The image quality of ground based telescopes can be significantly improved with the use of Adaptive Optics (AO). The AO system corrects the wavefront distortions introduced by the turbulent atmosphere. The European Southern Observatory (ESO) is currently upgrading the Very Large Telescope (VLT) with a Four Laser Guide Star AO system. TNO builds the Optical Tube Assemblies (OTA) for this Four Laser Guide Star Facility (4LGSF).
AO systems require a reference source for wavefront correction. On 10 meter class telescopes, sufficiently bright stars are only available for about 1% of the astronomical targets, therefore the application of natural guide star adaptive optics is very limited. This limitation can be overcome by using an artificial laser guide star. Laser guide stars work by projecting a powerful laser beam to the upper reaches of the atmosphere. The beam excites the sodium atoms in this atmospheric layer at an altitude of 90 km, producing an artificial star in the sky. A wavefront sensor is used to detect how the atmospheric conditions affects the light from the laser guide star. A powerful computer calculates the required real time corrections, which are applied using a deformable mirror with hundreds of small actuators.
ESO has developed a compact laser guide star unit, whereby a small powerful laser is combined with a telescope that launches the beam, creating a single modular unit that can be mounted directly on a large telescope.
Four Laser Guide Stars for VLT
The 4 LGSF consists of a high power 25 W CW 589 nm laser, a Beam Conditioning and Diagnostics System (BCDS) and an Optical Tube Assembly (OTA). OTA is a 20x Galilean beam expander, with a 15 mm diameter input beam and a steerable 300 mm diameter collimated output beam. OTA employs four optical elements, a Quarter Wave Plate, a small double concave L1 lens, a Field Selector Mirror and a highly aspherical 380 diameter mm L2 lens. The design is passively athermalized over a large temperature range as well as under the influence of thermal gradients.
To achieve 4.8 arcmin radius field of view on-sky, the FSM has to tilt up to ±6.1 mrad, in combination with less than 1.5 µrad RMS absolute accuracy. The FSM design consists of a Zerodur mirror, bonded to a membrane spring and strut combination to allow only tip and tilt. Since the range is too large for piezos, two (self-locking) spindle drives actuate the mirror, using a stiffness based transmission to increase resolution. Absolute accuracy is achieved with two differential inductive sensor pairs.
The L2 has a convex aspherical side with a radius of 637.381 mm and a conic constant of -0.447, resulting in a departure from the best-fit-sphere of 320 µm. The final corrective polishing is done at TNO. The convex aspherical surface of the L2 is particularly difficult to measure. The TNO NANOMEFOS machine allows for measuring the surface form directly, making the production much more efficient. TNO designed a custom 589 nm coating and applied it using its coating facility, resulting in a measured reflectivity of less than 0.2% for both sides. The thermal behaviour of the system has been analyzed by combining optical, lumped mass and FE analyses. Extensive thermal and high power laser testing has shown the system performs as required. The thermally induced defocus is less than 90 nm under transient conditions. The transmitted wavefront of the assembled optics is less than 25 nm RMS over the entire operational range (0 – 15 °C and 0 – 60 ° tilt).
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