The Cologne spectrometer THIS (Tuneable Heterodyne Infrared Spectrometer) opens the mid-infrared wavelength region from 5 to 17 µm for ultra-high-resolution spectroscopy. With a current bandwidth of 14 km/sec and a frequency resolution of R=2*10<sup>7</sup> it is the only widely tuneable and transportable infrared heterodyne receiver. A Quantum-Cascade laser is used as local oscillator (LO). To provide optimum beam combination a Fabry-Perot ringresonator is used to superimpose the LO and the radiation. Frequency mixing is done by a Mercury-Cadmium-Telluride photomixer and spectral analysis is performed by an Acousto-Optical spectrometer. The system noise temperature is about three times the quantum limit giving THIS a sensitivity equivalent to CO<sub>2</sub>-laser based heterodyne systems. Various measurements at different ground based telescopes including the analysis of trace gases in Earth's atmosphere, observations of molecular features in sunspots, and detection of non-LTE CO2 emission from the atmosphere of Venus' have been performed and demonstrate the instrument's capabilities for astronomical observations at ground based telescopes and the stratospheric observatory SOFIA in the near future. Possible targets for future observations with THIS will be discussed.
We present the newly developed transportable setup of the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS) designed for astronomical observations aboard the Stratospheric Observatory For Infrared Astronomy (SOFIA). With THIS a competitive tuneable heterodyne spectrometer for the mid-infrared is available that will allow measurements in a wide field of astronomical applications. Frequency tuneability over a wide range provided by the use of semiconductor lasers as local oscillators (LO) allows a variety of molecules in the mid infrared to be observed under very high frequency resolution. With the use of newly developed quantum-cascade lasers (QCL) a sensitivity close to the quantum limit will be in reach.