Design, manufacturing, alignment, and testing of the Back Telescope assembly of the MeteoSat Third Generation InfraRed Sounder (MTG-IRS)

The Back Telescope Assembly (BTA) is one of the optical sub-systems composing the IRS instrument. It consists in an afocal off-axis four-mirror telescope whose primary function is to transfer the optical beam between the interferometer and cold detector units. The design, manufacturing and AIT activities are performed under the responsibility of AMOS (Liege, Belgium) with OHB System (Wessling, Germany) as the prime contractor of the IRS instrument. In order to meet the challenging performance requirements, an innovative telescope design with four-mirror anastigmat optical configuration has been developed, using a full aluminum solution, i.e. mirrors and structure. It is manufactured and aligned under ambient conditions while it is operated close to 0°C in a non-uniform thermal environment. The aluminum structure of the BTA supports the 4 mirrors and is interfaced via 6 Titanium needles to the CFRP optical bench of the IRS instrument. The aluminum mirrors and the BTA structure are realized by the single-point diamond turning technique in AMOS manufacturing facilities. In view of its qualification, the BTA models underwent an extensive testing campaign including: thermal-vacuum cycles, mechanical loads testing and model correlation, thermal balance and model correlation, measurement of the optical performances (wavefront error, magnification, optical axis orientation including its stability and pupil position) under ambient and operational conditions. The results of the tests allow to demonstrate the compliance of the BTA to the requirements.