The Long wave (11 -16 µm) spectrograph for the EChO M3 Mission Candidate study

The results for the design study of the Long Wave Infrared Module (LWIR), a goal spectroscopic channel for the EChO ESA medium class candidate mission, are presented. The requirements for the LWIR module were to provide coverage of the 11–16 μm spectral range at a moderate resolving power of at least R = 30, whilst minimising noise contributions above photon due to the thermal background of the EChO instrument and telescope, and astrophysical sources such as the zodiacal light. The study output module design is a KRS-6 prism spectrograph with aluminium mirror beam expander and coated germanium lenses for the final focusing elements. Thermal background considerations led to enclosing the beam in a baffle cooled to approximately 25–29 K. To minimise diffuse astrophysical background contributions due to the zodiacal light, anamorphic designs were considered in addition to the elliptical input beam provided by the EChO telescope. Given the requirement that measurements in this waveband place on the performance of the infrared detector array, an additional study on the likely scientific return with lower resolving power (R < 30) is included. If specific high priority molecules on moderately warm giant planets (e.g. CO2, H2O) are targeted, the LWIR channel can still provide improvements in determining the atmospheric temperature structure and molecular abundances. Thus, the inclusion of even a coarse-resolution (R≈10) LWIR module would still make an important contribution to measurements of exoplanet atmospheres made by EChO.