A major milestone has been reached in the project to transform Unit Telescope 4 (Yepun) of ESO’s Very Large Telescope (VLT) into a fully adaptive telescope, with the installation of a deformable secondary mirror on the telescope.
Adaptive Optics systems compensate for the blurring effect of the Earth’s atmosphere that degrades the sharpness of images that can be obtained with a telescope — even a very large and sophisticated one like the VLT. The installation of the new Adaptive Optics Facility (AOF) as an integral part of UT4 will enable the telescope to obtain even sharper images.
The deformable secondary mirror consists of a 22 mm-thick flexible mirror sitting on a mount that holds 1170 actuators which change the shape of the mirror to correct for the effect of turbulence in the atmosphere. At just over one metre in diameter, this is the largest adaptive optics mirror ever produced. The deformable mirror has demanded cutting-edge technology, and its development journey began several years ago. Construction of the “reference optic” — essentially the mirror support — was completed in 2010, and the first version of the thin mirror itself was delivered to ESO’s Headquarters in Garching at the end of 2011, followed by an improved version in 2014. After extensive performance testing In Garching, the deformable secondary mirror assembly was shipped to Chile and has now been mounted on the 8.2-metre diameter Yepun.
Optical alignment and other tests have recently been performed with the deformable mirror — set in rigid mode — mounted on the telescope, and it has passed with flying colours.
In addition to replacing the telescope’s conventional secondary mirror with this new deformable mirror, the AOF system also includes the Four Laser Guide Star Facility (4LGSF), which saw first light in April 2016, and the installation of adaptive optics (AO) modules at the various foci of the secondary mirror. The AO modules are GRAAL (GRound laser Adaptive optics Assisted by Lasers), that feeds corrected images into HAWK-I, and GALACSI (Ground Atmospheric Layer Adaptive Corrector for Spectroscopic Imaging), which pre-filters images for the MUSE spectrograph.
Other tools to optimise the operation of the AOF have been developed and are now operational. These include the Astronomical Site Monitor software that monitors the atmosphere to determine the altitude at which the turbulence is occurring, and the Laser Traffic Control system that prevents the 4LGSF beams from interfering with aeroplanes or telescopes in space.
With the advent of the AOF system, the VLT will continue to extend its list of world-class scientific results and experience with the system at Paranal will contribute to overcoming the technical challenges presented by ESO’s next major project, building the European Extremely Large Telescope.