Stable mounting of beamsplitters for an interferometer

The Basic Angle Monitoring (BAM) system for satellite GAIA (2012–2018) will measure variation on the angle between the lines-of-sight between two telescopes with 2.5 prad uncertainty. It is a laser-interferometer system consisting of two optical benches with a number of mirrors and beamsplitters. The optical components need to be stable with respect to each other within 0.17 pm in position and 60 nrad in angle during measurements over a period of 6 h with 0.1 mK thermal stability. This paper aims at finding the most suitable mounting plane of the fused silica beamsplitters mounted onto the silicon carbide optical bench in the BAM system. These beamsplitters must be clamped mechanically. Based on a force stability analysis, mounting in the plane of light is a more stable solution than mounting on the reflective surface. However, when making a conceptual design the difficulty is making a design which has sufficient alignment stability to survive launch vibrations and a cool-down trajectory is more difficult.