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Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

The Instrument

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Image of the optical receiver mounted at the NASA Infrared
Telescope Facility on Mauna Kea/Hawaii in 2008.

THIS - A brief description

The Tuneable Heterodyne Infrared Spectrometer THIS is the first tuneable, transportable and competitive heterodyne receiver designed for atmospheric and astrophysical observations in the mid infrared spectral region (7-17 µm) providing an ultra high resolution of R=107 and a bandwidth of 3 GHz double sideband. With a weight of approx. 80 kg and dimensions of 45x60x60 cm3 it can easily be used at many telescopes.

In 2010, a second heterodyne receiver called iCHIPS (Infrared Compact Heterodyne Instrument for Planetary Science) has been commissioned. The physical dimensions of iCHIPS are roughly half of THIS thus providing even more flexibility. Its main application will be for observations of the Earth' atmosphere as well as laboratory work.


Schematic of the instrument THIS


Heterodyning, in principle, means superimposing a signal from the sky with a signal coming from a local oscillator (LO) emitting at a wavelength close to the one from the source. When detected by a non-linear mixer/detector the output of the mixer is a signal at the difference frequency (GHz) of both signals still containing the whole spectral information from the source.



The transmittance of the Earth's atmosphere is poor in the mid IR especially between 9 and 10 microns, mainly due to ozone absorption. Infrared heterodyne spectroscopy fully resolves individual absorption features allowing to peek through atmospheric windows providing the only ground-based direct access to features in planetary atmospheres (when they are Doppler shifted away from their telluric counterparts toward regions of higher atmospheric transmittance) and relevant astrophysical regions.

The components of the instrument

The main components of the receiver are the

  • local oscillator: quantum cascade lasers are used in THIS

  • the beam combiner: to optimize the transmission/reflection ratio we use a confocal Fabry-Perot diplexer

  • the detector: a wide bandwidth mercury-cadmium-telluride detector with a quantum efficiency of up to 80% is used for signal detection

  • spectral analysis: an in-house build acousto-optical spectrometer with a resolution of ~1MHz and a bandwidth of 3 GHz is used for the analysis of the RF signal

  • sky pointing: accurate positioning of the instrument field-of-view is guruanteed by using an optical guide camera

Fields of interest:

  • analysis and monitoring of trace gases in atmospheres of planets and moons (Venus, Earth, Mars, Saturn, Jupiter, Titan, ...)

  • dynamics in planetary atmospheres (Mars, Venus, Earth)

  • solar and stellar features (solar oscillations, magnetic fields, molecules in sunspots/starspots)

  • dynamics in circumstellar envelopes

  • the interstellar medium

  • protoplanetary discs etc.