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Full lists of authors: Thomas, N.; Hussmann, H.; Spohn, T.; Lara, L. M.; Christensen, U.; Affolter, M.; Bandy, T.; Beck, T.; Chakraborty, S.; Geissbuehler, U.; Gerber, M.; Ghose, K.; Gouman, J.; HosseiniArani, S.; Kuske, K.; Peteut, A.; Piazza, D.; Rieder, M.; Servonet, A.; Althaus, C. Behnke, T.; Gwinner, K.; Hüttig, C.; Kallenbach, R.; Lichopoj, A.; Lingenauber, K.; Lötzke, H. -G.; Lüdicke, F.; Michaelis, H.; Oberst, J.; Schrödter, R.; Stark, A.; Steinbrügge, G.; del Togno, S.; Wickhusen, K.; Castro, J. M.; Herranz, M.; Rodrigo, J.; Perplies, H.; Weigel, T.; Schulze-Walewski, S.; Blum, S.; Casciello, A.; Rugi-Grond, E.; Coppoolse, W.; Rech, M.; Weidlich, K.; Leikert, T.; Henkelmann, R.; Trefzger, B.; Metz, B.--This is an Open Access article distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/.Open Access funding provided by Universitat Bern., The BepiColombo Laser Altimeter (BELA) is the first European laser altimeter constructed for interplanetary flight. BELA uses a 50 mJ pulsed Nd:YAG laser operating at 10 Hz with a 20 cm aperture receiver to perform the ranging. The instrument also uses a digital approach for range detection and pulse analysis. The ranging accuracy is expected to be better than 2 metres and ∼20 cm in optimum conditions. With the given, only slightly elliptical, orbit, BELA should return a consistent data set for the most if not all of the planet. The instrument is required to function in an extreme environment with the thermal issues being particularly demanding. Novel solutions have been taken to resolve these issues. BELA is described in detail and its predicted performance outlined on the basis of pre-flight testing. © The Author(s) 2021, This work was supported by the Swiss State Secretariat for Education, Research and Innovation (SERI) under the PRODEX scheme (contract number C90191). The opinions expressed and arguments employed herein do not necessarily reflect the official views of the Swiss Government. DLR would like to thank all involved collaborators in the BELA transmitter team. Financial support was provided by the DLR Space Administration on behalf of the Federal Ministry of Economics and Technology of Germany. We acknowledge the financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award SEV-2017-0709, and from the research projects ESP 2014-54062-R, ESP 2016-76076-R and PGC 2018-099425-B-I00. Open Access funding provided by Universitat Bern.

The BepiColombo Spacecraft can’t tolerate to absorb a major fraction of the off-axis sunlight through larger payload apertures. Fortunately, there are solutions to design baffles such that they reflect the incoming radiation back through the front aperture rather than absorbing it. A Design Study, sponsored by ESA and performed by Contraves Space together with SAGEM Defense Securite, has analysed the potential of various solutions and assessed the options to manufacture them. The selected configuration has been analysed in detail for the optical, mechanical and thermal performance as well as the impact on mass and power dissipation. The size of the baffle was adapted to the needs of the BepiColombo Laser Altimeter (BELA) payload.

High precision space optics, such as spectrometers, relay optics, and filters, require ultra stable, lightweight platforms. These equipped platforms have on one side to survive the launch loads, on the other side they have to maintain their stability also under the varying thermal loads occurring in space. Typically such platforms have their equipment (prisms, etalons, beam expanders, etc.) mounted by means of classical bonding, hydro-catalytic bonding or optical contacting. Therefore such an optical bench requires to provide an excellent flatness, minimal roughness and is usually made of the same material as the equipment it carries (glass, glass ceramics). For space systems, mass is a big penalty, therefore such optical platforms are in most cases light weighted by means of machining features (i.e. pockets). Besides of being not extremely mass efficient, such pockets reduce the load carrying capability of the base material significantly. The challenge for Oerlikon Space, in this context, was to develop, qualify and deliver such optical benches, providing a substantial mass reduction compared to actual light weighted systems, while maintaining most of the full load carrying capacity of the base material. Additionally such a substrate can find an attractive application for mirror substrates. The results of the first development and of the first test results will be presented.

RUAG Space, together with THALES SESO, initiated the development of light weighted sandwich structures for optical applications, already some years ago. The results of the use of this type of structure applied for optical benches and the first outlook for their use as mirror substrates were published in previous papers. This paper is going to present the results of the polishing activities performed on a 650 mm diameter mirror manufactured with Zerodur face sheets and a low density aluminum core. This substrate showed a mass density of 15 kg/m2. The excellent optical quality achieved proves the suitability of this technology for several applications, in particular for scanning mirrors for space and possibly for moveable mirror in ground based astronomical telescopes. With the emerging need for extremely high flatness under thermal loads (radius of curvature < 400km) activities have been initiated to identify materials for the core of the substrate closer matching the extremely low CTE offered by materials like Zerodur. The progresses made in this field are presented and an outlook for future activities is provided.

The BepiColombo Laser Altimeter (BELA) shall profile the surface of planet Mercury and operates on the day side as well as on the night side. Because of the high thermal loads, most interior surfaces of the front optics are highly reflective and specular, including the baffle. This puts a handicap on the straylight performance, which is needed to limit the solar background. We present the design measures used to reach an attenuation of about 10 -8 . We resume the method of backward straylight analysis which starts the rays at the detector and analyses the results in object space. The backward analysis can be quickly compiled and challenges computer resources rather than labor effort. This is very useful in a conceptual design phase when a design is iterated and trade-offs are to be performed. For one design, we compare the results with values obtained from a forward analysis.