Novel AO on the Pathway to ELTs: MCAO with LINC-NIRVANA on LBT…Lessons Foreseen?

LINC-NIRVANA is a near infrared interferometric imager that will achieve ELT-like spatial resolution for panoramic imagery on the Large Binocular Telescope (LBT). The LBT is a unique platform since its two, co-mounted 8.4-meter primary mirrors, coupled with fully adaptive secondary mirrors, present a time and view-direction independent entrance pupil. This allows Fizeau-mode beam combination, giving 23-meter equivalent spatial resolution and the collecting area of a 12-m telescope. In order to achieve diffraction limited image quality and maximum sky coverage, in particular for finding fringe-tracking reference stars, LINC-NIRVANA employs unique multi-conjugate adaptive optics (MCAO). The NIRVANA system comprises a total of five control loops for atmospheric turbulence: sequential ground and high-layer NGS AO correction for each telescope, coupled together through a common delay line to remove differential atmospheric piston and vibration. The MCAO operates in layeroriented, multiple field-of-view mode with up to 12 ground-layer and 8 high-layer natural stars per telescope. LINC-NIRVANA is a pathfinder for ELT instrumentation and AO systems in more ways than merely spatial resolution: in terms of physical size, complexity, alignment tolerances, and integration challenges, LINC-NIRVANA serves as an instructive precursor for future efforts. In this paper, we provide an update on the integration and testing of the instrument, and attempt to foresee and forestall future lessons learned.