First light of the OVLA active mirror with its surface heating system

摘要

The Optical Very Large Array (OVLA) project consists in a kilometric-size optical interferometer of 27 mobile 1.5m telescopes designed to provide high-resolution IR and visible snap-shot images. An OVLA prototype telescope has been developed at the Observatoire de Haute-Provence. It features a 1.5m meniscus-shaped f/1.7 primary mirror weighting 200kg including its active cell with 32 actuators. The mirror blank made of 24mm-thick ordinary window glass is very cheap but extremely sensitive to temperature variations because of its large CTE (3 times larger than Pyrex). Indeed, the mirror shows a Zl 1=3150 run rms wavefront error due to a 0.5℃ thermal gradient generated between its front and back side by an unbalanced heat dissipation towards the night sky and the ground. This spherical aberration, too large to be corrected by the actuators, is compensated by an uniform electrical current generated through the aluminum coating by 42 peripheral electrodes. We also describe the electrodes control hardware and present some results obtained during the first light of the telescope. Lastly, we propose a possible upgraded surface heating system to adjust thermally other optical aberrations.