Wavefront corrective lens for the Subaru Laser Launching Telescope

The optical quality of the LLT unit of SUBARU NAOJ telescope was improved by new athermalized supports of the optics, in order to operate at the best performance at temperatures below 0°C. The ultimate wavefront correction of the whole LLT, that expands a laser beam from 40 mm to 500 mm, was made by Ion Beam Figuring on the small 40 mm LLT entrance window, in accordance to the WFE measured in operating conditions. The correction of small optics including high spatial frequencies, resulting by the LLT expanding ratio, was possible by a special technique of IBF process developed at the Astronomical Observatory of Brera (INAF-OAB), using a concentrator of the ion beam size, able to force the broader beam emitted from an ion source into a smaller spot having large removal rate.