GREAT, the German REceiver for Astronomy at Terahertz frequencies, is a first generation SOFIA dual channel heterodyne PI−instrument for high resolution spectroscopy. The system is developed by a consortium of German research institutes. The receiver will allow simultaneous observations in two out of the following three far−infrared frequency bands: * a low−frequency (1.4−1.9 THz) channel for e.g. the fine-structure lines of ionized nitrogen [NII] at 205μm and ionized carbon [CII] at 158μm; * a mid−frequency (2.4−2.7 THz) channel for e.g. the 112μm transition of HD; and * a high−frequency (4.7 THz channel) for the 63 μm fine−structure line of neutral atomic oxygen. Hot electron bolometers (HEB) mixers provide state of the art sensitivity. A spectral resolving power of up to 108 is achieved with chirp transform spectrometers, and a total bandwidth of 4 GHz at 1 MHz resolution is reached with wide band acousto-optical spectrometers. The modular concept of GREAT allows to observe with any combination of two out of the three channels aboard SOFIA. A more complete frequency coverage of the THz regime by adding additional GREAT channels is possible in the future. The adaptation of new LO−, mixer− or backend−techniques is easily possible. We describe details of the receiver and the results of first performance tests of the system at 1.9 THz. As an outlook to future developments we show first results obtained with phase locking a quantum cascade laser, the most promising option for future high power local oscillators in the Terahertz regime.
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