For sensitive infra-red long-baseline interferometry, it is crucial to control the differential piston between the apertures. Classically this is achieved with a fringe tracker which measures the movement of the interferometric fringes. In this paper, we describe a new method to reconstruct the piston variation introduced by atmospheric turbulence with real-time data from adaptive optics wave-front sensing. Concurrently, the dominant wind speed vector can also be retrieved. The method is analyzed in simulation for atmospheric turbulence of various strength, and wind vectors varying with layer altitude. The results from the simulations show that this method could help to reliably retrieve the piston variation and wind speed from wavefront sensor data. The method is related to concepts of predictive control AO algorithms and reconstruction of the point spread function.
One year and an half after ARGOS first light, the Large Binocular Telescope (LBT) laser guided ground-layer adaptive optics (GLAO) system has been operated on both sides of the LBT. The system fulfills the GLAO promise and typically delivers an improvement by a factor of 2 in FWHM over the 4'×4' field of view of both Luci instruments, the two near-infrared imagers and multi-object spectrographs.
In this paper, we report on the first on-sky results and analyze the performances based on the data collected so far. We also discuss adaptive optics procedures and the joint operations with Luci for science observations.
ARGOS is the Laser Guide Star and Wavefront sensing facility for the Large Binocular Telescope. With first laser light on sky in 2013, the system is currently undergoing commissioning at the telescope. We present the overall status and design, as well as first results on sky. Aiming for a wide field ground layer correction, ARGOS is designed as a multi- Rayleigh beacon adaptive optics system. A total of six powerful pulsed lasers are creating the laser guide stars in constellations above each of the LBTs primary mirrors. With a range gated detection in the wavefront sensors, and the adaptive correction by the deformable secondary’s, we expect ARGOS to enhance the image quality over a large range of seeing conditions. With the two wide field imaging and spectroscopic instruments LUCI1 and LUCI2 as receivers, a wide range of scientific programs will benefit from ARGOS. With an increased resolution, higher encircled energy, both imaging and MOS spectroscopy will be boosted in signal to noise by a large amount. Apart from the wide field correction ARGOS delivers in its ground layer mode, we already foresee the implementation of a hybrid Sodium with Rayleigh beacon combination for a diffraction limited AO performance.
Commissioning time for an instrument at an observatory is precious, especially the night time. Whenever
astronomers come up with a software feature request or point out a software defect, the software engineers have
the task to find a solution and implement it as fast as possible. In this project phase, the software engineers
work under time pressure and stress to deliver a functional instrument control software (ICS). The shortness of
development time during commissioning is a constraint for software engineering teams and applies to the ARGOS
project as well. The goal of the ARGOS (Advanced Rayleigh guided Ground layer adaptive Optics System)
project is the upgrade of the Large Binocular Telescope (LBT) with an adaptive optics (AO) system consisting
of six Rayleigh laser guide stars and wavefront sensors. For developing the ICS, we used the technique Test-
Driven Development (TDD) whose main rule demands that the programmer writes test code before production
code. Thereby, TDD can yield a software system, that grows without defects and eases maintenance. Having
applied TDD in a calm and relaxed environment like office and laboratory, the ARGOS team has profited from
the benefits of TDD. Before the commissioning, we were worried that the time pressure in that tough project
phase would force us to drop TDD because we would spend more time writing test code than it would be worth.
Despite this concern at the beginning, we could keep TDD most of the time also in this project phase
This report describes the practical application and performance of TDD including its benefits, limitations
and problems during the ARGOS commissioning. Furthermore, it covers our experience with pair programming
and continuous integration at the telescope.
ARGOS the Advanced Rayleigh guided Ground layer adaptive Optics System for the LBT (Large Binocular Telescope)
is built by a German-Italian-American consortium. It will be a seeing reducer correcting the turbulence in the lower
atmosphere over a field of 2' radius. In such way we expect to improve the spatial resolution over the seeing of about a
factor of two and more and to increase the throughput for spectroscopy accordingly. In its initial implementation,
ARGOS will feed the two near-infrared spectrograph and imager - LUCI I and LUCI II.
The system consist of six Rayleigh lasers - three per eye of the LBT. The lasers are launched from the back of the
adaptive secondary mirror of the LBT. ARGOS has one wavefront sensor unit per primary mirror of the LBT, each of the
units with three Shack-Hartmann sensors, which are imaged on one detector.
In 2010 and 2011, we already mounted parts of the instrument at the telescope to provide an environment for the main
sub-systems. The commissioning of the instrument will start in 2012 in a staged approach. We will give an overview of
ARGOS and its goals and report about the status and new challenges we encountered during the building phase. Finally
we will give an outlook of the upcoming work, how we will operate it and further possibilities the system enables by
design.
Present and future adaptive optics systems aim for the correction of the atmospheric turbulence over a large
field of view combined with large sky coverage. To achieve this goal the telescope is equipped with multiple laser
beacons. Still, to measure tip-tilt aberrations a natural guide star is used. For some fields such a tilt-star is not
available and a correction on the laser beacons alone is applied. For this method to work well the laser beacons
must not be affected by telescope vibrations on their up-link path.
For the ARGOS system the jitter of the beacons is specified to be below 0.05. To achieve this goal a vibration compensation system is necessary to mitigate the mechanical disturbances. The ARGOS vibration compensation system is an accelerometer based feed forward system. The accelerometer measurements are fed into a real time controller. To achieve high performance the controller of the system is model based. The output is applied to a fast steering mirror. This paper presents the concept of the ARGOS vibration compensation, the hardware, and laboratory results.
ARGOS, the laser-guided adaptive optics system for the Large Binocular Telescope (LBT), is now under construction at
the telescope. By correcting atmospheric turbulence close to the telescope, the system is designed to deliver high
resolution near infrared images over a field of 4 arc minute diameter. Each side of the LBT is being equipped with three
Rayleigh laser guide stars derived from six 18 W pulsed green lasers and projected into two triangular constellations
matching the size of the corrected field. The returning light is to be detected by wavefront sensors that are range gated
within the seeing-limited depth of focus of the telescope. Wavefront correction will be introduced by the telescope's
deformable secondary mirrors driven on the basis of the average wavefront errors computed from the respective guide
star constellation. Measured atmospheric turbulence profiles from the site lead us to expect that by compensating the
ground-layer turbulence, ARGOS will deliver median image quality of about 0.2 arc sec across the JHK bands. This will
be exploited by a pair of multi-object near-IR spectrographs, LUCIFER1 and LUCIFER2, with 4 arc minute field already
operating on the telescope. In future, ARGOS will also feed two interferometric imaging instruments, the LBT
Interferometer operating in the thermal infrared, and LINC-NIRVANA, operating at visible and near infrared
wavelengths. Together, these instruments will offer very broad spectral coverage at the diffraction limit of the LBT's
combined aperture, 23 m in size.
We describe the optical design of a calibration unit for the off-axis laser guide stars at the Large Binocular Telescope's ARGOS facility. Artificial stars with the desired wavefront are created using a computer generated hologram.
ARGOS is the Laser Guide Star adaptive optics system for the Large Binocular Telescope. Aiming for a wide field
adaptive optics correction, ARGOS will equip both sides of LBT with a multi laser beacon system and corresponding
wavefront sensors, driving LBT's adaptive secondary mirrors. Utilizing high power pulsed green lasers the artificial
beacons are generated via Rayleigh scattering in earth's atmosphere. ARGOS will project a set of three guide stars above
each of LBT's mirrors in a wide constellation. The returning scattered light, sensitive particular to the turbulence close to
ground, is detected in a gated wavefront sensor system. Measuring and correcting the ground layers of the optical
distortions enables ARGOS to achieve a correction over a very wide field of view. Taking advantage of this wide field
correction, the science that can be done with the multi object spectrographs LUCIFER will be boosted by higher spatial
resolution and strongly enhanced flux for spectroscopy. Apart from the wide field correction ARGOS delivers in its
ground layer mode, we foresee a diffraction limited operation with a hybrid Sodium laser Rayleigh beacon combination.
ARGOS is the ground layer adaptive optics system planned for the LBT. The goal of such a ground layer
adaptive optics system is to provide a maximum homogeneity of the point spread function over the full field of
view. Controllers for optimized correction with an adaptive optics system with guide star and science target at
different field angles are well known in the case of a single guide star. As ARGOS uses three laser guide stars and
one auxiliary natural guide star a weighting scheme is required to optimize the homogeneity using all available
information. Especially the tip and tilt modes measured by the one single off axis guide star and estimated
thereof over the field will need to be improved by incorporation of the laser measurements. I will present the
full scheme for an optimized controller for the ARGOS system. This controller uses the wavefront signals of
the three lasers to additionally reconstruct the lower atmosphere. Information on the higher atmosphere will be
provided by a DIMM-MASS instrument. The control scheme is tested analytically and the variation of the point
spread function is then measured over the full field.
ARGOS is the ground layer adaptive optics system planned for the LBT. Although ground layer adaptive optics
systems provide a relatively homogeneous point spread function over the full field of view there will be still
variations of the order of a few 10%. In the case of a laser based GLAO system these variations are mostly due
to the facts that only one auxiliary guide star is used and to the cone effect. Astrometry and photometry on
AO corrected images are effected by these changes in the shape of the point spread function with field angle.
To reduce this effect for the ARGOS system a scheme for point spread function reconstruction from wavefront
sensor data is developed. The scheme uses the wavefront sensor data twofold: To reconstruct the wavefront
and to measure the atmospheric profile via SLODAR. The reconstruction scheme is tested in simulations of the
full system for various seeing conditions and guide star angles. The quality of the reconstruction is tested in
simulation.
Effective calibration procedures play an important role for the efficiency and performance of astronomical
instrumentation. We report on the calibration scheme for ARGOS, the Laser Guide Star (LGS) facility at the LBT. An
artificial light source is used to feign the real laser beacons and perform extensive testing of the system, independent of
the time of day and weather conditions, thereby greatly enhancing the time available for engineering. Fibre optics and
computer generated holograms (CGHs) are used to generate the necessary wavefront. We present the optomechanical
design, and discuss the expected accuracy, as well as tolerances in assembly and alignment.
The 8 m SUBARU telescope atop Mauna Kea on Hawaii will shortly be equipped with a 188 actuator adaptive optics system (AO 188). Additionally it will be equipped with a Laser guide star (LGS) system to increase the sky coverage of that system. One of the additional tip-tilt sensor which is required to operate AO 188 in LGS mode will be working in the infrared to further enhance the coverage in highly obscured regions of the sky. Currently, various options for this sensor are under study, however the baseline design is a pyramid wavefront sensor. It is currently planned to have this sensor be able to provide also information on higher modes in order to feed AO 188 alone, i.e. without the LGS when NIR-bright guide stars are available. In this paper, we will present the results of the basic design tradeoffs, the performance analysis, and the project plan. Choices to be made concern the number of subapertures available across the primary mirror, the number of corrected modes, control of the AO system in combination with and without LGS, the detector of the wavefront sensor, the operation wavelength range and so forth. We will also present initial simulation results on the expected performance of the device, and the overall timeline and project structure.
In this paper we present an overview of the construction and implementation of the unmodulated infrared pyramid wavefront sensor PYRAMIR at the Calar Alto 3.5 m telescope. PYRAMIR is an extension of the existing visible Shack-Hartmann adaptive optics system ALFA, which allows wavefront sensing in the near-infrared wavefront regime. We describe the optical setup and the calibration procedure of the pyramid wavefront sensor. We discuss possible drawbacks of the calibration and show the results gained on Calar Alto.
PYRAMIR is a pyramid wavefront sensor (PWFS) for the 97-actuator AO system installed on the Calar Alto 3.5 m
telescope. With its linear pupil sampling of 18 pixels, its maximum loop frequency of 140 Hz, and its sensing
wavelength range from 1.1 micron to 2.4 micron it should be able to deliver reasonably high Strehl ratios at the sensing
wavelength. This feature is still unique in the world of pyramid sensors. The first on-sky test of the system was carried
out in March 2006. In this paper we will present the first results of this test. Strehl measurements medium atmospheric
conditions, using reference stars of mJ=8mag and mJ=4 mag and were performed during this first on-sky run. A detailed
comparison to simulation results will also be presented in order to confirm whether the system works up to expectances.
While this experiment has not yet the potential to show for the very first time the superiority of the pyramid principle
over corresponding Hartmann-Shack systems in a real telescope environment, it was confirmed that PYRAMIR
performs up to expectances and a detailed comparison to the Shack-Hartmann system can be carried out in the next run.
This paper describes the whole process of designing, manufacturing and assembling the optics for an infrared pyramid wavefront sensor, called PYRAMIR. This sensor is built to work with the adaptive optical system at the 3.5 m telescope of the Calar Alto Observatory, Spain, which controls a 97 actuator deformable mirror. PYRAMIR is working in combination with an infrared science camera, which is used for observations. Since the wavefront sensor works in the near infrared (1.0 μm to 2.4 μm), the detector, the optics and all the mechanics are cooled to liquid nitrogen temperature. For this cryogenic condition, special care has to be taken for the optical design and the mounting of the lenses. We describe in detail the process from infrared optical design and cryo-mechanical engineering, to the final assembly of the opto-mechanical units and testing in the lab. Technical solutions are illustrated and the final performance is demonstrated.
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