This paper describes the current optical and mechanical designs of NFIRAOS (Narrow Field InfraRed Adaptive Optics System, pronounced nefarious). The main subsystems are the science path optics, the laser guide star (LGS) wavefront sensors (WFSs), the visible natural guide star (NGS) truth WFSs, the IR acquisition camera, and a source and calibration unit. The science optics deliver a diffraction limited f/15 beam with a two arcminute field of view (FOV) to one of three instruments mounted to NFIRAOS. The LGS system relies on an asterism of five laser guide stars oriented in a 35 arcsecond radius pentagon with a sixth guide star at the center. The LGS optics are comprised of six separate optical trains that feed individual WFSs. Each optical train includes three zoom mechanisms catering to sodium layer height variations of 85-235 km. The visible WFS system includes an atmospheric dispersion corrector (ADC); the NGS WFS, used only for NGS mode; the moderate order radial (MOR) truth WFS, used for fast tracking of radially symmetric aberrations while in LGS mode; and the high-order low-bandwidth (HOL) truth WFS, used for sensing high-order LGS WFS offsets. The majority of NFIRAOS is cooled to -30 C to reduce background emissivity. Within the thermal enclosure are standard optical benches which are semi-kinematically mounted to a sub-structure, which is in turn connected via bipod flexures to the external NFIFAOS structure. This protects the optics benches from thermal distortion while maintaining alignment to instruments and TMT.
