FORUM payload study for phase A/B1 by a consortium led by OHB System AG and TAS-UK

In 2019, the FORUM mission (Far-infrared Outgoing Radiation Understanding and Monitoring) has been selected by ESA as Earth Explorer 9 mission. FORUM aims to record the emission spectrum of the Earth's stratosphere and troposphere in the spectral range of 100 to 1600 cm-1 (i.e. 6.25mm to 100 mm). It will measure the spectral features of the far-infrared contribution to the Earth radiation budget with the focus on the water vapour contribution, the cirrus cloud properties, and the ice/snow surface emissivity. FORUM's primary optical instrument is a non-imaging Fourier Transform Spectrometer (FTS) scanning the Earth atmosphere in a step-and-stare acquisition mode sounding a 15 km ground sampling with a ground sampling distance of 100 km. A consortium led by TAS-UK as mission and platform prime and OHB as instrument prime are currently developing the satellite and payload together with Leonardo Italy and MICOS during an ESA phase A / B1 study. This consortium is one of the 2 independent consortia selected for FORUM's phase A/B1. The paper will describe the design concept of the FORUM FTS instrument with main focus on the optical design. The key optical subsystems are the pointing unit and a double pendulum interferometer with an entrance three-mirror-anastigmatic telescope. The prediction of the performance with respect to threshold and goal requirements will be presented. Furthermore, the status of the beam splitter pre-development will be reported, in particular the development of an optical element based on a CVD-diamond substrate and a technology to manufacture broad band anti-reflective microstructures on a diamond surface. The microstructures are designed to suppress parasitic reflections in the spectral range between 6.25mm and 100mm. The highest efficiency occurs between 13 and 30mm, where the Fresnel reflection is reduced from 17% down to below 3.5%.