the University of Lethbridge, optimized the optical model for FTS-2 using Zemax to maximize the instrument Field Of View (FOV) and efficiency over the desired resolution range. We present an overview of the optical design, and the projected performance of the IFTS in terms of achievable FOV, spot pattern and vignetting, at Zero Path Difference (ZPD) and for the 2 resolution m- des (~0.1 and 0.006 cm^-1).