D. Peter

Massive binaries in the Cepheus OB2/3 region - Constraining the formation mechanism of massive stars

Context. Two different formation scenarios for stars of masses larger than 10 M exist. Although simulations within both scenarios are capable of producing stars up to the highest observed masses, the relevance of the two formation scenarios for massive star-formation is not yet clear. Aims. We aim to detect companions to massive stars to constrain the binary parameters of the multiple systems. These findings will help to constrain the formation of massive stars. Methods. We performed z′-band observations of massive and intermediate-mass stars in the Cep OB2/3 associations with the Lucky imaging camera AstraLux on the 2.2 m telescope of the Calar Alto observatory. The analysis aimed at detecting binary systems with separations out to 2. ′′2 (∼1700 AU), the inner limit depending on the contrast. The maximum contrast of 7 mag in z′ (corresponding to a mass contrast of 17:1 versus an O9V primary) requires a minimum separation of 0. ′′7 (∼550 AU). Results. We found 28 new companions for our sample of 148 intermediate-mass and massive stars in Cep OB2/3. The companion star fraction of the massive stars is 0.7; about 50% of the systems are triples. The mass function of the companions to the massive stars is strongly top-heavy. We found that the sample parameters closely agreed with those found in the Orion Trapezium cluster. Conclusions. The multiplicity of massive stars seems to be significantly higher than that of intermediate-mass stars, independently of the environment. The comparison of our findings with the theories of massive star-formation favor the formation of massive stars by the fragmentation of proto-stellar cores combined with competitive accretion.

ARGOS - The laser guide star system for the LBT

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 LBTs adaptive secondary mirrors. Utilizing high power pulsed green lasers the artificial beacons are generated via Rayleigh scattering in earths atmosphere. ARGOS will project a set of three guide stars above each of LBTs 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.

PYRAMIR: first on-sky results from an infrared pyramid wavefront sensor

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.

Status of the ARGOS project

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.

Practical experience with test-driven development during commissioning of the multi-star AO system ARGOS

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.

Vibration control for the ARGOS laser launch path

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.

Calibration strategy and optics for ARGOS at the LBT

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.

An optimized controller for ARGOS: using multiple wavefront sensor signals for homogeneous correction over the field

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.

SUPY: an infrared pyramid wavefront sensor for Subaru

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.

P-REx: the piston drift reconstruction experiment

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.