José Manuel Rebordão

An amplitude segmentation method based on the distribution function of an image

Abstract A method for image segmentation and compression based on the intrinsic properties of the distribution function of an image is presented. This method does not rely on the existence of modes on the histogram. The number of free parameters is reduced, which makes this algorithm essentially automatic and not time consuming.

mWorld: A multiuser 3D virtual environment

mWorld integrates 3D virtual-scene generation tools and synchronous, asynchronous, and time-dependent data communication techniques to enable cooperative 3D design and visualization across networks.

ESPRESSO: the Echelle spectrograph for rocky exoplanets and stable spectroscopic observations

ESPRESSO, the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, will combine the efficiency of modern echelle spectrograph design with extreme radial-velocity precision. It will be installed on ESOs VLT in order to achieve a gain of two magnitudes with respect to its predecessor HARPS, and the instrumental radialvelocity precision will be improved to reach cm/s level. Thanks to its characteristics and the ability of combining incoherently the light of 4 large telescopes, ESPRESSO will offer new possibilities in various fields of astronomy. The main scientific objectives will be the search and characterization of rocky exoplanets in the habitable zone of quiet, nearby G to M-dwarfs, and the analysis of the variability of fundamental physical constants. We will present the ambitious scientific objectives, the capabilities of ESPRESSO, and the technical solutions of this challenging project.

Accuracy of frequency-sweeping interferometry for absolute distance metrology

Coherent interferometric absolute distance metrology is one of the most interesting techniques for length metrology. Without movement, measurements are made without ambiguity, by using either one or several synthetic wavelengths resulting from the beating of two or more wavelengths (multiple-wavelength interferometry), or a frequency sweep (frequency-sweeping interferometry). Sensors based on the latter are relatively simple devices and can fulfill an important role in dimensional metrology. In addition, their parameterization flexibility allows trade-offs to be performed, either technology-driven or application-related. We present in detail a theoretical model of frequency-sweeping interferometry and its uncertainty budget, discuss different parameterizations, present a method for drift error compensation, and demonstrate and evaluate sensor performance and robustness with a prototype sensor composed of a mode-hop-free frequency-sweep external-cavity diode laser, a high-finesse Fabry-Perot interferometer (to measure accurately the frequency sweep range), homodyne detection, and data processing. Results have shown that the inaccuracy will not exceed 10 μm for distances up to 1 m, at an affordable degree of complexity.

A Discretized Linear Elastic Model for Cloth Buckling and Drape

We present a computational model for plain woven fabrics. The model is able to represent known elastic behavior in deformation such as planar extension, shearing and out-of-plane bending, drape, and buckling. Buckling behavior is present both in shear and compression. The cloth is assumed to be an orthotropic linear elastic continuum, dis cretized by a mesh of triangles. Each triangle links three particles and is able to measure the stress and strain of the underlying medium. For the planar deformation, we assume the hypothesis of the plate under plane stress from the classical theory of elasticity. For the out-of-plane deformation, we allow linear elasticity and nonlinear displacement in bend ing, by modeling curvature interactions along the edges of neighboring triangles. To formulate the dynamic equilibrium of the mechanical system, we use Newtons second law. Nonlinear elastic material behavior is made possible by piecewise linear approxi mation of measured data. Visual results of cloth under various deformation conditions are presented—in tensile, draping, and buckling situations.

Uncovering the kiloparsec-scale stellar ring of NGC 5128 ,

We reveal the stellar light emerging from the kiloparsec-scale, ring-like structure of the NGC 5128 (Centaurus A) galaxy in unprecedented detail. We use arcsecond-scale resolution near infrared images to create a “dust-free” view of the central region of the galaxy, which we then use to quantify the shape of the revealed structure. At the resolution of the data, the structure contains several hundreds of discreet, point-like or slightly elongated sources. The typical extinction-corrected surface brightness of the structure is KS ≈ 16. 5m ag/arcsec 2 , and we estimate the total near infrared luminosity of the structure to be M ≈− 21 mag. We use diffraction limited (FWHM resolution of ≈0.1 �� , or 1.6 pc) near infrared data taken with the NACO instrument on the VLT to show that the structure decomposes into thousands of separate, mostly point-like sources. According to the tentative photometry, the most luminous sources have MK ≈− 12 mag, making them red supergiants or relatively low-mass star clusters. We also discuss the large-scale geometry implied by the reddening signatures of dust in our near infrared images.

Absolute distance metrology with frequency sweeping interferometry

Coherent interferometric absolute distance metrology is one of the most interesting techniques for length metrology. Without any movement, measurements are made without ambiguity, by using either one or several synthetic wavelengths resulting from the beating of two or more wavelengths (multiple wavelength interferometry) or, in the case of frequency sweeping interferometry (FSI), from a frequency sweep. FSI-based sensors are relatively simple devices and can fulfill an important role on the metrology chain, even for very small relative errors in the context of demanding applications (such as space). In addition, their parameterization flexibility allows tradeoffs to be performed, either technology driven or application related. In the context of the ESA/Darwin technology package, we implemented a FSI sensor composed of a mode-hop free frequency sweep external cavity diode laser, a high finesse Fabry-Perot interferometer to measure accurately the frequency sweep range, homodyne detection and data processing. In this paper, we present in detail the uncertainty budget for the FSI final uncertainty, give examples of different parameterizations, and demonstrate and evaluate sensor performances and robustness for the high precision optical metrology for the Darwin satellite configuration.

3D Finite Element Model for Writing Long-Period Fiber Gratings by CO2 Laser Radiation

In the last years, mid-infrared radiation emitted by CO2 lasers has become increasing popular as a tool in the development of long-period fiber gratings. However, although the development and characterization of the resulting sensing devices have progressed quickly, further research is still necessary to consolidate functional models, especially regarding the interaction between laser radiation and the fibers material. In this paper, a 3D finite element model is presented to simulate the interaction between laser radiation and an optical fiber and to determine the resulting refractive index change. Dependence with temperature of the main parameters of the optical fiber materials (with special focus on the absorption of incident laser radiation) is considered, as well as convection and radiation losses. Thermal and residual stress analyses are made for a standard single mode fiber, and experimental results are presented.

System overview of the VLTI Spectro-Imager

The VLTI Spectro Imager project aims to perform imaging with a temporal resolution of 1 night and with a maximum angular resolution of 1 milliarcsecond, making best use of the Very Large Telescope Interferometer capabilities. To fulfill the scientific goals (see Garcia et. al.), the system requirements are: a) combining 4 to 6 beams; b) working in spectral bands J, H and K; c) spectral resolution from R= 100 to 12000; and d) internal fringe tracking on-axis, or off-axis when associated to the PRIMA dual-beam facility. The concept of VSI consists on 6 sub-systems: a common path distributing the light between the fringe tracker and the scientific instrument, the fringe tracker ensuring the co-phasing of the array, the scientific instrument delivering the interferometric observables and a calibration tool providing sources for internal alignment and interferometric calibrations. The two remaining sub-systems are the control system and the observation support software dedicated to the reduction of the interferometric data. This paper presents the global concept of VSI science path including the common path, the scientific instrument and the calibration tool. The scientific combination using a set of integrated optics multi-way beam combiners to provide high-stability visibility and closure phase measurements are also described. Finally we will address the performance budget of the global VSI instrument. The fringe tracker and scientific spectrograph will be shortly described.

Primary laser vibration metrology: evaluation of the rocking motion impact in the accuracy of acceleration measurements

Optical interferometry for the absolute calibration of standard accelerometers is based on displacement amplitude measurements considering a uniaxial sinusoidal excitation movement at a given frequency. In reality, the movement generated by a shaker also contains components perpendicular to the oscillation axis, introducing a rocking motion effect. In the primary calibration of vibrations by laser interferometry, the rocking motion is a critical issue to be considered for high accuracy measurements. The knowledge of the impact of this effect in the performances of acceleration amplitude measurement is fundamental for the definition of a robust calibration approach. Generally, this effect increases with the excitation frequency and, beyond a certain threshold, its influence in the final result may become quite relevant. In this work, we study the influence of the rocking motion in the calibration of one accelerometer with two shaker models. The study comprises a nominal acceleration of 100 m.s-2 for frequencies between 1 kHz and 9 kHz, considering a sinusoidal excitement. An interferometric system based on heterodyne detection was used for the high frequency regime. Measurements were performed for 12 incidence points equally spaced along the border of the surface of a dummy mass attached to the standard accelerometer, and the corresponding average was estimated, allowing the characterisation of the rocking motion effect and the estimation of the corresponding component in the expanded uncertainty budget.