The MCAO systems within LINC-NIRVANA: control aspects beyond wavefront correction

LINC-NIRVANA is the near-infrared homothetic imaging camera for the Large Binocular Telescope. Once operational, it will provide an unprecedented combination of angular resolution, sensitivity and field of view. Its layer-oriented MCAO systems (one for each arm of the interferometer) are conjugated to the ground layer and an additional layer in the upper atmosphere. In this contribution MCAO wavefront control is discussed in the context of the overall control scheme for LINC-NIRVANA. Special attention is paid to a set of auxiliary control tasks which are mandatory for MCAO operation: The Fields of View of each wavefront sensor in the instrument have to be derotated independent from each other and independently from the science field. Any wavefront information obtained by the sensors has to be matched to the time invariant modes of the deformable mirrors in the system. The tip/tilt control scheme is outlined, in which atmospheric, but also instrumental tip/tilt corrections are sensed with the high layer wavefront sensor and corrected by the adaptive secondary mirror of the LBT. Slow image motion effects on the science detector have to be considered, which are caused by flexure in the non-common path between AO and the science camera, atmospheric differential refraction, and alignment tolerances of the derotators. Last but not least: The sensor optics (pyramids) have to be accurately positioned at the images of natural reference stars.

[1]  R. Ragazzoni,et al.  LINC-NIRVANA: optical design of an interferometric imaging camera , 2006, SPIE Astronomical Telescopes + Instrumentation.

[2]  Armando Riccardi,et al.  First light AO (FLAO) system for LBT: final integration, acceptance test in Europe, and preliminary on-sky commissioning results , 2010, Astronomical Telescopes + Instrumentation.

[3]  W A Traub,et al.  Combining beams from separated telescopes. , 1986, Applied optics.

[4]  W. Gässler,et al.  A very wide field wavefront sensor for a very narrow field interferometer , 2010, Astronomical Telescopes + Instrumentation.

[5]  Peter Bizenberger,et al.  The LINC-NIRVANA IR cryostat , 2006, SPIE Astronomical Telescopes + Instrumentation.

[6]  Roberto Ragazzoni,et al.  Imaging beyond the fringe: an update on the LINC-NIRVANA Fizeau interferometer for the LBT , 2010, Astronomical Telescopes + Instrumentation.

[7]  Roberto Ragazzoni,et al.  Adaptive optics for 100-m-class telescopes: new challenges require new solutions , 2000, Astronomical Telescopes and Instrumentation.

[8]  Roberto Ragazzoni,et al.  The MCAO wavefront sensing system of LINC-NIRVANA: status report , 2006, SPIE Astronomical Telescopes + Instrumentation.

[9]  John M. Hill,et al.  Large Binocular Telescope project , 2000, Astronomical Telescopes + Instrumentation.

[10]  Roberto Ragazzoni,et al.  Numerical control matrix rotation for the LINC-NIRVANA multiconjugate adaptive optics system , 2010, Astronomical Telescopes + Instrumentation.

[11]  Jacques M. Beckers,et al.  Increasing the size of the isoplanatic patch with multiconjugate adaptive optics. , 1988 .

[12]  R. Ragazzoni Pupil plane wavefront sensing with an oscillating prism , 1996 .