Grism performance for mid-IR (5 - 40 micron) spectroscopy

This paper addresses the performance of a suite of grisms as part of an Astrobiology Science and Instrument Development (ASTID) Program to implement a moderate resolution spectroscopic capability in the mid/far-IR facility instrument FORCAST for the Stratospheric Observatory For Infrared Astronomy (SOFIA). A moderate resolution mid-IR spectrometer on SOFIA will offer advantages not available to either ground or space-based instruments after the Spitzer Space Telescope ceases operation in ~2008. SOFIA will begin operations in 2008 and will have an operational lifetime of ~20 years. From aircraft altitudes, it will be possible to cover a wide range of wavelengths, particularly in the critical 5-9 micron band, where detection of astrobiologically interesting molecules have key spectral signatures that are not accessible from the ground The FORCAST grism suite consists of six grisms: four monolithic Si grisms and two KRS-5 grisms. These devices will allow long-slit low-resolution (R = 100-300) and short-slit, cross-dispersed high-resolution spectroscopic modes (R = 800-1200) over select wavelengths in the 5-40 μm spectral range and enable observing programs to gather both images and spectra in a single SOFIA flight. The silicon grisms demonstrate a new family of dispersive elements with good optical performance for spectroscopy from 1.2-8 μm and beyond 18 μm. After SOFIA flies, the grism modes in FORCAST will complement other first generation instruments on SOFIA and provide follow-up capability of bright sources observed with Infrared Spectrograph (IRS) on Spitzer. This paper highlights the design of the grism suite for FORCAST and the current laboratory cryogenic performance of the silicon grisms.