First flight model optical performance test on the Flexible Combined Imager telescope assembly of Meteosat Third Generation

Abstract

The Flexible Combined Imager (FCI), main instrument of the Meteosat Third Generation Imager (MTG-I) satellite, will image the on-Earth instrument swath with high image quality and highly stable line of sight. It is composed by a Three Mirror Anastigmatic (TMA) telescope assembly, followed by a Spectral Separation and Detection Assembly (SSDA) which has the function to separate the different spectral channels and focus them onto the detector. The Telescope assembly, developed by OHB System AG in Oberpfaffenhofen, will be the subject of this article, presenting first its optical design concept and its main characteristics. Thereafter, the FCI Telescope Assembly (FCI-TA) alignment will be presented, followed by the full test campaign accomplished to characterize the full optical performance of the FCI-TA. The telescope optical design is based on an off-axis field TMA telescope. It is composed by three aspheric off-axis mirrors, having a focal length of 1650 mm, an F/# of 5.5 and an entrance pupil diameter of 302 mm. A fourth, folding, mirror deflects the beam onto the TA focal plane, while in front of the first TMA mirror there is a Scan mirror, in order to let the telescope scan the whole Earth disc. In mid-2019 OHB ended the alignment of this first flight model of FCI-TA. We will show in this article the good results in terms of optical performance obtained from that alignment with a worst WFE of 63 nm and a LOS error of 7.8 arcsec.