Ultra high spectral resolution observations of planetary atmospheres using the Cologne tuneable heterodyne infrared spectrometer

High-resolution spectroscopy is a versatile tool to study planetary atmospheres. In the mid infrared wavelength regime the highest possible spectral resolution is provided by applying heterodyne techniques. At spectral resolution of more than 105 observations of fully resolved molecular features are possible allowing retrieval of many physical parameters from single lines. In addition, due to the fact that many of the observed species are abundant also in the Earth's atmosphere, high-resolution measurements allow to peak through the telluric features and lead to less ambiguity than low-resolution data. The paper will present a detailed description of the Cologne-based receiver THIS, the only tuneable heterodyne infrared spectrometer for application to astronomy offering access to the 7-17μm wavelength region at a resolution of up to 3×107 and a bandwidth of 3 GHz. An overview of the science addressable with THIS will be given as well as a brief introduction to the theory of heterodyning in the infrared including a comparison to direct detection instrumentation.