This article describes the operation of the near-infrared wavefront sensing based Adaptive Optics (AO) system CIAO. The Coudé Infrared Adaptive Optics (CIAO) system is a central auxiliary component ...of the Very Large Telescope (VLT) interferometer (VLTI). It enables in particular the observations of the Galactic Center (GC) using the GRAVITY instrument. GRAVITY is a highly specialized beam combiner, a device that coherently combines the light of the four 8-m telescopes and finally records interferometric measurements in the K-band on 6 baselines simultaneously. CIAO compensates for phase disturbances caused by atmospheric turbulence, which all four 8 m Unit Telescopes (UT) experience during observation. Each of the four CIAO units generates an almost diffraction-limited image quality at its UT, which ensures that maximum flux of the observed stellar object enters the fibers of the GRAVITY beam combiner. We present CIAO performance data obtained in the first 3 years of operation as a function of weather conditions. We describe how CIAO is configured and used for observations with GRAVITY. In addition, we focus on the outstanding features of the near-infrared sensitive Saphira detector, which is used for the first time on Paranal, and show how it works as a wavefront sensor detector.
ABSTRACT
For sensitive optical interferometry, it is crucial to control the evolution of the optical path difference (OPD) of the wavefront between the individual telescopes of the array. The OPD ...between a pair of telescopes is induced by differential optical properties such as atmospheric refraction, telescope alignment, etc. This has classically been measured using a fringe tracker that provides corrections to a piston actuator to account for this difference. An auxiliary method, known as the Piston Reconstruction Experiment (P-REx), has been developed to measure the OPD, or differential ‘piston’ of the wavefront, induced by the atmosphere at each telescope. Previously, this method was outlined and results obtained from Large Binocular Telescope adaptive optics data for a single telescope aperture were presented. P-REx has now been applied off-line to previously acquired Very Large Telescope Intereferometer (VLTI)’s GRAVITY Coudé Infrared Adaptive Optics wavefront sensing data to estimate the atmospheric OPD for the six baselines. Comparisons with the OPD obtained from the VLTI GRAVITY fringe tracker were made. The results indicate that the telescope and instrumental noise of the combined VLTI and GRAVITY systems dominates over the atmospheric turbulence contributions. However, good agreement between simulated and on-sky P-REx data indicates that if the telescope and instrumental noise was reduced to atmospheric piston noise levels, P-REx has the potential to reduce the OPD root mean square of piston turbulence by up to a factor of 10 for frequencies down to 1 Hz. In such conditions, P-REx will assist in pushing the sensitivity limits of optical fringe tracking with long baseline interferometers.
ABSTRACT
We analyse high spatial resolution near infrared (NIR) imaging of NGC 6384, a Milky Way like galaxy, using ARGOS commissioning data at the Large Binocular Telescope (LBT). ARGOS provides a ...stable PSFFWHM = 0${^{\prime\prime}_{.}}$2–0${^{\prime\prime}_{.}}$3 AO correction of the ground layer across the LUCI 2 NIR camera 4 × 4 arcmin field by using six laser guide stars (three per telescope) and a natural guide star for tip-tilt sensing and guiding. Enabled by this high spatial resolution, we analyse the structure of the nuclear star cluster (NSC) and the central kiloparsec of NGC 6384. We find via 2D modelling that the NSC (reff ≃ 10 pc) is surrounded by a small (reff ≃ 100 pc) and a larger Sersić (reff ≃ 400 pc), all embedded within the NGC 6384 large-scale boxy/X-shaped bulge and disc. This proof-of-concept study shows that with the high spatial resolution achieved by ground-layer AO we can push such analysis to distances previously only accessible from space. SED-fitting to the NIR and optical HST photometry allowed to leverage the age–metallicity–extinction degeneracies and derive the effective NSC properties of an young to old population mass ratio of $8{{\ \rm per\ cent}}$ with ${\cal M}_{\rm \star ,old} \simeq 3.5\times 10^7\, \mathrm{M}_\odot$, Ageold, young = 10.9 ± 1.3 Gyr and 226 Myr $\pm 62{{\ \rm per\ cent}}$, metallicity M/H = −0.11 ± 0.16, and $0.33\pm 39{{\ \rm per\ cent}}$ dex, and E(B − V) = 0.63 and 1.44 mag.
This paper presents a unified approach to load generation in IP-based networks supported by a Unified Load Generator UniLoG which incorporates a formal automata-based load specification technique. ...The load specification technique is applied to two exemplarily chosen models for VoIP and MPEG-coded video traffic sources in order to use them for load generation in UniLoG. The performance characteristics of UniLoG modules, which are responsible for the injection of real traffic loads at different interfaces in IP networks, are discussed and the practical use of UniLoG is demonstrated in the context of a comprehensive QoS study of video streaming via an IEEE 802.11g WLAN under various background loads.
For sensitive optical interferometry, it is crucial to control the evolution of the optical path difference (OPD) of the wavefront between the individual telescopes of the array. The OPD between a ...pair of telescopes is induced by differential optical properties such as atmospheric refraction, telescope alignment, etc. This has classically been measured using a fringe tracker that provides corrections to a piston actuator to account for this difference. An auxiliary method, known as the Piston Reconstruction Experiment (P-REx) has been developed to measure the OPD, or differential 'piston' of the wavefront, induced by the atmosphere at each telescope. Previously, this method was outlined and results obtained from LBT adaptive optics (AO) data for a single telescope aperture were presented. P-REx has now been applied off-line to previously acquired VLT's GRAVITY CIAO wavefront sensing data to estimate the atmospheric OPD for the six VLTI baselines. Comparisons with the OPD obtained from the VLTI GRAVITY fringe tracker were made. The results indicate that the telescope and instrumental noise of the combined VLTI and GRAVITY systems dominate over the atmospheric turbulence contributions. However, good agreement between simulated and on-sky P-REx data indicates that if the telescope and instrumental noise were reduced to atmospheric piston noise levels, P-REx has the potential to reduce the OPD root mean square of piston turbulence by up to a factor of 10 for frequencies down to 1 Hz. In such conditions, P-REx will assist in pushing the sensitivity limits of optical fringe tracking with long baseline interferometers.
This article describes the operation of the near-infrared wavefront sensing based Adaptive Optics (AO) system CIAO. The Coudé Infrared Adaptive Optics (CIAO) system is a central auxiliary component ...of the Very Large Telescope (VLT) interferometer (VLTI). It enables in particular the observations of the Galactic Center (GC) using the GRAVITY instrument. GRAVITY is a highly specialized beam combiner, a device that coherently combines the light of the four 8-m telescopes and finally records interferometric measurements in the K-band on 6 baselines simultaneously. CIAO compensates for phase disturbances caused by atmospheric turbulence, which all four 8 m Unit Telescopes (UT) experience during observation. Each of the four CIAO units generates an almost diffraction-limited image quality at its UT, which ensures that maximum flux of the observed stellar object enters the fibers of the GRAVITY beam combiner. We present CIAO performance data obtained in the first 3 years of operation as a function of weather conditions. We describe how CIAO is configured and used for observations with GRAVITY. In addition, we focus on the outstanding features of the near-infrared sensitive Saphira detector, which is used for the first time on Paranal, and show how it works as a wavefront sensor detector.
We analyse high spatial resolution near infra-red (NIR) imaging of NGC6384, a Milky Way like galaxy, using ARGOS commissioning data at the Large Binocular Telescope (LBT). ARGOS provides a stable ...PSF\(_{\rm FWHM}\!=\!0.2"\!-\!0.3"\) AO correction of the ground layer across the LUCI2 NIR camera \(4'\!\times4'\) field by using six laser guide stars (three per telescope) and a natural guide star for tip-tilt sensing and guiding. Enabled by this high spatial resolution we analyse the structure of the nuclear star cluster (NSC) and the central kiloparsec of NGC6384. We find via 2D modelling that the NSC (\(r_{\rm eff}\!\simeq\!10\)pc) is surrounded by a small (\(r_{\rm eff}\!\simeq\!100\)pc) and a larger Sersić (\(r_{\rm eff}\!\simeq\!400\)pc), all embedded within the NGC\,6384 large-scale boxy/X-shaped bulge and disk. This proof-of-concept study shows that with the high spatial resolution achieved by ground-layer AO we can push such analysis to distances previously only accessible from space. SED-fitting to the NIR and optical HST photometry allowed to leverage the age-metallicity-extinction degeneracies and derive the effective NSC properties of an young to old population mass ratio of \(8\%\) with \({\cal M}_{\rm\star,old}\!\simeq\!3.5\times10^7M_\odot\), Age\(_{\rm old,\ young}\!=\!10.9\pm1.3\)Gyr and 226\,Myr \(\pm62\%\), metallicity M/H\(=\!-0.11\pm0.16\) and \(0.33\pm39\%\)dex, and \(E(B\!-\!V)\!=\!0.63\) and 1.44mag.
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