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.
LINC-NIRVANA (LN) is the near-infrared, Fizeau-type imaging interferometer for the Large Binocular Telescope
(LBT) on Mt. Graham, Arizona, USA (3267m of elevation). The instrument is currently being built by a consortium of
German and Italian institutes under the leadership of the Max Planck Institute for Astronomy (MPIA) in Heidelberg,
Germany. It will combine the radiation from both 8.4m primary mirrors of LBT in such a way that the sensitivity of a
11.9m telescope and the spatial resolution of a 22.8m telescope will be obtained within a 10.5arcsec x 10.5arcsec
scientific field of view. Interferometric fringes of the combined beams are tracked in an oval field with diameters of 1
and 1.5arcmin. In addition, both incoming beams are individually corrected by LN’s multi-conjugate adaptive optics
(MCAO) system to reduce atmospheric image distortion over a circular field of up to 6arcmin in diameter.
This paper gives a comprehensive technical overview of the instrument comprising the detailed design of LN’s four
major systems for interferometric imaging and fringe tracking, both in the NIR range of 1 - 2.4μm, as well as
atmospheric turbulence correction at two altitudes, both in the visible range of 0.6 - 0.9μm. 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 (AIV) process will be 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.
LINC-NIRVANA is an instrument to combine the light from both LBT primary mirrors in an imaging Fizeau interferometer. The goals in terms of resolution and field of view are quite ambitious, which leads to a complex instrument consisting of a bunch of subsystems. The layer oriented MCAO system alone is already quite complicated and to get everything working together properly is not a small challenge. As we are reaching the completion of LINC-NIRVANA's subsystems, it becomes more and more important to define a strategy to align all these various subsystems. The specific layout of LINC-NIRVANA imposes some restrictions and difficulties on the sequence and the method of this alignment. The main problem for example is that we have to get two perfectly symmetrical focal planes to be able to properly combine them interferometrically. This is the major step on which all further alignment is based on, since all the subsystems (collimator and camera optics, wavefront sensors, cold IR optics, etc.) rely on these focal planes as a reference. I will give a small introduction on the optics of the instrument and line out the resulting difficulties as well as the strategy that we want to apply in order to overcome these.
One possible key reference element in optical alignment is represented by the rotational stage, a mechanical bearing, or
any similar suitable device having enough accuracy and precision so that optical tolerances are reasonably relaxed wrt
imperfections in the rotational movement. This allows a safe, reliable, easy to reproduce, determination of both rays
parallel to the axis or to their centering within almost any plane. An image derotator, that in its simplest form is made up
by three flat mirrors arranged in a so called K-mirror layout, moving together on a precision rotating stage, seems to be
the most safe, strong, and self built-in alignment tool. Moreover you can use the mechanical part as well as the optical
one. Care has to be given when internally and externally aligning has to be accomplished within a certain degree of
precision. To further make the situation more complex, the technical overall requirements can be tight enough that the
distribution of the error budget among the various components (imperfect mechanical rotation, imperfect internal
alignment, flexures during rotations) is not due to a single item. In this case, in fact, a number of tips and tricks can be
useful to find out which is the best approach to follow. The specific case of the two K-mirrors on board LINCNIRVANA
is here illustrated in a few lessons.
LINC-NIRVANA is the Fizeau beam combiner for the LBT, with the aim to retrieve the sensitivity of a 12m telescope
and the spatial resolution of a 22.8m one. Despite being only one of the four wavefront sensors of a layer-oriented
MCAO system, the GWS, which is retrieving the deformation introduced by the lower atmosphere, known to be the main
aberration source, reveals a noticeable internal opto-mechanical complexity.
The presence of 12 small devices used to select up to the same number of NGSs, with 3 optical components each,
moving in a wide annular 2'-6' arcmin Field of View and sending the light to a common pupil re-imager, and the need to
obtain and keep a very good super-imposition of the pupil images on the CCD camera, led to an overall alignment
procedure in which more than a hundred of degrees of freedom have to be contemporary adjusted.
The rotation of the entire WFS to compensate for the sky movement, moreover, introduces a further difficulty both in the
alignment and in ensuring the required pupil superposition stability.
A detailed description of the alignment procedure is presented here, together with the lessons learned managing the
complexity of such a WFS, which led to considerations regarding future instruments, like a possible review of numerical
versus optical co-add approach, above all if close to zero read-out noise detectors will be soon available.
Nevertheless, the GWS AIV has been carried out and the system will be soon mounted at LBT to perform what is called
the Pathfinder experiment, which consists in ground-layer correction, taking advantage of the Adaptive Secondary
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.
LINC-NIRVANA is a near infrared interferometric imager with a pair of layer-oriented multi-conjugate adaptive
optics systems (ground layer and high layer) for the Large Binocular Telescope. To prepare for the commissioning
of LINC-NIRVANA, we have integrated the high layer wavefront sensor and its associated deformable mirror (a
Xinetics-349) in a laboratory, located at Max Planck Institute for Astronomy, in Heidelberg, Germany. Together
with a telescope simulator, which includes a rotating field and phase screens that introduce the effects of the
atmosphere, we tested the acquisition of multiple guide stars, calibrating the system with the push-pull method,
and characterizing the wavefront sensor together with the deformable mirror. We have closed the AO loop with
up to 200 Zernike modes and with multiple guide stars. The AO correction demonstrated that uniform correction
can be achieved in a large field of view. We report the current status and results of the experiment.
LINC-NIRVANA<sup></sup> (LN) is an instrument for the Large Binocular Telescope<sup></sup> (LBT). Its purpose is to combine the light
coming from the two primary mirrors in a Fizeau-type interferometer. In order to compensate turbulence-induced
dynamic aberrations, the layer oriented adaptive optics system of LN<sup></sup> consists of two major subsystems for each side:
the Ground-Layer-Wavefront sensor (GLWS) and the Mid- and High-Layer Wavefront sensor (MHLWS). The MHLWS
is currently set up in a laboratory at the Max-Planck-Institute for Astronomy in Heidelberg. To test the multi-conjugate
AO with multiple simulated stars in the laboratory and to develop the necessary control software, a dedicated light
source is needed. For this reason, we designed an optical system, operating in visible as well as in infrared light, which
imitates the telescope's optical train (f-ratio, pupil position and size, field curvature). By inserting rotating surface etched
glass phase screens, artificial aberrations corresponding to the atmospheric turbulence are introduced. In addition,
different turbulence altitudes can be simulated depending on the position of these screens along the optical axis. In this
way, it is possible to comprehensively test the complete system, including electronics and software, in the laboratory
before integration into the final LINC-NIRVANA setup. Combined with an atmospheric piston simulator, also this effect
can be taken into account. Since we are building two identical sets, it is possible to feed the complete instrument with
light for the interferometric combination during the assembly phase in the integration laboratory.
The LINC-NIRVANA wavefront sensors are in their AIT phase. The first Ground-layerWavefront Sensor (GWS)
is shaping in the Adaptive Optics laboratory of the Astronomical Observatory of Padova, while both the Mid-
High Wavefront Sensors (MHWSs) have been aligned and tested as stand-alone units in the Observatory of
Bologna (MHWS#1 aligned to LINC-NIRVANA post focal relay optics).
LINC-NIRVANA is a Fizeau infrared interferometer equipped with advanced, MultiConjugated Adaptive
Optics (MCAO) for the Large Binocular Telescope. The aim of the instrument is to allow true interferometric
imagery over a 10" square Field of View (FoV), getting the sensitivity of a 12m telescope and the spatial resolution
of a 22.8m one. Thanks to the MCAO concept, LINC-NIRVANA will use up to 20 Natural Guide Stars (NGS)
which are divided, according to Layer-Oriented Multiple Field of View technique, between the GWSs and the
MHWSs. To find such a large number of references, the AO systems will use a wide FoV of 6' in diameter and
the light coming from the references used by each WFS will optically sum on its CCD camera.
The MHWSs will detect the deformations due to the high layers and will select up to 8 NGSs in the inner 2'
The GWSs, instead, will reconstruct the deformations introduced by the lower atmosphere, which was found
out to be the main source of seeing. Their peculiarity is the highest number of references (up to 12) ever used
in a single instrument, selected in an annular 2'-6' FoV.
A joint project among INAF--Osservatorio Astronomico di Bologna (Italy), Università di Bologna--Dipartimento di
Astronomia (Italy) and Max-Planck-Institut für Astronomie (Heidelberg, Germany) led in about one year to the
construction of two infrared test cameras for the LBT Observatory. Such cameras will be used to test the performance
achieved by the telescope adaptive optics system as well as to prepare the telescope pointing model and to completely
test all the focal stations at the Gregorian focus.
In the present article the design and the integration of the two test cameras are described. The achieved performances are
presented as well.
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.
The very large telescope (VLT) interferometer (VLTI) in its current operating state is equipped with high-order
adaptive optics (MACAO) working in the visible spectrum. A low-order near-infrared wavefront sensor (IRIS)
is available to measure non-common path tilt aberrations downstream the high-order deformable mirror. For
the next generation of VLTI instrumentation, in particular for the designated GRAVITY instrument, we have
examined various designs of a four channel high-order near-infrared wavefront sensor. Particular objectives of
our study were the specification of the near-infrared detector in combination with a standard wavefront sensing
system. In this paper we present the preliminary design of a Shack-Hartmann wavefront sensor operating in
the near-infrared wavelength range, which is capable of measuring the wavefronts of four telescopes simultaneously.
We further present results of our design study, which aimed at providing a first instrumental concept for