Jürgen Berwein

Commissioning multi-conjugate adaptive optics with LINC-NIRVANA on LBT

This paper reports on early commissioning of LINC-NIRVANA (LN), an innovative Multi-Conjugate Adaptive Optics (MCAO) system for the Large Binocular Telescope (LBT). LN uses two, parallel MCAO systems, each of which corrects turbulence at two atmospheric layers, to deliver near diffraction-limited imagery over a two-arcminute field of view. We summarize LN’s approach to MCAO and give an update on commissioning, including the achievement of First Light in April 2018. This is followed by a discussion of challenges that arise from our particular type of MCAO and the solutions implemented. We conclude with a brief look forward to the remainder of commissioning and future upgrades.

Wavefront sensing in a partially illuminated, rotating pupil

LINC-NIRVANA is the near-infrared interferometric imaging camera for the Large Binocular Telescope. Once operational, it will provide an unprecedented combination of angular resolution, sensitivity, and field of view. Its pyramid-based layer-oriented MCAO systems are conjugated to the ground layer and to an additional layer in the upper atmosphere. The Groundlayer Wavefront Sensor optically coadds the light of up to 12 reference stars in the pupil, the Highlayer Wavefront Sensor optically combines the light of up to 8 reference stars in its metapupil. Each Wavefront Sensor has its own associated field derotator. It introduces a dependency of the sensor-actuator relation on the angle of the field derotator, which requires regular updates of the reconstructor in closed loop. In addition, the Highlayer Wavefront Sensor has to be able to reconstruct the incoming wavefronts by analyzing an only partially illuminated metapupil. The distribution of illuminated subapertures depends on the distribution of reference stars. For each pointing, a specific reconstruction matrix has to be generated, which only considers the illuminated subapertures. In this contribution we will present the concept of LINC-NIRVANAs wavefront reconstruction mechanism and report on laboratory and on-sky tests.

Installation and commissioning of the LINC-NIRVANA near-infrared MCAO imager on LBT

This paper reports on the installation and initial commissioning of LINC-NIRVANA (LN), an innovative high resolution, near-infrared imager for the Large Binocular Telescope (LBT). We present the delicate and difficult installation procedure, the culmination of a re-integration campaign that was in full swing at the last SPIE meeting. We also provide an update on the ongoing commissioning campaigns, including our recent achievement of First Light. Finally, we discuss lessons learned from the shipment and installation of a large complex instrument.

LINC-NIRVANA for the large binocular telescope: setting up the world’s largest near infrared binoculars for astronomy

LINC-NIRVANA (LN) is the near-infrared, Fizeau-type imaging interferometer for the large binocular telescope (LBT) on Mt. Graham, Arizona (elevation of 3267 m). The instrument is currently being built by a consortium of German and Italian institutes under the leadership of the Max Planck Institute for Astronomy in Heidelberg, Germany. It will combine the radiation from both 8.4 m primary mirrors of LBT in such a way that the sensitivity of a 11.9 m telescope and the spatial resolution of a 22.8 m telescope will be obtained within a 10.5×10.5 arcsec 2 scientific field of view. Interferometric fringes of the combined beams are tracked in an oval field with diameters of 1 and 1.5 arcmin. In addition, both incoming beams are individually corrected by LN’s multiconjugate adaptive optics system to reduce atmospheric image distortion over a circular field of up to 6 arcmin in diameter. A comprehensive technical overview of the instrument is presented, comprising the detailed design of LN’s four major systems for interferometric imaging and fringe tracking, both in the near infrared range of 1 to 2.4 μm, as well as atmospheric turbulence correction at two altitudes, both in the visible range of 0.6 to 0.9 μm. The resulting performance capabilities and a short outlook of some of the major science goals will be presented. In addition, the roadmap for the related assembly, integration, and verification process are discussed. To avoid late interface-related risks, strategies for early hardware as well as software interactions with the telescope have been elaborated. The goal is to ship LN to the LBT in 2014.

Acquiring multiple stars with the LINC-NIRVANA Pathfinder

The LINC-NIRVANA Pathfinder1 (LN-PF), a ground-layer adaptive optics (AO) system recently commissioned at the Large Binocular Telescope (LBT), is one of 4 sensors that provide AO corrected images to the full LINC-NIRVANA instrument. With first light having taken place on November 17, 2013,2, 3 the core goals for the LN-PF have been accomplished. In this report, we look forward to one of the LN-PF extended goals. In particular, we review the acquisition mechanism required to place each of several star probes on its corresponding star in the target asterism. For emerging AO systems in general, co-addition of light from multiple stars stands as one of several methods being pursued to boost sky coverage. With 12 probes patrolling a large field of view (an annulus 6-arcminutes in diameter), the LN-PF will provide a valuable testbed to verify this method.

The MCAO systems within LINC-NIRVANA: control aspects in addition to 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.

The calibration procedure of the LINC-NIRVANA ground and high layer WFS

LINC–NIRVANA (LN) is a MCAO module currently mounted on the Rear Bent Gregorian focus of the Large Binocular Telescope (LBT). It mounts a camera originally design to realize the interferometric imaging focal station of the LBT. LN follows the LBT strategy having two twin channels: a double Layer Oriented multi-conjugate adaptive optics system assists the two arms, supplying high order wave-front correction. In order to counterbalance the field rotation a mechanical derotation is applied for the two ground wave-front sensors, and an optical (K-mirror) one for the two high layers sensors, fixing the positions of the focal planes with respect to the pyramids aboard the wavefront sensors. The derotation introduces a pupil images rotation on the wavefront sensors changing the projection of the deformable mirrors on the sensor consequently. The soft real-time computer load the matrix corresponding to the needed at one degree step. Calibrations were performed in daytime only and using optical fibers.

Solving The Multi-Conjugated Adaptive Optics Partial Illumination Issue With Laboratory Results

Multi-Conjugate Adaptive Optics can produce uniform wide-field correction. However, partial illumination makes it challenging to estimate the true aberrations at specific high atmospheric layers. We have tested and will present lab results for a solution to this issue for LINC-NIRVANA, an instrument at the Large Binocular Telescope.

Aligning the LINC-NIRVANA Natural Guide Stars MCAO system

LINC-NIRVANA (LN) is an instrument built to be a Fizeau interferometric imager for the Large Binocular Telescope that will achieve ELT-like spatial resolution. Of course achieving this outstanding resolution requires a very complex instrument, assuring the delivery of plane wavefronts, parallel input beams, homoteticity and zero Optical Path Difference. LN will be one of the most complex ground-based instruments ever built, consisting of a Multi-Conjugate Adaptive Optics (MCAO) system, a fringe tracker, a beam combiner and a Near-InfraRed science camera, for a total of more than 250 indivudual lenses and mirrors.The MCAO sub-unit itself is the state of the art in the sector of wide field adaptive optics. It consists of 4 Wavefront Sensors (WFSs), two for each arm of the telescope, to sense the turbulence at the ground layer and at 7.1 km above the telescope. They operate in a layer oriented, Multiple Field of View mode, using up to 12 Natural Guide Stars (NGSs) for the ground layer correction and up to 8 NGSs for the mid layer correction.The ambitious nature of LN, which compels us to meet very tight requirements, together with the high number of subsystems lead to a challenging alignment procedure of the instrument. Despite of the complexity, the Alignment, Integration and Verification phase of the instrument has been recently completed with success in Heidelberg and LN is currently on its way to the LBT, where it will be re-aligned and finally mounted at one of the bend focal stations of the telescope. In this paper the integration and alignment procedure of the MCAO subsystem to the rest of LN is described and results are presented.

Pathfinder first light: alignment, calibration, and commissioning of the LINC-NIRVANA ground-layer adaptive optics subsystem

We present descriptions of the alignment and calibration tests of the Pathfinder, which achieved first light during our 2013 commissioning campaign at the LBT. The full LINC-NIRVANA instrument is a Fizeau interferometric imager with fringe tracking and 2-layer natural guide star multi-conjugate adaptive optics (MCAO) systems on each eye of the LBT. The MCAO correction for each side is achieved using a ground layer wavefront sensor that drives the LBT adaptive secondary mirror and a mid-high layer wavefront sensor that drives a Xinetics 349 actuator DM conjugated to an altitude of 7.1 km. When the LINC-NIRVANA MCAO system is commissioned, it will be one of only two such systems on an 8-meter telescope and the only such system in the northern hemisphere. In order to mitigate risk, we take a modular approach to commissioning by decoupling and testing the LINC-NIRVANA subsystems individually. The Pathfinder is the ground-layer wavefront sensor for the DX eye of the LBT. It uses 12 pyramid wavefront sensors to optically co-add light from natural guide stars in order to make four pupil images that sense ground layer turbulence. Pathfinder is now the first LINC-NIRVANA subsystem to be fully integrated with the telescope and commissioned on sky. Our 2013 commissioning campaign consisted of 7 runs at the LBT with the tasks of assembly, integration and communication with the LBT telescope control system, alignment to the telescope optical axis, off-sky closed loop AO calibration, and finally closed loop on-sky AO. We present the programmatics of this campaign, along with the novel designs of our alignment scheme and our off-sky calibration test, which lead to the Pathfinder’s first on-sky closed loop images.