22 research outputs found
Direct imaging with highly diluted apertures. I. Field of view limitations
Future optical interferometric instrumentation mainly relies on the
availability of an efficient cophasing system: once available, what has so far
postponed the relevance of direct imaging with an interferometer will vanish.
This paper focuses on the actual limits of snapshot imaging, inherent to the
use of a sparse aperture: the number of telescopes and the geometry of the
array impose the maximum extent of the field of view and the complexity of the
sources. A second limitation may arise from the beam combination scheme.
Comparing already available solutions, we show that the so called
hypertelescope mode (or densified pupil) is ideal. By adjusting the direct
imaging field of view to the useful field of view offered by the array, the
hypertelescope makes an optimal use of the collected photons. It optimizes
signal to noise ratio, drastically improves the luminosity of images and makes
the interferometer compatible with coronagraphy, without inducing any loss of
useful field of view.Comment: 13 pages, 6 figures, accepted for publication in MNRAS.
Full-resolution version available at
http://www.obs-hp.fr/~lardiere/publi/2006-Lardiere-MNRAS.pd
First images on the sky from a hyper telescope
We show star images obtained with a miniature ``densified pupil imaging
interferometer'' also called a hyper-telescope. The formation of such images
violates a ``golden rule of imaging interferometers'' which appeared to forbid
the use of interferometric arrangements differing from a Fizeau interferometer.
These produce useless images when the sub-apertures spacing is much wider than
their size, owing to diffraction through the sub-apertures. The hyper-telescope
arrangement solves these problems opening the way towards multi-kilometer
imaging arrays in space. We experimentally obtain an intensity gain of 24 +- 3X
when a densified-pupil interferometer is compared to an equivalent Fizeau-type
interferometer and show images of the double star alpha Gem. The initial
results presented confirm the possibility of directly obtaining high resolution
and high dynamic range images in the recombined focal plane of a large
interferometer if enough elements are used.Comment: 6 pages, LaTeX, standard A&A macros + BibTeX macros. Accepted for
publication in Astronomy and Astrophysics Supplement
Direct imaging with highly diluted apertures. II. Properties of the point spread function of a hypertelescope
In the future, optical stellar interferometers will provide true images
thanks to larger number of telescopes and to advanced cophasing subsystems.
These conditions are required to have sufficient resolution elements (resel) in
the image and to provide direct images in the hypertelescope mode. It has
already been shown that hypertelescopes provide snapshot images with a
significant gain in sensitivity without inducing any loss of the useful field
of view for direct imaging applications. This paper aims at studying the
properties of the point spread functions of future large arrays using the
hypertelescope mode. Numerical simulations have been performed and criteria
have been defined to study the image properties. It is shown that the choice of
the configuration of the array is a trade-off between the resolution, the halo
level and the field of view. A regular pattern of the array of telescopes
optimizes the image quality (low halo level and maximum encircled energy in the
central peak), but decreases the useful field of view. Moreover, a
non-redundant array is less sensitive to the space aliasing effect than a
redundant array.Comment: 10 pages paper with referee in A&
Tests with a Carlina-type diluted telescope; Primary coherencing
Studies are under way to propose a new generation of post-VLTI
interferometers. The Carlina concept studied at the Haute- Provence Observatory
is one of the proposed solutions. It consists in an optical interferometer
configured like a diluted version of the Arecibo radio telescope: above the
diluted primary mirror made of fixed cospherical segments, a helium balloon (or
cables suspended between two mountains), carries a gondola containing the focal
optics. Since 2003, we have been building a technical demonstrator of this
diluted telescope. First fringes were obtained in May 2004 with two
closely-spaced primary segments and a CCD on the focal gondola. We have been
testing the whole optical train with three primary mirrors. The main aim of
this article is to describe the metrology that we have conceived, and tested
under the helium balloon to align the primary mirrors separate by 5-10 m on the
ground with an accuracy of a few microns. The servo loop stabilizes the mirror
of metrology under the helium balloon with an accuracy better than 5 mm while
it moves horizontally by 30 cm in open loop by 10-20 km/h of wind. We have
obtained the white fringes of metrology; i.e., the three mirrors are aligned
(cospherized) with an accuracy of {\approx} 1 micron. We show data proving the
stability of fringes over 15 minutes, therefore providing evidence that the
mechanical parts are stabilized within a few microns. This is an important step
that demonstrates the feasibility of building a diluted telescope using cables
strained between cliffs or under a balloon. Carlina, like the MMT or LBT, could
be one of the first members of a new class of telescopes named diluted
telescopes.Comment: 18 pages, 17 figures, A&A, accepte
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L1 transfer in the acquisition of manner and path in Spanish by native speakers of English
In this article the authors argue that L1 transfer from English is not only important in the early stages of L2 acquisition of Spanish, but remains influential in later stages if there is not enough positive evidence for the learners to progress in their development (Lefebvre, White, & Jourdan, 2006). The findings are based on analyses of path and manner of movement in stories told by British students of Spanish (N = 68) of three different proficiency levels. Verbs that conflate motion and path, on the one hand, are mastered early, possibly because the existence of Latinate path verbs, such as enter and ascend in English, facilitate their early acquisition by British learners of Spanish. Contrary to the findings of Cadierno (2004) and Cadierno and Ruiz (2006), the encoding of manner, in particular in boundary crossing contexts, seems to pose enormous difficulties, even among students who had been abroad on a placement in a Spanish-speaking country prior to the data collection. An analysis of the frequency of manner verbs in Spanish corpora shows that one of the key reasons why students struggle with manner is that manner verbs are so infrequent in Spanish. The authors claim that scarce positive evidence in the language exposed to and little or no negative evidence are responsible for the long-lasting effect of transfer on the expression of manner
Study of centroiding algorithms to optimize Shack-Hartmann WFS in the context of ELTs
Along with the expected ELTs comes the diversification of Adaptive Optics (AO) systems. Most
AO are beginning to make extensive use of laser guide stars (LGS), to increase sky coverage and
produce a bright beacon in order to reduce wavefront measurement errors and improve
performance. However, because of the larger size of the next generation of telescopes, the
elongation seen in a Shack-Hartmann Wavefront Sensor is larger, i.e up to 7” for a 30m
telescope. With such an elongation, both photon and read noise will increase and distribution
variations of sodium atoms in the sodium layer start to matter. In this paper, we conduct
ShackHartmann simulations at the sub-aperture level made with real sodium profiles taken at
Lick Observatory. We will compare focus on two methods of centroiders: the matched filter and
the correlation. We then compare results with data taken form a real bench at Uvic in the
particular case of NFIRAOS AO system for TMT. The goal is to better understand the impact of
such variations on the final error budget for the WFS
Characterization and mitigation of Laser–Guide–Star–induced aberrations
Sodium Laser Guide Stars (LGS) induce optical aberrations in adaptive optics (AO) systems.
The artificial star is elongated due to the sodium layer thickness, and the variations of the
sodium layer altitude and atom density profile induce errors on centroid measurements of
elongated spots. In AO systems, these errors generate spurious optical aberrations, termed LGS
aberrations, especially with ELTs for which the spot elongation is greater. According to
analytical models and experimental results obtained with the University of Victoria LGS bench
demonstrator, we characterized the main LGS aberrations and studied two options to mitigate
them: the Radial Thresholding and the temporal filtering