Athermal design of the optical tube assemblies for the ESO VLT Four Laser Guidestar Facility

TNO is developing the Optical Tube Asssemblies (OTAs) for the ESO VLT Four Laser Guide Star Facility. The OTAs are Galilean 20x beam expanders, expanding a O15 mm input beam to a steerable O300 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. A design that is passively athermalized over a large temperature range as well as under the influence of thermal gradients has been developed. Extensive thermal testing has shown the system performs as required. This paper describes the design and test results. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).