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In order to measure atmospheric concentrations of carbon monoxide, methane, water and carbon dioxide from spaceborne
platforms, Short-Wave Infrared (SWIR) immersed grating spectrometers are employed. Due to the need to
minimise detector dark current and internal black body radiation from the spectrometer’s own structure, these
instruments are operated at cryogenic temperatures. ESA’s Sentinel 5-Precursor is a small satellite science mission; the
platform comprises the Tropospheric Monitoring Instrument (TROPOMI), which includes a SWIR module.
Optical mounts have been developed for the SWIR module which meet the requirements to cope with the differences in
thermal expansion between the optical elements and their structural mounts over cryogenic temperature ranges, be robust
against the mechanical environment during launch, and maintain optical alignment stability with a tight volume
constraint.
Throughout the design of the SWIR spectrometer, flexures were deployed to control deformations due to thermal
expansion, the design of interfaces between materials of differing coefficient of thermal expansion was carefully
managed, and the geometry of adhesive pads was tightly controlled. Optical mounting concepts were evaluated using
finite element analysis (FEA). A breadboard programme was undertaken to verify these concepts. FEA and breadboard
results were correlated to provide confidence in the design.
The breadboard programme consisted of thermal cycling and pull-testing of adhesive joints, as well as environmental
and optical testing of representative subsystems.
Analysis and breadboarding demonstrated that the optical mounting design will survive the mechanical and thermal
environments, and verified the stability of the optical alignment requirements.
Novel optical mounting structures have been designed, analysed, assembled, tested and integrated into the optical
assemblies of the TROPOMI SWIR spectrometer, creating a compact and robust state of the art instrument. These
concepts are applicable to instruments for astronomical missions aiming to characterise exoplanets, as well as Earth
observation missions.
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