Performance Of Large Chemically Etched Silicon Grisms For Infrared Spectroscopy - Art. No. 62695r

FORCAST is a mid/far-IR camera for use on NASA's SOFIA airborne observatory. We are fabricating monolithic silicon grisms to retrofit a spectroscopic capability for this facility-class instrument without affecting the imaging optics. The grisms will operate in the 5-8, 17-28, and 28-37 mu m wavelength ranges. We will cover the 5-8 mu m range in one exposure at a resolving power R=1200 with a 2 arcsecond slit using two grisms with one serving as a cross-disperser. For the 17-28 and 28-37 mu m ranges, the resolving powers are R similar to 140, 250 when used in low order with a slit of 3 arcseconds. We illustrate aspects of fabrication and testing during the grism development, and summarize the performance of the gratings at near- and mid-IR wavelengths. These gratings rely on procedures that can be used for modest sized (similar to 10 cm) silicon pieces, thereby providing dispersive elements with good optical performance and large slit width-resolving power products from 1.2-8.1 mu m and beyond 17 mu m.