115 research outputs found
Tomographic reconstruction for Wide Field Adaptive Optics systems: Fourier domain analysis and fundamental limitations
Several Wide Field of view Adaptive Optics (WFAO) concepts like
Multi-Conjugate AO (MCAO), Multi-Object AO (MOAO) or Ground-Layer AO (GLAO) are
currently studied for the next generation of Extremely Large Telescopes (ELTs).
All these concepts will use atmospheric tomography to reconstruct the turbulent
phase volume. In this paper, we explore different reconstruction algorithms and
their fundamental limitations. We conduct this analysis in the Fourier domain.
This approach allows us to derive simple analytical formulations for the
different configurations, and brings a comprehensive view of WFAO limitations.
We then investigate model and statistical errors and their impact on the phase
reconstruction. Finally, we show some examples of different WFAO systems and
their expected performance on a 42m telescope case.Comment: 40 pages, 23 figures, accepted for publication in JOSA-
Fast minimum variance wavefront reconstruction for extremely large telescopes
We present a new algorithm, FRiM (FRactal Iterative Method), aiming at the
reconstruction of the optical wavefront from measurements provided by a
wavefront sensor. As our application is adaptive optics on extremely large
telescopes, our algorithm was designed with speed and best quality in mind. The
latter is achieved thanks to a regularization which enforces prior statistics.
To solve the regularized problem, we use the conjugate gradient method which
takes advantage of the sparsity of the wavefront sensor model matrix and avoids
the storage and inversion of a huge matrix. The prior covariance matrix is
however non-sparse and we derive a fractal approximation to the Karhunen-Loeve
basis thanks to which the regularization by Kolmogorov statistics can be
computed in O(N) operations, N being the number of phase samples to estimate.
Finally, we propose an effective preconditioning which also scales as O(N) and
yields the solution in 5-10 conjugate gradient iterations for any N. The
resulting algorithm is therefore O(N). As an example, for a 128 x 128
Shack-Hartmann wavefront sensor, FRiM appears to be more than 100 times faster
than the classical vector-matrix multiplication method.Comment: to appear in the Journal of the Optical Society of America
Modeling of pulsed laser guide stars for the Thirty Meter Telescope project
The Thirty Meter Telescope (TMT) has been designed to include an adaptive
optics system and associated laser guide star (LGS) facility to correct for the
image distortion due to Earth's atmospheric turbulence and achieve
diffraction-limited imaging. We have calculated the response of mesospheric
sodium atoms to a pulsed laser that has been proposed for use in the LGS
facility, including modeling of the atomic physics, the light-atom
interactions, and the effect of the geomagnetic field and atomic collisions.
This particular pulsed laser format is shown to provide comparable photon
return to a continuous-wave (cw) laser of the same average power; both the cw
and pulsed lasers have the potential to satisfy the TMT design requirements for
photon return flux.Comment: 16 pages, 20 figure
Ground-layer wavefront reconstruction from multiple natural guide stars
Observational tests of ground layer wavefront recovery have been made in open
loop using a constellation of four natural guide stars at the 1.55 m Kuiper
telescope in Arizona. Such tests explore the effectiveness of wide-field seeing
improvement by correction of low-lying atmospheric turbulence with ground-layer
adaptive optics (GLAO). The wavefronts from the four stars were measured
simultaneously on a Shack-Hartmann wavefront sensor (WFS). The WFS placed a 5 x
5 array of square subapertures across the pupil of the telescope, allowing for
wavefront reconstruction up to the fifth radial Zernike order. We find that the
wavefront aberration in each star can be roughly halved by subtracting the
average of the wavefronts from the other three stars. Wavefront correction on
this basis leads to a reduction in width of the seeing-limited stellar image by
up to a factor of 3, with image sharpening effective from the visible to near
infrared wavelengths over a field of at least 2 arc minutes. We conclude that
GLAO correction will be a valuable tool that can increase resolution and
spectrographic throughput across a broad range of seeing-limited observations.Comment: 25 pages, 8 figures, to be published in Astrophys.
A resolved, au-scale gas disk around the B[e] star HD 50138
articleHD 50138 is a B[e] star surrounded by a large amount of circumstellar gas and dust. Its spectrum shows characteristics which may indicate either a pre- or a post-main-sequence system. Mapping the kinematics of the gas in the inner few au of the system contributes to a better understanding of its physical nature. We present the first high spatial and spectral resolution interferometric observations of the Brγ line of HD 50138, obtained with VLTI/AMBER. The line emission originates in a region more compact (up to 3 au) than the continuum-emitting region. Blue- and red-shifted emission originates from the two different hemispheres of an elongated structure perpendicular to the polarization angle. The velocity of the emitting medium decreases radially. An overall offset along the NW direction between the line- and continuum-emitting regions is observed. We compare the data with a geometric model of a thin Keplerian disk and a spherical halo on top of a Gaussian continuum. Most of the data are well reproduced by this model, except for the variability, the global offset and the visibility at the systemic velocity. The evolutionary state of the system is discussed; most diagnostics are ambiguous and may point either to a post-main-sequence or a pre-main-sequence nature.National Science FoundationGeorgia State UniversityW.M. Keck FoundationDavid and Lucile Packard FoundationFizeau ProgramCNRS-PICS progra
The FALCON concept: multi-object adaptive optics and atmospheric tomography for integral field spectroscopy. Principles and performances on an 8 meter telescope
Integral field spectrographs are major instruments to study the mechanisms
involved in the formation and the evolution of early galaxies. When combined
with multi-object spectroscopy, those spectrographs can behave as machines used
to derive physical parameters of galaxies during their formation process. Up to
now, there is only one available spectrograph with multiple integral field
units, e.g. FLAMES/GIRAFFE on the VLT. However, current ground based
instruments suffer from a degradation of their spatial resolution due to
atmospheric turbulence. In this article we describe the performance of FALCON,
an original concept of a new generation multi-object integral field
spectrograph with adaptive optics for the ESO Very Large Telescope. The goal of
FALCON is to combine high angular resolution (0.25 arcsec) and high spectral
resolution (R > 5000) in J and H bands over a wide field of view (10x10
arcmin2) in the VLT Nasmyth focal plane. However, instead of correcting the
whole field, FALCON will use multi-object adaptive optics (MOAO) to perform
locally on each scientific target the adaptive optics correction. This requires
then to use atmospheric tomography in order to use suitable natural guide stars
for wavefront sensing. We will show that merging MOAO and atmospheric
tomography allows us to determine the internal kinematics of distant galaxies
up to z=2 with a sky coverage of 50%, even for objects observed near the
galactic pole. The application of such a concept to Extremely Large Telescopes
seems therefore to be a very promising way to study galaxy evolution from z = 1
to redshifts as high as z = 7.Comment: Monthly Notices of the Royal Astronomical Society, accepte
Cell–cell and cell–matrix dynamics in intraperitoneal cancer metastasis
The peritoneal metastatic route of cancer dissemination is shared by cancers of the ovary and gastrointestinal tract. Once initiated, peritoneal metastasis typically proceeds rapidly in a feed-forward manner. Several factors contribute to this efficient progression. In peritoneal metastasis, cancer cells exfoliate into the peritoneal fluid and spread locally, transported by peritoneal fluid. Inflammatory cytokines released by tumor and immune cells compromise the protective, anti-adhesive mesothelial cell layer that lines the peritoneal cavity, exposing the underlying extracellular matrix to which cancer cells readily attach. The peritoneum is further rendered receptive to metastatic implantation and growth by myofibroblastic cell behaviors also stimulated by inflammatory cytokines. Individual cancer cells suspended in peritoneal fluid can aggregate to form multicellular spheroids. This cellular arrangement imparts resistance to anoikis, apoptosis, and chemotherapeutics. Emerging evidence indicates that compact spheroid formation is preferentially accomplished by cancer cells with high invasive capacity and contractile behaviors. This review focuses on the pathological alterations to the peritoneum and the properties of cancer cells that in combination drive peritoneal metastasis
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