The ESA study carried out by the Concurrent Design Facility at ESTEC provides a reference design for the EChO telescope and spectrometer. This reference design will be further studied by ESA, while Industry and the Instrument Consortia will study and propose alternative designs in coming months.
The current reference telescope design is based on a simple Cassegrain telescope, with a 20"x20" field of view that is diffraction-limited at visible wavelengths. The primary mirror has an entrance pupil diameter of 1.26 metres (effective focal length of ~ 10.7 metres) providing a collecting area of ~1.1 square metres after allowing for obscuration by the secondary mirror support structure and the central aperture. The telescope will be passively cooled to below 50 Kelvin in order to minimize the thermal background.
ECHO will cover the 0.4 – 11 micron waveband (goal to extend to 16 microns) with continuous and simultaneous spectral coverage. This is unique among current and proposed facilities for transit spectroscopy studies.
This broad spectral coverage gives access to an extensive range of spectral features from many key molecules that can be used to study the atmospheres of exoplanets covering a range of masses and physical temperatures. Crucially, with coverage in the optical as well as in the near- and thermal-infrared it will be possible to distinguish between small variations in the combined signal of the host star and the exoplanet and those due to variations in the output of the host star.
The waveband will be split into a number of distinct channels using a series of beam splitters, with the goal to achieve a sensitivity that is limited only by astronomical noise (i.e. photon noise from the host star and zodiacal light) in each of the channels using appropriate detector technology. A resolving power of R~300 or better will be required at wavelengths below 5 microns and R~30 (goal R~300) for wavelengths above.
The spectrometer, along with its detectors, will be cooled to below 50 Kelvin in order to achieve photon-noise limited sensitivities. The required detector temperatures will strongly depend on the choice of detector and thus on the particular wavelength channel.