A silicon and KRS-5 grism suite for FORCAST on SOFIA

We have designed and fabricated a suite of grisms for use in FORCAST, a mid-infrared camera scheduled as a first-light instrument on SOFIA. The grism suite gives SOFIA a new capability: low and moderate resolution spectroscopy from 5μm to 37μm, without the addition of a new instrument. One feature of the optical design is that it includes a mode using pairs of cross-dispersed grisms, providing continuous wavelength coverage over a broad range at higher resolving power. We fabricated four silicon (n = 3.44) grisms using photolithographic techniques and purchased two additional mechanically ruled KRS-5 (n = 2.3) grisms. One pair of silicon grisms permits observations of the 5 - 8μm band with a long slit at R~ 200 or, in a cross-dispersed mode, at resolving powers up to 1500. In the 8 - 14μm region, where silicon absorbs heavily, the KRS-5 grisms produce resolving powers of 300 and 800 in long-slit and cross-dispersed mode, respectively. The remaining two silicon grisms cover 17 - 37μm at resolving powers of 140 and 250. We have thoroughly tested the silicon grisms in the laboratory, measuring efficiencies in transmission at 1.4 - 1.8μm. We report on these measurements as well as on cryogenic performance tests of the silicon and KRS-5 devices after installation in FORCAST.