Experimental verification of the virtual wavefront sensor concept

Ground-based Extremely Large Telescopes with aperture diameters of 30-60 m will reach their ultimate imaging capabilities by means of Adaptive Optics (AO) systems. One of the fundamental limitations of AO systems working with Natural Guide Stars (NGSs) is very low sky coverage, especially in the visible. The use of laser guide stars (LGSs) created at a finite distance above the telescope aperture could, in principle, relax this limitation, but owing to proximity of the LGSs, a problem of their imaging through the telescope optics arises. To resolve the LGS re-imaging problem, one may adopt the virtual wavefront sensing concept, which employs two wavefront sensors (WFSs): the primary WFS located in the first available telescope focus for measuring wavefront errors induced by atmospheric turbulence and, possibly by the first deformable mirror (DM), and a test-source WFS located at the final telescope focus for measuring contributions from additional DMs. To verify the virtual WFS concept, an optical system has been designed for an experimental setup. It contains three artificial reference sources; a beam splitter forming reference-source arm and test-source arm, atmospheric module with three phase screens followed by a scaled model of a 10-m telescope with two DMs positioned in two separate arms and conjugated to different heights. A single WFS module combining images from the two arms plays a role of the primary and test WFSs. An acquisition camera is employed to monitor image correction made with two DMs. The foundation of the virtual WFS concept is described and its two approaches are outlined in relation to the optical design of the setup. The validity of the experimental verification under the simplified conditions is discussed together with further work addressing the critical issues of the concept, which have not been covered in the present experiment.