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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 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.