Frank Kittmann

First laboratory results with the LINC-NIRVANA high layer wavefront sensor

In the field of adaptive optics, multi-conjugate adaptive optics (MCAO) can greatly increase the size of the corrected field of view (FoV) and also extend sky coverage. By applying layer oriented MCAO (LO-MCAO) [4], together with multiple guide stars (up to 20) and pyramid wavefront sensors [7], LINC-NIRVANA (L-N for short) [1] will provide two AO-corrected beams to a Fizeau interferometer to achieve 10 milliarcsecond angular resolution on the Large Binocular Telescope. This paper presents first laboratory results of the AO performance achieved with the high layer wavefront sensor (HWS). This sensor, together with its associated deformable mirror (a Xinetics-349), is being operated in one of the L-N laboratories. AO reference stars, spread across a 2 arc-minute FoV and with aberrations resulting from turbulence introduced at specific layers in the atmosphere, are simulated in this lab environment. This is achieved with the Multi-Atmosphere Phase screen and Stars (MAPS) [2] unit. From the wavefront data, the approximate residual wavefront error after correction has been calculated for different turbulent layer altitudes and wind speeds. Using a somewhat undersampled CCD, the FWHM of stars in the nearly 2 arc-minute FoV has also been measured. These test results demonstrate that the high layer wavefront sensor of LINC-NIRVANA will be able to achieve uniform AO correction across a large FoV.

LINC-NIRVANA Pathfinder: testing the next generation of wave front sensors at LBT

LINC-NIRVANA will employ four wave front sensors to realize multi-conjugate correction on both arms of a Fizeau interferometer for LBT. Of these, one of the two ground-layer wave front sensors, together with its infrared test camera, comprise a stand-alone test platform for LINC-NIRVANA. Pathfinder is a testbed for full LINC-NIRVANA intended to identify potential interface problems early in the game, thus reducing both technical, and schedule, risk. Pathfinder will combine light from multiple guide stars, with a pyramid sensor dedicated to each star, to achieve ground-layer AO correction via an adaptive secondary: the 672-actuator thin shell at the LBT. The ability to achieve sky coverage by optically coadding light from multiple stars has been previously demonstrated; and the ability to achieve correction with an adaptive secondary has also been previously demonstrated. Pathfinder will be the first system at LBT to combine both of these capabilities. Since reporting our progress at A04ELT2, we have advanced the project in three key areas: definition of specific goals for Pathfinder tests at LBT, more detail in the software design and planning, and calibration. We report on our progress and future plans in these three areas, and on the project overall.

Calibrating the interaction matrix for the LINC-NIRVANA high layer wavefront sensor

LINC-NIRVANA is a near-infrared Fizeau interferometric imager that will operate at the Large Binocular Telescope. In preparation for the commissioning of this instrument, we conducted experiments for calibrating the high-layer wavefront sensor of the layer-oriented multi-conjugate adaptive optics system. For calibrating the multi-pyramid wavefront sensor, four light sources were used to simulate guide stars. Using this setup, we developed the push-pull method for calibrating the interaction matrix. The benefits of this method over the traditional push-only method are quantified, and also the effects of varying the number of push-pull frames over which aberrations are averaged is reported. Finally, we discuss a method for measuring mis-conjugation between the deformable mirror and the wavefront sensor, and the proper positioning of the wavefront sensor detector with respect to the four pupil positions.

Ground layer correction: the heart of LINC-NIRVANA

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.

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.

Multiple guide star acquisition software for LINC-NIRVANA

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 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. The wavefront sensors can use up to 12 natural guide stars for wavefront sensing. Up to 12 opto-mechanical units have to be accurately positioned to coincide with the positions of the natural guide stars in the focal plane. A positioning software will coordinate the motion of these units. It has to fulfill a number of requirements: Collisions between the opto-mechanical units have to be prevented at any time. The units shall be positionable as close as possible to each other without touching their neighbors. To reduce the acquisition overhead, the units shall move in parallel. Different positioning modes have to be supported: Guide star acquisition, but also positioning model corrections and common offsets will be commanded. In this presentation we will outline the requirements and use cases of the positioning software. The logic that will be used to prevent collisions will be discussed as well as the algorithm that can be used to assign the opto-mechanical units to the guide stars.

Integration of the Mid-High Wavefront Sensor to the Linc-Nirvana post-focal relay

LINC-NIRVANA is an infrared camera working in Fizeau interferometric mode. The beams coming from the two primary mirrors of the LBT are corrected for the effects of the atmospheric turbulence by two Multi-Conjugate Adaptive Optics (MCAO) systems, working in a scientific field of view of 2 arcminutes. One single arm MCAO system includes two wave-front sensors, driving two deformable mirrors, one for the ground layer correction (LBT secondary mirror) and one for the correction of a mid-high layer (up to a maximum distance of 15 km). The first of the two Mid-High Wavefront Sensors (MHWS) was integrated and tested as a stand-alone unit in the laboratory at INAF-Osservatorio Astronomico di Bologna, where the telescope was simulated by means of a simple afocal system illuminated by a set of optical fibers. Then the module was delivered to the MPIA laboratories in Heidelberg, where is going to be integrated and aligned to the post-focal optical relay of one LINC-NIRVANA arm, including the deformable mirror. A number of tests are in progress at the moment of this writing, in order to characterize and optimize the system functionalities and performance. A report is presented about the status of this work.

Design and implementation of a service-oriented driver architecture for LINC-NIRVANA

LINC-NIRVANA (LN) is a German-Italian Fizeau (imaging) interferometer for the Large Binocular Telescope (LBT). The Instrument Control Software (ICS) of this instrument is a hierarchical, distributed software package, which runs on several computers. In this paper we present the bottom layer of the hierarchy - the Basic Device Application (BASDA) layer. This layer simplifies the development of the ICS through a general driver architecture, which supports different types of hardware. This generic device architecture provides a high level interface to encapsulate the hardware dependent driver. The benefit of such a device architecture is to keep the basic device-driver layer flexible and independent from the hardware, and to keep the hardware transparent to the ICS. Additionally, the basic device-driver layer supports interfaces to IDL based applications for calibration and laboratory testing of astronomical instruments, and interfaces to engineering GUIs that allow to maintain the software components easily.

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.