2,915 research outputs found
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
FALCON: a concept to extend adaptive optics corrections to cosmological fields
FALCON is an original concept for a next generation spectrograph at ESO VLT
or at future ELTs. It is a spectrograph including multiple small integral field
units (IFUs) which can be deployed within a large field of view such as that of
VLT/GIRAFFE. In FALCON, each IFU features an adaptive optics correction using
off-axis natural reference stars in order to combine, in the 0.8-1.8 \mu m
wavelength range, spatial and spectral resolutions (0.1-0.15 arcsec and
R=10000+/-5000). These conditions are ideally suited for distant galaxy
studies, which should be done within fields of view larger than the galaxy
clustering scales (4-9 Mpc), i.e. foV > 100 arcmin2. Instead of compensating
the whole field, the adaptive correction will be performed locally on each IFU.
This implies to use small miniaturized devices both for adaptive optics
correction and wavefront sensing. Applications to high latitude fields imply to
use atmospheric tomography because the stars required for wavefront sensing
will be in most of the cases far outside the isoplanatic patch.Comment: To appear in the Backaskog "Second Workshop on ELT" SPIE proceeding
Extragalactic Fields Optimized for Adaptive Optics
In this paper we present the coordinates of 67 55' x 55' patches of sky which
have the rare combination of both high stellar surface density (>0.5
arcmin^{-2} with 13<R<16.5 mag) and low extinction (E(B-V)<0.1). These fields
are ideal for adaptive-optics based follow-up of extragalactic targets. One
region of sky, situated near Baade's Window, contains most of the patches we
have identified. Our optimal field, centered at RA: 7h24m3s, Dec: -1deg27'15",
has an additional advantage of being accessible from both hemispheres. We
propose a figure of merit for quantifying real-world adaptive optics
performance, and use this to analyze the performance of multi-conjugate
adaptive optics in these fields. We also compare our results to those that
would be obtained in existing deep fields. In some cases adaptive optics
observations undertaken in the fields given in this paper would be orders of
magnitude more efficient than equivalent observations undertaken in existing
deep fields.Comment: 28 pages, 15 figures, 1 table; accepted for publication in PAS
Laser Tomography Adaptive Optics (LTAO): A performance study
We present an analytical derivation of the on-axis performance of Adaptive
Optics systems using a given number of guide stars of arbitrary altitude,
distributed at arbitrary angular positions in the sky. The expressions of the
residual error are given for cases of both continuous and discrete turbulent
atmospheric profiles. Assuming Shack-Hartmann wavefront sensing with circular
apertures, we demonstrate that the error is formally described by integrals of
products of three Bessel functions. We compare the performance of Adaptive
Optics correction when using natural, Sodium or Rayleigh laser guide stars. For
small diameter class telescopes (~5m), we show that a few number of Rayleigh
beacons can provide similar performance to that of a single Sodium laser, for a
lower overall cost of the instrument. For bigger apertures, using Rayleigh
stars may not be such a suitable alternative because of the too severe cone
effect that drastically degrades the quality of the correction.Comment: accepted for publication in JOS
Ground-based adaptive optics coronagraphic performance under closed-loop predictive control
The discovery of the exoplanet Proxima b highlights the potential for the
coming generation of giant segmented mirror telescopes (GSMTs) to characterize
terrestrial --- potentially habitable --- planets orbiting nearby stars with
direct imaging. This will require continued development and implementation of
optimized adaptive optics systems feeding coronagraphs on the GSMTs. Such
development should proceed with an understanding of the fundamental limits
imposed by atmospheric turbulence. Here we seek to address this question with a
semi-analytic framework for calculating the post-coronagraph contrast in a
closed-loop AO system. We do this starting with the temporal power spectra of
the Fourier basis calculated assuming frozen flow turbulence, and then apply
closed-loop transfer functions. We include the benefits of a simple predictive
controller, which we show could provide over a factor of 1400 gain in raw PSF
contrast at 1 on bright stars, and more than a factor of 30 gain on
an I = 7.5 mag star such as Proxima. More sophisticated predictive control can
be expected to improve this even further. Assuming a photon noise limited
observing technique such as High Dispersion Coronagraphy, these gains in raw
contrast will decrease integration times by the same large factors. Predictive
control of atmospheric turbulence should therefore be seen as one of the key
technologies which will enable ground-based telescopes to characterize
terrrestrial planets.Comment: Accepted to JATI
Study of a MEMS-based Shack-Hartmann wavefront sensor with adjustable pupil sampling for astronomical adaptive optics
We introduce a Shack-Hartmann wavefront sensor for adaptive optics that enables dynamic control of the spatial sampling of an incoming wavefront using a segmented mirror microelectrical mechanical systems (MEMS) device. Unlike a conventional lenslet array, subapertures are defined by either segments or groups of segments of a mirror array, with the ability to change spatial pupil sampling arbitrarily by redefining the segment grouping. Control over the spatial sampling of the wavefront allows for the minimization of wavefront reconstruction error for different intensities of guide source and different atmospheric conditions, which in turn maximizes an adaptive optics system's delivered Strehl ratio. Requirements for the MEMS devices needed in this Shack-Hartmann wavefront sensor are also presented
Optical Design and Active Optics Methods in Astronomy
Optical designs for astronomy involve implementation of active optics and
adaptive optics from X-ray to the infrared. Developments and results of active
optics methods for telescopes, spectrographs and coronagraph planet finders are
presented. The high accuracy and remarkable smoothness of surfaces generated by
active optics methods also allow elaborating new optical design types with high
aspheric and/or non-axisymmetric surfaces. Depending on the goal and
performance requested for a deformable optical surface analytical
investigations are carried out with one of the various facets of elasticity
theory: small deformation thin plate theory, large deformation thin plate
theory, shallow spherical shell theory, weakly conical shell theory. The
resulting thickness distribution and associated bending force boundaries can be
refined further with finite element analysis. Keywords: active optics, optical
design, elasticity theory, astronomical optics, diffractive optics, X-ray
optic
Adaptive Optics for Astronomy
Adaptive Optics is a prime example of how progress in observational astronomy
can be driven by technological developments. At many observatories it is now
considered to be part of a standard instrumentation suite, enabling
ground-based telescopes to reach the diffraction limit and thus providing
spatial resolution superior to that achievable from space with current or
planned satellites. In this review we consider adaptive optics from the
astrophysical perspective. We show that adaptive optics has led to important
advances in our understanding of a multitude of astrophysical processes, and
describe how the requirements from science applications are now driving the
development of the next generation of novel adaptive optics techniques.Comment: to appear in ARA&A vol 50, 201
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