A LWIR, contrast-mode, spectral radiometer has been installed on the 1.6 meter tele-scope of the DARPA Maui Optical Station (AMOS) Observatory located on Mt. Haleakala, Maui, Hawaii. The instrument covers the spectral range from 3 to 23 micrometers using a pair of Si:As detectors which are cooled to 10°K with a helium based closed cycle refrigerator. Two rotating Circular Variable Filters cover the 3 to 5 and 8 to 14 micrometer atmospheric windows with a 2% spectral resolution. Other wavelengths are covered with fixed filters. Details of the optical and dewar design are presented.