An environmentally sealed, densely packaged optical assembly utilizing an all-aluminum alloy primary optical train has been developed as part of the Infrared Calibrated Airborne Spatial Measurement System (IR CASMS) for deployment in the retractable turret of a C-130 aircraft. The primary optical train, consisting of a Kennedy scanner with a rotating pentagon prism, a Ritchey-Chretien telescope, and a clamshell-type relay assembly, is aligned on a common optical bench that is hard-mounted to the aircraft turret using kinematic mounting techniques. The single-point diamond-machined mirrors and mounts, utilizing a bolt-together design to permit mirror maintenance and eliminate potting instabilities, make use of integrally machined mounting pads and datums that permit alignment by means of an autocollimating alignment telescope. Remotely actuated miniature precision mechanisms for focusing the telescope, changing optical bandpass filters, and presenting field-filling blackbody energy to the 120 element HgCdTe detector array have been designed for optimum packaging efficiency. Two calibrated blackbodies, one thermoelectrically cooled, the other employing an etched-foil heater, are isolated from the desiccated optical environment in a sealed, heat-rejecting plenum and are coupled to the primary optical train by means of zinc selenide relay lenses. This paper presents a functional description of the IR CASMS optical assembly and discusses the construction and alignment details of the optical train, blackbodies, mechanisms, and environmental enclosure.