This paper describes the end-to-end opto-mechanical design of the SAFARI instrument on SPICA and the analysis of the spectrometer optical performances. SAFARI instrument is a high sensitivity grating-based spectrometer operating in the 34-230 μm wavelength range. The scientific drivers lead to the implementation of two modes of operation. The Low- Resolution (LR) or nominal mode (R~300) and the High-Resolution (HR), that implies to include a Martin-Puplett Fourier Transform Spectrometer (MP-FTS) to achieve the required spectral resolution (R~2000-11000). The optical system is all-reflective and consists of three main modules. The input optics module (IOM) is an unobscured reflective Offner relay. In the IOM a Beam Steering Mirror (BSM) is included for spatial modulation and to allow efficient sky mapping. The Band and Mode Distributing Optics (BMDO) module splits the radiation band into the four different spectral bands and includes the MP-FTS. The field image existing at the output of the BMDO constitutes the entrance to the Grating Module Optics (GM). These modules provide spectral dispersion by means of linear and reflective diffraction gratings and the final image onto the detectors. Performances of the GMs are high demanding with a detector divided into 2 sub-bands with a different pixel size for each sub-band.
SPICA provided the next step in mid- and far-infrared astronomical research and was a candidate of ESA's fifth medium class Cosmic Vision mission. SAFARI is one of the spectroscopic instruments on board SPICA. The Focal Plane Unit (FPU) design and analysis represent a challenge both from the mechanical and thermal point of view, as the instrument is working at cryogenic temperatures between 4.8K and 0.05K. Being a large instrument, with a current best estimate of 148,7kg of mass, its design will have to be optimized to fit within the mission´s mass and volume budget. The FPU will also have to be designed for its modularity and accessibility due to the large number of subsystems that SAFARI had to accommodate, highlighting Fourier Transform Spectrometer Mechanism (FTSM) and the three grating-based point source spectrometer modules (GM) which operates at 1.7K in the FPU, the latter representing 60% of the total mass of the instrument
SAFARI is a point source spectrometer for the SPICA mission, which provides far-infrared spectroscopy and high sensitivity. 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. SAFARI is a grating-based spectrometer with two modes of operation, Low Resolution (LR), or nominal mode (R~300) and High Resolution, (HR) (R~2000-11000). The SAFARI shall provide point source spectroscopy with diffraction-limited capability in four spectral bands over 34-230μm and a field of view (FoV) on sky over 2’×2’. Due to the complexity of the optical design of the SAFARI instrument a modular design was selected. Four principal modules are defined: Calibration Module (CS), Input Optics Module (IOM), Beam and Mode Distribution (BMDO) and Grating Modules (GMs). The present work is focused in the last module. Dispersive optical systems inherently demand the need of volume allocation for the optical system, being this fact somehow proportional to the wavelength and the required resolving power. The image sampling and the size of the detector elements are key drivers in this optical modular design. The optimization process has been performed taking into account the conceptual design parameters obtained during this phase such as collimator and camera optics focal lengths, subsystem diameters and periods and AOIs of the diffraction gratings.
ELMER is an optical instrument for the GTC designed to observe between 370 and 1000 nm. The observing modes for the instrument at Day One shall be: imaging, long slit spectroscopy, slit-less multi-object spectroscopy, fast photometry, fast short-slit spectroscopy and mask multi-object spectroscopy. It will be installed at the Nasmyth-B focal station at Day One, but it has also been designed to operate at the Folded Cassegrain focal station. The physical configuration of the instrument consists of a front section where the focal plane components are mounted (cover masks and slits) and a rear section with the rest of the components (field lens, folder mirrors, collimator, shutter, filters, prisms, grisms, camera and cryostat). Both sections are connected through a hexapod type structure.
An accurate behavior model of the instrument has been developed to optimize the design of the structural parts. The geometry of the hexapod configuration has been adjusted to reduce the ratio between the lateral deflection of the rear section and its rotation in order to minimize the image motion due to the deflections of the instrument. Special effort has been devoted to the design of the drives of the four wheels, each one driven by a preloaded worm gear.
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