Optical measurements of phase steps in segmented mirrors - fundamental precision limits

Phase steps are an important type of wavefront aberrations generated by large telescopes with segmented mirrors. In a closed-loop correction cycle these phase steps have to be measured with the highest possible precision using natural reference stars, that is with a small number of photons. In this paper the classical Fisher information of statistics is used for calculating the Cramer-Rao bound, which determines the limit to the precision with which the height of the steps can be estimated in an unbiased fashion with a given number of photons and a given measuring device. Four types of measurement devices are discussed: a Shack-Hartmann sensor with one small cylindrical lenslet covering a sub-aperture centred over a border, a modified Mach-Zehnder interferometer, a Foucault test, and a curvature sensor. The Cramer-Rao bound is calculated for all sensors under ideal conditions, that is narrowband measurements without additional noise or disturbances apart from the photon shot noise. This limit is compared with the ultimate quantum statistical limit for the estimate of such a step which is independent of the measuring device. For the Shack-Hartmann sensor, the effects on the Cramer-Rao bound of broadband measurements, finite sampling, and disturbances such as atmospheric seeing and detector readout noise are also investigated. The methods presented here can be used to compare the precision limits of various devices for measuring phase steps and for optimising the parameters of the devices. Under ideal conditions the Shack-Hartmann and the Foucault devices nearly attain the ultimate quantum statistical limits, whereas the Mach-Zehnder and the curvature devices each require approximately twenty times as many photons in order to reach the same precision.Comment: 23 pages, 19 figures, to be submitted to Journal of Modern Optic

Extinction controlled Adaptive Mask Coronagraph Lyot and Phase Mask dual concept for wide extinction area

A dual coronagraph based on the Adaptive Mask concept is presented in this paper. A Lyot coronagraph with a variable diameter occulting disk and a nulling stellar coronagraph based on the Adaptive Phase Mask concept using polarization interferometry are presented in this work. Observations on sky and numerical simulations show the usefulness of the proposed method to optimize the nulling efficiency of the coronagraphs. In the case of the phase mask, the active control system will correct for the detrimental effects of image instabilities on the destructive interference (low-order aberrations such as tip-tilt and focus). The phase mask adaptability both in size, phase and amplitude also compensate for manufacturing errors of the mask itself, and potentially for chromatic effects. Liquid-crystal properties are used to provide variable transmission of an annulus around the phase mask, but also to achieve the achromatic π phase shift in the core of the PSF by rotating the polarization by 180°.A compressed mercury (Hg) drop is used as an occulting disk for the Lyot mask, its size control offers an adaptation to the seeing conditions and provides an optimization of the Tip-tilt correction

Preferências estabelecidas e personalidade: uma abordagem comportamental

TCC (graduação) - Universidade Federal de Santa Catarina. Centro Sócio-Econômico. Economia.Este estudo tem o objetivo de contribuir para o entendimento da dissonância cognitiva que surge quando preferências estabelecidas com relação a produtos são alteradas à medida em que os consumidores se deparam com informação mais completa do que a existente na escolha inicial. Buscou-se verificar a probabilidade de abandono da escolha preliminar na amostra de consumidores analisada. A contribuição se refere à mediação da habilidade cognitiva e da personalidade nessas decisões. No experimento, os participantes com fraca preferência na escolha inicial – e baixa habilidade cognitiva, além de baixos escores em traços de honestidade-humildade, extroversão e conscienciosidade – tendem a abandonar a escolha inicial. Já os participantes com escores mais altos em todas essas características tendem a não abandonar a sua escolha preliminar

Accretion-ejection morphology of the microquasar SS 433 resolved at sub-au scale

This is the author accepted manuscript. the final version is available from EDP Sciences via the DOI in this recordWe present the first optical observation of the microquasar SS 433 at sub-milliarcsecond (mas) scale obtained with the GRAVITY instrument on the Very Large Telescope interferometer (VLTI). The 3.5-h exposure reveals a rich K-band spectrum dominated by hydrogen Brγand He i lines, as well as (red-shifted)emission lines coming from the jets. The K-band-continuum-emitting region is dominated by a marginally resolved point source (<1 mas) embedded inside a diffuse background accounting for 10% of the total flux. The jet line positions agree well with the ones expected from the jet kinematic model, an interpretation also supported by the consistent sign (i.e., negative/positive for the receding/approaching jet component) of the phase shifts observed in the lines. The significant visibility drop across the jet lines, together with the small and nearly identical phases for all baselines, point toward a jet that is offset by less than 0.5 mas from the continuum source and resolved in the direction of propagation, with a typical size of 2 mas. The jet position angle of ~80° is consistent with the expected one at the observation date. Jet emission so close to the central binary system would suggest that line locking, if relevant to explain the amplitude and stability of the 0.26c jet velocity, operates on elements heavier than hydrogen. The Brγprofile is broad and double peaked. It is better resolved than the continuum and the change of the phase signal sign across the line on all baselines suggests an East-West-oriented geometry similar to the jet direction and supporting a (polar) disk wind origin.Centre National d’Etudes Spatiales (CNES)Programme National Hautes Energies (PNHE)Humboldt FoundationNAS

Lessons learned with the Active Phasing Experiment: comparison of four optical phasing sensors on a segmented Very Large Telescope

The adaptive optics capabilities are strongly limited by the quality of the phasing of the primary mirror of the extremely large telescope. Up to date, the Keck telescopes are the only segmented telescope phased with a quality enabling the application of adaptive optics. The Active Phasing Experiment has been installed at the Namyth focus of the Very Large Telescope Melipal during the last 6 months. Its purpose is to understand and compare different technological concepts for an optical phasing sensor dedicated to the European Extremely Large Telescope. The pupil of the telescope is segmented in 61 hexagonal segments by projecting it on an Active Segmented Mirror. The ASM is controlled by a dual wavenlength interferometer made by Fogale Nanotech with a nanometric precision. The segmented pupil is distributed in parallel to four optical phasing sensors. They are a pyramid sensor, a curvature sensor, a phase filtering sensor and a ShackHartmann sensor. They have been developed respectively by Istituto Nazionale di Astrofisica in Florenze, Instituto Astrofisica Canarias in Tenerife, Laboratoire d’Astrophysique de Marseille and ESO. The global behaviour of the optical phasing sensors will be described and preliminary results of the Active Phasing Experiments obtained on sky will be explained. The extrapolation of the results to the EELT and the potential consequences for the adaptive optics will be given. The Active Phasing Experiment has been financed by the European Union and the European Southern Observatory via the Sixth European Union Framework Program for Research and Technological Development under the contract number 011863

On-sky results of the ZEUS phasing sensor, closed-loop precision in the context of multi-wavelength measurements

International audienceThe Active Phasing Experiment (APE) was designed to test four different phasing techniques and to validate wavefront control concepts for Extremely Large Telescopes. One of the sensors is the ZErnike Unit for Segment phasing (ZEUS), which was successfully tested on-sky along with the rest of the APE experiment at one of the Nasmyth platforms of the Very Large Telescope (VLT) in 2009. During the four observing campaigns, multiple results were obtained in open-loop and in closed-loop at different wavelengths. We present in this paper an analysis of the multi-wavelength data in terms of piston measurement precision at the edges of the segments and on the reconstructed wavefront, and an analysis of the evolution of these errors in successive closed-loop runs at different wavelengths. This work demonstrates how the applied multi-wavelength algorithm leads to convergence, allowing phasing of segments with piston errors of several microns