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.
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.
The LBT (Large Binocular Telescope), located at about 3200m on Mount Graham (Tucson, Arizona) is an innovative project undertaken by institutions from Europe and USA. LINC-NIRVANA is an instrument which provides MCAO (Multi-Conjugate Adaptive Optics) and interferometry, combining the light from the two 8.4m telescopes coherently. This configuration offers 23m-baseline optical resolution and the sensitivity of a 12m mirror, with a 2 arc-minute diffraction limited field of view. The integration, alignment and testing of such a big instrument requires a well-organized choreography and AIV planning which has been developed in a hierarchical way. The instrument is divided in largely independent systems, and all of them consist of various subsystems. Every subsystem integration ends with a verification test and an acceptance procedure. When a certain number of systems are finished and accepted, the instrument AIV phase starts. This hierarchical approach allows testing at early stages with simple setups. The philosophy is to have internally aligned subsystems to be integrated in the instrument optical path, and extrapolate to finally align the instrument to the Gregorian bent foci of the telescope. The alignment plan was successfully executed in Heidelberg at MPIA facilities, and now the instrument is being re-integrated at the LBT over a series of 11 campaigns along the year 2016. After its commissioning, the instrument will offer MCAO sensing with the LBT telescope. The interferometric mode will be implemented in a future update of the instrument. This paper focuses on the alignment done in the clean room at the LBT facilities for the collimator, camera, and High-layer Wavefront Sensor (HWS) during March and April 2016. It also summarizes the previous work done in preparation for shipping and arrival of the instrument to the telescope. Results are presented for every step, and a final section outlines the future work to be done in next runs until its final commissioning.
Telescopes or instruments equipped with Multi-Conjugate Adaptive Optics (MCAO) provide uniform turbulence correction over a wide Field of View (FoV), thereby overcoming the problems of isoplanatism and enabling previously challenging science. LINC-NIRVANA (LN), the German-Italian near-infrared high-resolution imager for the Large Binocular Telescope (LBT), has an advanced and unique MCAO module, which uses the Optical Co-addition of Layer- Oriented Multiple-FoV Natural Guide Star approach to MCAO with pyramid wavefront sensing. The layer-oriented wavefront correction can be performed by conjugating the Deformable Mirrors (DM) and the respective Wavefront Sensors (WFS) to the corresponding atmospheric layers. LN corrects for the aberrations in two different layers. The ground layer, conjugated to the telescope pupil ~100m above LBT, is corrected by the Ground-layer Wavefront Sensors (GWS) driving the LBT adaptive secondary mirrors, and a higher layer ∼7.1km above the telescope is corrected by the High-layer Wavefront Sensors (HWS) driving a pair of Xinetics DMs on the LN bench.
At the ground layer, the footprints of the stars overlap completely and every star footprint illuminates the entire pupil-plane. However, for a higher layer, the footprints do not overlap completely and each star illuminates a different region of the conjugated plane. Lack of stars, therefore, results in some regions in this "meta-pupil"-plane not being illuminated, implying no information regarding the aberrations in these areas. The optimum way of correcting the high layer, given this limited information, is the crux of the "partial illumination issue". In this paper, we propose a solution for this issue and discuss laboratory results from the aligned LN bench in the lab. Currently, LN has completed the re-integration and re-alignment at LBT. In early June 2016, we tested our partial illumination algorithm in the instrument’s final configuration in the LBT mountain lab, using simulated stars. On sky testing will begin in late 2016.
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-NIRVANA's wavefront reconstruction mechanism and report on laboratory and on-sky tests.
Spreading the PSF over a quite large amount of pixels is an increasingly used observing technique in order to reach
extremely precise photometry, such as in the case of exoplanets searching and characterization via transits observations.
A PSF top-hat profile helps to minimize the errors contribution due to the uncertainty on the knowledge of the detector
flat field. This work has been carried out during the recent design study in the framework of the ESA small mission
CHEOPS. Because of lack of perfect flat-fielding information, in the CHEOPS optics it is required to spread the light of
a source into a well defined angular area, in a manner as uniform as possible. Furthermore this should be accomplished
still retaining the features of a true focal plane onto the detector. In this way, for instance, the angular displacement on
the focal plane is fully retained and in case of several stars in a field these look as separated as their distance is larger
than the spreading size. An obvious way is to apply a defocus, while the presence of an intermediate pupil plane in the
Back End Optics makes attractive to introduce here an optical device that is able to spread the light in a well defined
manner, still retaining the direction of the chief ray hitting it. This can be accomplished through an holographic diffuser
or through a lenslet array. Both techniques implement the concept of segmenting the pupil into several sub-zones where
light is spread to a well defined angle. We present experimental results on how to deliver such PSF profile by mean of
holographic diffuser and lenslet array. Both the devices are located in an intermediate pupil plane of a properly scaled
laboratory setup mimicking the CHEOPS optical design configuration.
The delivered image quality of ground-based telescopes depends greatly on atmospheric turbulence. At every observatory, the majority of the turbulence (up to 60-80% of the total) occurs in the ground layer of the atmosphere, that is, the first few hundred meters above the telescope pupil. Correction of these perturbations can, therefore, greatly increase the quality of the image. We use Ground-layer Wavefront Sensors (GWSs) to sense the ground layer turbulence for the LINC-NIRVANA (LN) instrument, which is in its final integration phase before shipment to the Large Binocular Telescope (LBT) on Mt. Graham in Arizona.19 LN is an infrared Fizeau interferometer, equipped with an advanced Multi-Conjugate Adaptive Optics (MCAO) module, capable of delivering images with a spatial resolution equivalent to that of a ~23m diameter telescope. It exploits the Layer-Oriented, Multiple Field of View, MCAO approach3 and uses only natural guide stars for the correction. The GWS has more than 100 degrees of freedom. There are opto-mechanical complexities at the level of sub- systems, the GWS as a whole, and at the interface with the telescope. Also, there is a very stringent requirement on the superposition of the pupils on the detector. All these conditions make the alignment of the GWS very demanding and crucial. In this paper, we discuss the alignment and integration of the left-eye GWS of LN and detail the various tests done in the lab at INAF-Padova to verify proper system operation and performance.
The LINC-NIRVANA Pathfinder experiment is a test-bed to verify a very complex sub-system: the Ground-layer Wavefront Sensor, or GWS. Pathfinder will test the GWS in its final working environment and demonstrate on-sky the performance achievable with a multiple natural guide star, ground-layer adaptive optics system with a very wide FoV. The GWS uses up to 12 natural guide stars within a 2.8'-6' annular field of view and drives the LBT adaptive secondary mirror to correct the lower layers of atmospheric turbulence. This paper will trace the path of the instrument on its way to First Light on-sky in November 2013, from its installation on the telescope to the calibrations to its final operation, focusing in particular on opto-mechanical and software aspects and how they lead to the main achieved results.