Opto-mechanical design and realization of a far-infrared grating module for future space missions

Bridging the gap between JWST and ALMA, the far-infrared wavelength range between 30 and 300 micron, contains a wealth of spectral features enabling deep studies of galaxy evolution and planet forming systems. One of the key diagnostic tools used by far-IR astronomers is spectroscopy, employing low resolution (R~100) grating modules to achieve the highest possible spectroscopic sensitivity and mapping speed. The next generation of space missions in this field, as exemplified by the Origins Space Telescope (OST), and recently proposed concepts like the PRIMA, FIRSST and SALTUS, will utilize grating spectrometers combined with ultra-sensitive large-format KID detector arrays. Such grating modules will enable a variety of instrument architectures offering powerful observing capabilities including post-dispersed Fourier Transformation Spectrometer (FTS), long-slit spectroscopic mapping, and high-resolution Fabry-Perot or a Virtually Imaged Phased Array (VIPA) based spectroscopy, where the grating is used for order-sorting. To fully exploit the astronomical potential these instruments require compact, cryogenic and wideband grating spectrometers with a large telecentric field of view. <br/> <br/> In this paper we present the opto-mechanical design and realization of a multi-purpose Grating Module Breadboard (GMBB), which supports arbitrary one-octave bandwidth diffraction gratings in the 25-400 micron wavelength band. The purpose of realizing this GMBB is to aid experimental verification of blazed grating developments, and to verify the optical interfaces and spectral characteristics of KID detector arrays. <br/> <br/> The driving concepts, methodologies, engineering solutions and finally the realization are discussed and supported by optical verification results. A simple and modular configuration containing a collimator unit and camera bay optics allows for easy adaptation to different wavelength/dispersion combinations by exchanging the grating and/or detector array. The opto-mechanical design is monolithic with highly accurate and reproducible kinematic optical mounts, while allowing mirror realization with the highest optical performance. Special attention is given to the development of grating production methods for high-efficiency blazed gratings optimized for specific wavelength bands.