Optical Tube Assemblies for the ESO VLT Four Laser Guide Star Facility
R. Henselmans, D. Nijkerk, M. Lemmen, N.Rijnveld, N. Doelman, F.Kamphues
ESO is implementing a new Adaptive Optics facility (AOF) on the Unit Telescope 4 (UT4) of the Very Large Telescope (VLT). For increased sky coverage, Four Laser Guide Star Facilities (4LGSF) will be installed. TNO is developing the Optical Tube Asssemblies (OTAs) for the 4LGSF.The OTAs are Galilean 20x beam expanders, expanding a ∅15 mm input beam to a steerable ∅300 mm output beam with a wavefront quality requirement of <50 nm rms. The allowed defocus under the influence of the changing environmental air temperature (0-15°C, -0.7°C/hr gradient) is only 0.2 waves. The thermal behaviour of the system has been analyzed by combining optical, lumped mass and FE analyses. The design is passively athermalized over a large temperature range as well as under the influence of thermal gradients. Extensive thermal and high power laser testing has shown the system performs as required. This poster describes the design and test results.