The warm calibration unit (WCU) is one subsystem of the future METIS instrument on the European Extremely Large Telescope (E-ELT). Operating at daytime temperature, the WCU is mounted above the main cryostat of METIS and will be employed as calibration reference for science observations, as well as for verification and alignment purposes during the AIT phase. The WCU is designed and constructed at the University of Cologne, partner in the METIS consortium. The WCU, together with the full METIS instrument, went recently through a successful preliminary design review (PDR) phase at ESO and is entering now the Phase C of the project. In this paper, we present the current status of the WCU and summarize the mostly mechanical and optical engineering work. We adopted a hexapod unit to interface with the METIS cryostat and a CFRP-based optical bench to optimally cope with alignment flexure. We develop the case for fiber-fed laser sources feeding the integrating sphere for spectral calibration of the LM-Spectrograph of METIS. We detail the activity foreseen for Phase C including the optical tolerances analysis, the eigenfrequency and earthquake analysis and a preparation of the sub-system MAIT work, finishing the paper with a short overview of the WCU future plans.
METIS, the mid-infrared imager and spectrograph for the wavelength range 2.9-14 µm (astronomical L-, M- and N bands), will be equipped with a calibration unit, developed at the University of Cologne, which task is to deliver simulated sources for the test and calibration of the main imaging and spectral functionalities of METIS. Our subsystem, as the full METIS instrument, is currently in the Phase C of the project, which leads to the Final Design Review expected by the end of 2021. In this contribution, we first briefly introduce the general concepts chosen for the Warm Calibration Unit (WCU) and then detail the laboratory work that is undertaken in Cologne to validate most of the concepts presented at the Preliminary Design Review. A core unit of the WCU is the integrating sphere combined with the black body, which is the hub delivering the calibration functionalities. We first report the measured spatial uniformity of the output port of the integrating sphere when fed with the black body source radiation. The measurement made using our uncooled thermal camera, evidences a spatial uniformity below 1% RMS. Longer integration times will further improve the final accuracy on this important parameter. We also take a closer look at the black body source and report on its flux temporal stability, which is found to be better than 1% over a 2h duration. We characterize time windows for different settings of the main WCU light source, which is the black body and stability and repeatability of the detected signal. Through different experiments we investigated the best options to manufacture the aperture mask that will be used to generate artificial point sources.
We present the preliminary design of the calibration unit of the future E-ELT instrument METIS. This independent subunit is mounted externally to the main cryostat of METIS and will function both as calibration reference for science observations, as well as verification and alignment tool during the AIT phase. In this paper, we focus on describing its preliminary layout and foreseen functionalities, based on the performance requirements defined at system level and the constraints imposed by warm IR background. We discuss the advantage of employing an integrating sphere as common radiation emitter, leading to a novel and versatile design, where the source’s spatio-spectral properties can be varied with high fidelity and repeatability. By combining only few tuneable sources and mechanisms we show how a large instrument such as METIS can be calibrated and tested, without the need of a complex cold calibration unit.
METIS, a mid-infrared imager and spectrograph for the wavelength range 2.9–19μm (astronomical L-, M-, N- and Q-band), will be one of the first three science instruments at the European Extremely Large Telescope (E-ELT). It will provide diffraction limited imaging, coronagraphy, high resolution integral field spectroscopy and low and medium resolution slit spectroscopy. Within the international METIS consortium, the 1st Institute of Physics of the University of Cologne in Germany is responsible for the design, manufacturing, integration and qualification of the Warm Calibration Unit (WCU) of the instrument. The WCU will be a self-contained unit operating at ambient temperature outside of the voluminous METIS dewar, feeding a variety of optical calibration and alignment signals into the optical path of METIS. The functionalities of the WCU will be used for routine daily daytime calibrations after astronomical observing nights and verification of the internal alignment of METIS during assembly, integration and verification (AIV). In this contribution we present the preliminary optical design and principle of operation of the WCU in its current state of the preliminary design phase of METIS.
The LINC-NIRVANA Fringe and Flexure Tracking System has nearly completed assembly in the lab in Cologne, and will soon be ready for shipment and integration into the full LINC-NIRVANA system at MPIA Heidelberg. This paper provides an overview of the final assembly and testing phase in Cologne, concentrating on those aspects that directly affect instrument performance, including the detector performance and stability of the detector positioning system.
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.
The super-massive 4 million solar mass black hole (SMBH) SgrA* shows variable emission from the millimeter to the X-ray domain. A detailed analysis of the infrared light curves allows us to address the accretion phenomenon in a statistical way. The analysis shows that the near-infrared flux density excursions are dominated by a single state power law, with the low states of SgrA* are limited by confusion through the unresolved stellar background. We show that for 8-10m class telescopes blending effects along the line of sight will result in artificial compact star-like objects of 0.5-1 mJy that last for about 3-4 years. We discuss how the imaging capabilities of GRAVITY at the VLTI, LINC-NIRVANA at the LBT and METIS at the E-ELT will contribute to the investigation of the low variability states of SgrA*.
The Fringe and Flexure Tracking System (FFTS) is meant to monitor and correct atmospheric piston varia tion and instrumental vibrations and flexure during near-infrared interferometric image acquisition of LING NIRVANA. In close work with the adaptive optics system the FFTS enables homothetic imaging for the Large Binocular Telescope. One of the main problems we had to face is the connection between the cryogenic upper part of the instrument, e.g. detector head, and the lower ambient temperature part. In this ambient temperature part the moving stages are situated that move the detector head in the given field of view (FOV). We show how we solved this problem using the versatile material glass fiber reinforced plastics (GFRP's) and report in what way this material can be worked. We discuss in detail the exquisite characteristics of this material which we use to combine the cryogenic and ambient environments to a fully working system. The main characteristics that we focus on are the low temperature conduction and the tensile strength of the GFRP's. The low temperature conduction is needed to allow for a low heat-exchange between the cryogenic and ambient part whereas the tensile strength is needed to support heavy structures like the baffle motor and to allow for a minimum of flexure for the detector head. Additionally, we discuss the way we attached the GFRP to the remaining parts of the FFTS using a two component encapsulant.
We present the latest status of the control system of the LN (LINC-NIRVANA) FFTS (Fringe and Flexure Tracker
System) for the LBT. The software concept integrates the sensor data and control of the various subsystems
and provides the interaction with the whole LN instrument. Varying conditions and multiple configurations for
observations imply a flexible interconnection of the control loops for the hardware manipulators with respect
to the time-critical data analysis of the fringe detection. In this contribution details of the implementation of
the algorithms on a real-time Linux PC are given. By considering the results from simulations of the system
dynamics, lab experiments, atmospheric simulations, and telescope characterization the optimal parameter setup
for an observation can be chosen and basic techniques for adaption to changing conditions can be derived.
LINC-NIRVANA is a near-Infrared homothetic, beam combining camera for the Large Binocular Telescope that offers Multi-Conjugate Adaptive Optics wavefront correction and fringe tracking to achieve a time-stable fringe pattern. Therefore, the trajectory of the reference source has to be followed as accurate as possible for a precise point spread function acquisition. The presented measurement campaign shows detector positioning errors exceeding the requirements significantly and indicates that these huge errors arise from the software, while the installed hardware matches the requirements.
LINC-NIRVANA (LN) is a German /Italian interferometric beam combiner camera for the Large Binocular Telescope. Due to homothetic imaging, LN will make use of an exceptionally large field-of-view. As part of LN, the Fringe-and-Flexure-Tracker system (FFTS) will provide real-time, closed-loop measurement and correction of pistonic and flexure signals induced by the atmosphere and inside the telescope-instrument system. Such
compensation is essential for achieving coherent light combination over substantial time intervals (~10min.).
The FFTS is composed of a dedicated near-infrared detector, which can be positioned by three linear stages within the curved focal plane of LN. The system is divided into a cryogenic (detector) and ambient (linear stages) temperature environment, which are isolated from each other by a moving baffie. We give an overview of the current design and implementation stage of the FFTS opto-mechanical components. The optical components represent an update of the original design to assess slow image motion induced by the LN instrument separately.
We review the status of hardware developments related to the Linc-Nirvana optical path difference (OPD) control. The
status of our telescope vibration measurements is given. We present the design concept of a feed-forward loop to damp
the impact of telescope mirror vibrations on the OPD seen by Linc-Nirvana. At the focus of the article is a description of
the actuator of the OPD control loop. The weight and vibration optimized construction of this actuator (aka piston
mirror) and its mount has a complex dynamical behavior, which prevents classical PI feedback control from delivering
fast and precise motion of the mirror surface. Therefore, an H-; optimized control strategy will be applied, custom
designed for the piston mirror. The effort of realizing a custom controller on a DSP to drive the piezo is balanced by the
outlook of achieving more than 5x faster servo bandwidths. The laboratory set-up to identify the system, and verify the
closed loop control performance is presented. Our goal is to achieve 30 Hz closed-loop control bandwidth at a precision of 30 nm.
The Fringe and Flexure Tracker System (FFTS) of the LINC-NIRVANA instrument is designed to monitor and
correct the atmospheric piston variations and the instrumental vibrations and flexure at the LBT during the
NIR interferometric image acquisition. In this contribution, we give an overview of the current FFTS control
design, the various subsystems, and their interaction details. The control algorithms are implemented on a realtime
computer system with interfaces to the fringe and flexure detector read-out electronics, the OPD vibration
monitoring system (OVMS) based on accelerometric sensors at the telescope structure, the piezo-electric actuator
for piston compensation, and the AO systems for offloading purposes. The FFTS computer combines data from
different sensors with varying sampling rate, noise and delay. This done on the basis of the vibration data and the
expected power spectrum of atmospheric conditions. Flexure effects are then separated from OPD signals and
the optimal correcting variables are computed and distributed to the actuators. The goal is a 120 nm precision
of the correction at a bandwidth of about 50 Hz. An end-to-end simulation including models of atmospheric
effects, actuator dynamics, sensor effects, and on-site vibration measurements is used to optimize controllers and
filters and to pre-estimate the performance under different observation conditions.
LINC-NIRVANA (LN) is a German/Italian interferometric beam combiner camera for the Large Binocular
Telescope. Due to homothetic imaging, LN will make use of an exceptionally large field-of-view. As part of LN,
the Fringe-and-Flexure-Tracker system (FFTS) will provide real-time, closed-loop measurement and correction
of pistonic and flexure signals induced by the atmosphere and inside the telescope-instrument system. Such
compensation is essential for achieving coherent light combination over substantial time intervals (~ 10min.).
The FFTS is composed of a dedicated near-infrared detector, which can be positioned by three linear stages
within the curved focal plane of LN. The system is divided into a cryogenic (detector) and ambient (linear
stages) temperature environment, which are isolated from each other by a moving baffle. We give an overview
of the current design and implementation stage of the FFTS opto-mechanical and electronic components. We
present recent important updates of the system, including the development of separated channels for the tracking
of piston and flexure. Furthermore, the inclusion of dispersive elements will allow for the correction of atmospheric
differential refraction, as well as the induction of artificial dispersion to better exploit the observational-conditions
parameter space (air mass, brightness).
LINC-NIRVANA is the NIR homothetic imaging camera for the Large Binocular Telescope (LBT). In close
cooperation with the Adaptive Optics systems of LINC-NIRVANA the Fringe and Flexure Tracking System
(FFTS) is a fundamental component to ensure a complete and time-stable wavefront correction at the position
of the science detector in order to allow for long integration times at interferometric angular resolutions. In this
contribution, we present the design and the realization of the ongoing FFTS laboratory tests, taking into account
the system requirements. We have to sample the large Field of View and to follow the reference source during
science observations to an accuracy of less than 2 microns. In particular, important tests such as cooling tests
of cryogenic components and tip - tilt test (the repeatability and the precision under the different inclinations)
are presented. The system parameters such as internal flexure and precision are discussed.
We present the latest status of the fringe detecting algorithms for the LINC-NIRVANA FFTS (Fringe and Flexure
Tracker System). The piston and PSF effects of the system from the top of the atmosphere through the telescopes and
multi-conjugate AO systems to the detector are discussed and the resulting requirements for the FFTS outlined.
LINC-NIRVANA is the near-infrared Fizeau interferometric imaging camera for the Large Binocular Telescope (LBT).
For an efficient interferometric operation of LINC-NIRVANA the Fringe and Flexure Tracking System (FFTS) is
mandatory: It is a real-time servo system that allows to compensate atmospheric and instrumental optical pathlength
differences (OPD). The thereby produced time-stable interference pattern at the position of the science detector enables
long integration times at interferometric angular resolutions.
As the development of the FFTS includes tests of control software and robustness of the fringe tracking concept in a
realistic physical system a testbed interferometer is set up as laboratory experiment.
This setup allows us to generate point-spread functions (PSF) similar to the interferometric PSF of the LBT via a
monochromatic (He-Ne laser) or a polychromatic light source (halogen lamp) and to introduce well defined, fast varying
phase offsets to simulate different atmospheric conditions and sources of instrumental OPD variations via dedicated
actuators.
Furthermore it comprises a piston mirror as actuator to counteract the measured OPD and a CCD camera in the focal
plane as sensor for fringe acquisition which both are substantial devices for a fringe tracking servo loop. The goal of the
setup is to test the performance and stability of different control loop algorithms and to design and optimize the control
approaches.
We present the design and the realization of the testbed interferometer and comment on the fringe-contrast behavior.
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
Fringe and Flexure Tracking System (FFTS) is mandatory for an efficient interferometric operation of LINC-NIRVANA.
It is tailored to compensate low-order phase perturbations in real-time to allow for a time-stable
interference pattern in the focal plane of the science camera during the integration. Two independent control
loops are realized within FFTS: A cophasing loop continuously monitors and corrects for atmospheric and
instrumental differential piston between the two arms of the interferometer. A second loop controls common
and differential image motion resulting from changing orientations of the two optical axes of the interferometer.
Such changes are caused by flexure but also by atmospheric dispersion.
Both loops obtain their input signals from different quadrants of a NIR focal plane array. A piezo-driven
piston mirror in front of the beam combining optics serves as actuator in the cophasing loop. Differential piston
is determined by fitting a parameterized analytical model to the observed point spread function of a reference
target. Tip-tilt corrections in the flexure loop are applied via the secondary mirrors. Image motion is sensed for
each optical axis individually in out-of-focus images of the same reference target.
In this contribution we present the principles of operation, the latest changes in the opto-mechanical design,
the current status of the hardware development.
LINC-NIRVANA is the NIR homothetic imaging camera for the Large Binocular Telescope (LBT). Its Fringe
and Flexure Tracking System (FFTS) is mandatory for an effcient interferometric operation of LINC-NIRVANA:
the task of this cophasing system is to assure a time-stable interference pattern in the focal plane of the camera.
A testbed interferometer, set up as laboratory experiment, is used to develop the FFTS control loop and
to test the robustness of the fringe tracking concept. The geometry of the resulting interferometric intensity
distribution in the focal plane of the implemented CCD corresponds to that of the LBT PSF. The setup allows to
produce monochromatic (He-Ne laser) and polychromatic (halogen lamp) PSFs and allows to actively introduce
well defined low-order phase perturbations, namely OPD and differential tip/tilt. Furthermore, all components
that are required in a fringe tracking servo loop are included: a sensor for fringe acquisition and an actuator
to counteract measured OPD. With this setup it is intended to determine the performance with which a fringe
tracking control loop is able to compensate defined OPD sequences, to test different control algorithms, and to
optimize the control parameters of an existing servo system.
In this contribution we present the design and the realization of the testbed interferometer. Key parameters
describing the white light testbed interferometer, such as fringe contrast and thermal sensitivity are discussed.
The effects of all controllable phase perturbations are demonstrated.
As a near-infrared (NIR) wide field interferometric imager offering an angular resolution of about 10 milliarcseconds
LINC-NIRVANA at the Large Binocular Telescope will be an ideal instrument for imaging the center of the
Milky Way especially in conjunction with mm/sub-mm interferometers like CARMA, ATCA or, in the near
future, ALMA. Sagittarius A* (Sgr A*) is the electromagnetc manifestation of the ~4×106M super-massive
black hole (SMBH) at the Galactic Center. First results from a mult-wavelength campaign focused on Sgr A*,
based on the VLT
and on CARMA, ATCA, and the IRAM 30m-telescope, in May 2007 show that the NIR
data are consistent with partially depolarized non-thermal emission from confined hot spots in relativistic orbits
around SgrA*. A 3mm flare following a May 2007 NIR flare is consistent with SSC emission from adiabatically
expanding plasma in a wind or jet. With the LBT and ALMA we will be able to study the spectral evolution
of NIR/sub-mm/mm flare emission in order to constrain the emission mechanism, the jet/wind physics, and
possibly determine the angular momentum of the SMBH. LINC/NIRVANA will also serve to investigate the
stellar population and dynamics in the cluster surrounding Sgr A*. A particular emphasis will lie on examining
dust embedded and young stars and to unravel the star formation history in the cluster.
For the 0.3 parsec core radius central star cluster the investigation of will be investigated.
LINC-NIRVANA is the NIR homothetic imaging camera for the Large Binocular Telescope (LBT). Its Fringe
and Flexure Tracking System (FFTS) is mandatory for an efficient interferometric operation of LINC-NIRVANA:
the task of this cophasing system is to assure a time-stable interference pattern in the focal plane of the camera.
Differential piston effects will be detected and corrected in a real-time closed loop by analyzing the PSF of
a guide star at a frequency of 100Hz-200Hz. A dedicated piston mirror will then be moved in a corresponding
manner by a piezo actuator. The long-term flexure tip/tilt variations will be compensated by the AO deformable
mirrors.
A testbed interferometer has been designed to simulate the control process of the movement of a scaled
piston mirror under disturbances. Telescope vibration and atmospheric variations with arbitrary power spectra
are induced into the optical path by a dedicated piezo actuator. Limiting factors of the control bandwith are
the sampling frequency and delay of the detector and the resonance frequency of the piston mirror. In our setup
we can test the control performance under realistic conditions by considering the real piston mirrors dynamics
with an appropriate software filter and inducing a artificial delay of the PSF detector signal. Together with
the expected atmospheric OPD variations and a realistic vibration spectrum we are able to quantify the piston
control performance for typical observation conditions. A robust control approach is presented as result from
in-system control design as provided by the testbed interferometer with simulated dynamics.
The Large Binocular Telescope (LBT) is an international collaboration, with partners from the United States, Italy, and
Germany. The telescope uses two 8.4-meter diameter primary mirrors to produce coherent images with the combined
light along with adaptive optics.
The correct functioning and optimum performance of the LBT is only achieved through a complex interplay of various
optical elements. Each of these elements has its individual vibration behaviour, and therefore it is necessary to
characterize the LBT as a distributed vibration system.
LINC-NIRVANA is a near-infrared image-plane beam combiner with advanced, multi-conjugated adaptive optics, and
one of the interferometric instruments for the Large Binocular Telescope (LBT). Its spectral range goes from 1.0 μm to
2.45 μm, therefore the requirements for the maximum optical path difference (OPD) are very tight (λ/10 ~ 100 nm). 1
During two dedicated campaigns, the vibrations introduced by various actuators were measured using different kinds of
sensors. The evaluation of the obtained data allows an estimation of the frequency and amplitude contributions of the
individual vibration sources.
Until the final state of the LBT is reached, further measurements are necessary to optimize and adapt the equipment and
also the investigated elements and configurations (measurement points and directions, number of sensors, etc.).
The correction of atmospheric differential piston and instrumental flexure effects is mandatory for interferometric operation of the LBT NIR interferometric imaging camera LINC-NIRVANA. The task of the Fringe and Flexure Tracking System (FFTS) is to detect and correct these effects in real-time. In the fringe tracking concept that we present, differential piston information is gathered in the image plane by analyzing the PSF of a reference star anywhere in the large field of view of the LBT. We have developed and tested a fast PSF analysis algorithm that allows to clearly identify differential piston even in the case of low S/N. We present performance estimates of the algorithm. Since the performance of the FFTS algorithm has a strong impact on the overall sky coverage of LINC-NIRVANA, we studied the required limiting magnitudes of the fringe tracking reference star for different scenarios. As the FFTS may not necessarily operate on the science target, but rather uses a suitable reference star at a certain angular distance to the science target, differences between piston values at the two positions add to the residual piston of the FFTS. We have dealt with the question of differential piston angular anisoplanatism and studied a possible improvement of the isopistonic patch size by the use of multi-conjugate adaptive optics (MCAO). In its final stage, LINC-NIRVANA will be equipped with such a system.
Current and future opportunities for interferometric observations of the Galactic Center in the near- and mid-infrared (NIR/MIR) wavelength domain are highlighted. Main emphasis is being put on the Large Binocular Telescope (LBT) and the Very Large Telescope Interferometer (VLTI). The Galactic Center measurements of stellar orbits and strongly variable NIR and X-ray emission from Sagittarius A* (SgrA*) at the center of the Milky Way have provided the strongest evidence so far that the dark mass concentration at this position is associated with a super massive black hole. Similar dark mass concentrations seen in many galactic nuclei are most likely super massive black holes as well. High angular resolution interferometric observations in the NIR/MIR will provide key information on the central massive black hole and the stellar cluster it is embedded in. These observations have already started: Recent results on the luminous dust enshrowded star IRS3 using MIDI at the VLTI are presented and future scientific possibilities in the GC using MIDI at the VLTI in the MIR and GRAVITY in the NIR are highlighted. As a NIR wide field interferometric imager offering an angular resolution of about 10 milliarcseconds LINC/NIRVANA at the Large Binocular Telescope will be an ideal instrument for imaging galactic nuclei including the center of the Milky Way.
The correction of atmospheric differential piston and instrumental flexure effects is mandatory for interferometric operation of the LBT NIR interferometric imaging camera LINC-NIRVANA. The task of the Fringe and Flexure Tracking System (FFTS) is to detect and correct these effects in a real-time closed loop. Being a Fizeau-Interferometer, the LBT provides a large field of view (FoV). The FFTS can make use of the large FoV and increase the sky coverage of the overall instrument if it is able to acquire the light of a suitable fringe tracking reference star within the FoV. For this purpose, the FFTS detector needs to be moved to the position of the reference star PSF in the curved focal plane and needs to precisely follow its trajectory as the field rotates. Sub-pixel (1 pixel = 18.5 micron) positioning accuracy is required over a travel range of 200mm x 300mm x 70mm. Strong are the constraints imposed by the need of a cryogenic environment for the moving detector. We present a mechanical design, in which the Detector Positioning Unit (DPU) is realized with off-the-shelf micro-positioning stages, which can be kept at ambient temperature. A moving baffle will prevent the intrusion of radiation from the ambient temperature environment into the cryogenic interior of the camera. This baffle consists of two nested disks, which synchronously follow any derotation - or repositioning trajectory of the DPU. The detector, its fanout board and a filter wheel are integrated into a housing that is mounted on top of the DPU and that protects the FFTS detector from stray light. Long and flexible copper bands allow heat transfer from the housing to the LINC-NIRVANA heat exchanger.
LINC-NIRVANA is the interferometric near-infrared imaging camera for the Large Binocular Telescope (LBT). Being able to observe at wavelength bands from J to K (suppported by an adaptive optics system operating at visible light) LINC-NIRVANA will provide an unique and unprecedented combination of high angular resolution (~ 9 milliarcseconds at 1.25μm), wide field of view (~ 100 arcseconds2 at 1.25μm), and large collecting area (~ 100m2).
One of the major contributions of the 1. Physikalische Institut of the University of Cologne to this project is the development and provision of the Fringe and Flexure Tracking System (FFTS). In addition to the single-eye adaptive optics systems the FFTS is a crucial component to ensure a time-stable wavefront correction over the full aperture of the double-eye telescope, a mandatory pre-requisite for interferometric observations.
Using a independent HAWAII 1 detector array at a combined focus close to the science detector, the Fringe and Flexure Tracking System analyses the complex two-dimensional interferometric point spread function (PSF) of a suitably bright reference source at frame rates of up to several hundred Hertz. By fitting a parameterised theoretical model PSF to the preprocessed image-data the FFTS determines the amount of pistonic phase difference and angular misalignment between the wavefronts of the two optical paths of LINC-NIRVANA. For every exposure the corrective parameters are derived in real-time and transmitted to a dedicated piezo-electric fast linear mirror for simple path lengths adjustments, and/or to the adaptive optics systems of the single-eye telescopes for more complicated corrections.
In this paper we present the basic concept and currect status of the opto-mechanical design of the Fringe and Flexure Tracker, the operating principle of the fringe and flexure tracking loops, and the encouraging result of a laboratory test of the piston control loop.
The Fringe and Flexure Tracking System (FFTS) is designed to correct
the atmospheric piston variations and the instrumental flexure during the NIR interferometric image acquisition of the LINC-NIRVANA camera at the LBT. The interferometric image quality depends on the performance of these corrections.
Differential piston and flexure effects will be detected and corrected in a real-time closed loop by analyzing the PSF of a guide star at a frequency of up to several hundred Hz. A dedicated piston mirror will then be moved in a corresponding manner by a piezo actuator.
The FFTS is expected to provide a residual piston of better then 0.1 λ at the central wavelength of the science band. Thus, the required correction bandwidth is 10-20 Hz as differential piston simulations of different seeing conditions indicate. Therefore, a sampling frequency of 100-200 Hz is required to correct OPD variations. The upper limit for the loop frequency is the resonance frequency of the mirror and the response function respectively.
The piston mirror as the actuator and the FFTS detector as the sensor
feedback are embedded in a very complex system. Many control loop aspects like sampling frequencies, delays, controller algorithm and control bandwidth have to be identified. With accurate simulations of the system the limits of atmospheric and instrumental conditions for reliable closed loops can be determined against the respective control parameters. We present strategies for the closed-loop control of the piston correction which are suitable to achieve the 0.1 λ requirement and the optimal overall imaging performance with a sufficient "all-purpose" control stability.
The MPIA is leading an international consortium of institutes in building an instrument called LINC-NIRVANA, the LBT INterferometric Camera and Near-IR / Visible Adaptive INterferometer for Astronomy. LINC-NIRVANA is a Fizeau interferometer for the Large Binocular Telescope doing imaging in the near infrared (J, H, K - band). Multi-conjugated adaptive objects is used to increase sky coverage and to get diffraction limited images over a 2 arcminute field of view. The LN Common Software provides a software infrastructure common to all partners and consists of a documented collection of common patterns in control systems and of services, which implement those patterns. The heart of LCSW is an object model of controlled devices, implemented as ICE network objects. A code generator creates application from templates for these network objects.
KEYWORDS: Computing systems, Image processing, Interferometry, Near infrared, Signal processing, Atmospheric corrections, Sensors, Control systems, Telescopes, Point spread functions
The correction of atmospheric differential piston and instrumental flexure effects is mandatory for optimum interferometric performance of the LBT NIR interferometric imaging camera LINC-NIRVANA. The task of the Fringe and Flexure Tracking System (FFTS) is to detect and correct these effects in a real-time closed loop. On a timescale of milliseconds, image data of the order of 4K bytes has to be retrieved from the FFTS detector, analyzed, and the results have to be sent to the control system. The need for a reliable communication between several processes within a confined period of time calls for solutions with good real-time performance. We investigated two soft real-time options for the Linux platform. The design we present takes advantage of several features that follow the POSIX standard with improved real-time performance, which were implemented in the new Linux kernel (2.6.12). Several concepts, such as synchronization, shared memory, and preemptive scheduling are considered and the performance of the most time-critical parts of the FFTS software is tested.
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