A heterodyne receiver designed for detection of stellar mid-infrared signals has been constructed as follows. Collection of light from an astronomical source is performed with a 1.65m aperture telescope of effective focal length f/89. A few centimeters beyond the focal point, the telescope beam is split into two parts by a dichroic beamsplitter which sends mid-infrared (N band) radiation on to the heterodyne detection system, and near-infrared (K band) radiation on to a camera for guiding and tip/tilt wavefront correction. Because the atmospheric index of refraction does not change significantly between the K and N bands, wavefront correction in the near-infrared is adequate to stabilize the mid-infrared signal. The mid-infrared stellar beam reflected off of the dichroic beamsplitter is then sent to a beamsplitter where it is combined with light from a CO2 laser (which has been properly shaped by a series of lenses in order to ensure the laser and telescope beams are of the same f-number). The combined beams propagate to a mirror which reflects the beam into a signal detection dewar through an antireflection (AR) coated f/1.5 aspheric ZnSe lens and an AR coated ZnSe window tilted at 5° from normal incidence to the optical axis in order to prevent backreflections.
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