Manufacturing by deep x-ray lithography of pyramid wavefront sensors for astronomical adaptive optics

In Astronomy the goal of the Adaptive Optics systems is the real time correction of the aberrations introduced from the turbolence of the air in the wavefront of the observed field of view. A wavefront sensor with pyramidic shape has being developed by a group of Italian researchers that offers the advantage of either variable gain against the wavefront deformation and tunable sampling of the telescope pupil. Single pyramid prototypes were made using the classical figuring and polishing techniques. This approach however, is not only very time consuming but also does not guarantee a uniform repeatability of the optical characteristics of the pyramids requested by this application (the implementation of a high number of these devices is needed for multi-conjugated adaptive optics). We therefore are investigating a manufacturing process for the pyramidal optical components based on the Deep X-ray Lithography (DXRL) technology, using X-ray synchrotron radiation, and exploiting a lithography dedicated bean-dine already operating at the ELETTRA Synchrotron in Trieste. This method foresees the irradiation of a transparent and amorphous plastic polymer like PMMA (Plexiglass) with a collimated high intensity X-ray beam. The radiation modifies the internal structure of the irradiated polymer that, once immersed into an appropriate chemical bath, permits its dissolution. In this way, by means of a mask and a series of exposures, it is possible to form a four-faces pyramid. After some preliminary tests,. a mechanical system has been built to manufacture the plastic pyramids with tight tolerances. The pyramids manufactured with DXRL can be seen as the final product to be implemented in an AO system. To manufacture large numbers of this components (mass production) we are investigating their molding starting from a pyramid template made with DXRL. The manufactured mold, made by electrodeposition of a metal on the template pyramid, could then be used for the production of pyramids by Injection molding or hot embossing.