Conceptual design of a compact space-borne IR high-resolution cross-dispersed spectrograph realized by germanium immersion grating and all-cordierite reflective optical system

We present a new concept of an unprecedentedly compact, 10 cm-sized (optics), IR high-resolution (λ~3μm, R~30,000) cross-dispersed spectrograph. The spectrograph has high-throughput and can cover an octave of wavelength in a single exposure without mechanisms and electric power, thus it is not only highly efficient but also stable and low power rather than existing IR high-resolution spectrometers for space application. The key technologies that make this possible are germanium (n~4) immersion grating with high diffraction efficiency and all-cordierite reflective optical system with diffraction-limited performances at 633 nm. In previous studies, we have successfully demonstrated that both systems work properly at cryogenic temperature. An immersion grating enables us to realize n-times smaller spectrographs than that with a classical reflective grating, and also it can offer the quasi-Littrow configuration without the white pupil. Cordierite is a low CTE ceramics and has higher specific stiffness and thermal conductivity than low CTE glasses. An optical system made entirely of this material enables us to utilize all the advantages of the reflective optical system (wide wavelength coverage, high throughput, correction of high-order aberrations, compact optical design, and constant optical path independent of environment conditions) without bothering about fabrication of mirrors of high surface accuracy and complex optical alignment demanded for the conventional reflective optical system. This CubeSat-class spectrograph could expand the opportunity of IR high-resolution remote-sensing spectroscopy of the Jupiter’s and Saturn’s moons for exploring molecules possibly related to prebiotic chemistry.