BOPPS Infra-Red Camera (BIRC) Overview ====================================== The BOPPS Infrared Camera (BIRC) is a multispectral infrared imager designed to operate in 8 wavelengths between 2.5 and 5.0 μm, with each spectral width being ~ 3% of the center wavelength, and the astronomical R-band near 640 nm. BIRC was designed to measure the water and CO2 emissions from comets at 2.73 and 4.3 μm, respectively, and the water-related infrared absorption feature in asteroids and the Moon from ~ 2.5 to 3.2 μm. This capability is obtained with a Teledyne H2RG cryocooled HgCdTe detector and an 80 cm telescope. The system produces an f/4 image over a field of view of 3 arcminutes, which subtends approximately 151 pixels on the 2K x 2K array, and employs shift/co-add algorithms to increase signal-to-noise for the observation of dim objects. The BIRC is comprised of a collimator subsystem and a camera. The collimator is designed to relay the beam from prime focus of the 80 cm main telescope to the camera after passing through the cryogenically cooled nine-position filter wheel. The collimator subsystem consists of an enclosed, cooled, nitrogen-purged box (the “cold box”) with a collimating mirror and three fold mirrors, all of which are coated with protected gold to reduce thermal self-emission. Light enters the cold box through a CaF2 window, and a collimated beam exits the cold box through another CaF2 window. The collimated beam passes through an evacuated, cryogenically cooled nine-position filter wheel and then enters the camera, where the light is focused on the Teledyne H2RG detector by a small Ritchey-Chretien telescope inside the evacuated and cryogenically cooled camera body. The ‘cold box’ is maintained at ~200K to reduce thermal self-emission to well below that which is contributed from the main telescope or from downwelling sky radiation. The spent liquid nitrogen is then used to purge the cold box with dry N2 so that frost does not form on the mirrors when the mission is being prepared for launch on the ground and during ascent. The filter wheel is cooled by liquid nitrogen to 150K or cooler, while the small Ritchey-Chretien telescope inside the camera is cooled by a mechanical cryocooler to ~100K. This cryocooler also maintains the detector at 70K or lower. Custom firmware provided by Teledyne Imaging Systems allows the BIRC flight software to readout a programmable area of interest, which was then defined to be the central 320 x 200 pixel region that contains the 3 arcmin field of view and additional pixels for dark calibration. It is this subframe that is generated by the BIRC instrument for all the image data. The average plate scale of the detector is 1.1572 arcsec/pixel with a standard deviation of 0.062205 arcsec/pixel.