Development and manufacturing of the receiver telescope for the Ganymede Laser Altimeter (GALA)

The GALA (Ganymede Laser Altimeter) is one of eleven scientific instruments of the ESA mission JUICE (Jupiter Icy Moons Explorer) with the goal of exploring the icy moons of Jupiter, with a special interest in Ganymede. By its atmosphere, magnetic field, and water abundance, Ganymede is similar to Earth [1]. GALA is a laser altimeter that generates a surface profile with a resolution of < 15 cm based on an emitted laser pulse that is reflected by the surface of the moon 500 km away [2]. The mechanical development of the receiver telescope with an extremely thin-walled primary mirror (thickness 4-8 mm; diameter similar to 300 mm) was driven by tough boundary conditions. These are a small envelope and mass budget with high mechanical loads, such as a quasi-static acceleration of 120 g during rocket launch and a temperature range from -50 degrees C up to 150 degrees C, at the same time. The athermal design is based on the use of a silicon particle reinforced aluminum compound (AlSi40) and an amorphous nickel-phosphorous plating to allow various shape correction and polishing processes. Another challenge was the high radiation load of 10(12) protons / cm(2) @ 10 MeV. Fraunhofer IOF developed and qualified a gold HR coating based on nanolaminate with R > 98% @ 1064 nm and high resistance. Thus, almost all process steps from development through manufacturing to integration and characterization could be carried out at Fraunhofer IOF. With a shape deviation of 27 nm RMS of the primary mirror and 8 nm RMS of the secondary mirror, a system performance of 90% encircled energy could be achieved with a pupil radius of 38 mu m. The telescope was handed over to HENSOLDT in spring 2020 and will start its eight-year journey to Jupiter in 2023 [1, 2].