Development of a light-weight beryllium Cassegrain telescope: from the optical design to the performance measurement

The BepiColombo Laser Altimeter (BELA) is selected to fly on board of the ESA's BepiColombo Mercury Planetary Orbiter (MPO). The instrument will be the first European planetary laser altimeter system. RUAG Space is the industrial prime for the Receiver part of the scientific instrument. The BELA Receiver is a joined effort of Swiss industries under the leading role of RUAG and University of Bern as co-Prime. A core element is the light weighted Receiver Telescope (RTL), to collect the laser pulse reflected from the planet’s surface. An innovative design was required to deal with the very challenging Mercury’s environmental conditions and with the very stringent instrument’s mass budget. The Optothermo- mechanical analyses lead to the design of a 1250mm focal length Cassegrain telescope made of Beryllium. It provides an aperture of 204 mm diameter and a 2 mm thick primary mirror for a total mass of less than 600gr. The manufacturing and the integration needed special developments. This paper presents the design analyses and the major challenges which had to be solved. Discussing some aspects of the telescope integration and test campaign, the finally achieved performances and lessons learnt will be presented.