TNO has developed the Optical Tube Assemblies (OTAs) for the ESO VLT Four Laser Guide Star Facility.
The OTAs are Galilean 20x beam expanders, expanding a ∅15 mm input beam (25W, 589 nm CW) to a
steerable ∅300 mm output beam. TNO has recently successfully completed acceptance testing of the four
units, showing compliance to the challenging requirements on output wavefront quality, thermally induced
defocus under operational conditions, absolute pointing accuracy and polarization extinction ratio (PER).
TNO applied its corrective polishing in combination with the NANOMEFOS measurement machine to
produce the ∅380 mm aspherical output lens, resulting in 20 nm rms output wavefront quality. 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 a thermally induced defocus of less than 0.15 waves
under the operational conditions of 0-15°C and upto -0.7°C/hr gradient. A custom tip-tilt mechanism was
designed to steer the output beam over a 4.8 arcmin radius, with less than 0.1" (3σ) accuracy at 1 Hz update
rate. The PER was also measured under operational (thermal and tilt) conditions and demonstrated to be well
above 99%. This paper describes the design, modelling and analysis, and the test results of these instruments.