32,319 research outputs found
Extremely fast focal-plane wavefront sensing for extreme adaptive optics
We present a promising approach to the extremely fast sensing and correction
of small wavefront errors in adaptive optics systems. As our algorithm's
computational complexity is roughly proportional to the number of actuators, it
is particularly suitable to systems with 10,000 to 100,000 actuators. Our
approach is based on sequential phase diversity and simple relations between
the point-spread function and the wavefront error in the case of small
aberrations. The particular choice of phase diversity, introduced by the
deformable mirror itself, minimizes the wavefront error as well as the
computational complexity. The method is well suited for high-contrast
astronomical imaging of point sources such as the direct detection and
characterization of exoplanets around stars, and it works even in the presence
of a coronagraph that suppresses the diffraction pattern. The accompanying
paper in these proceedings by Korkiakoski et al. describes the performance of
the algorithm using numerical simulations and laboratory tests.Comment: SPIE Paper 8447-7
Testing for flexible nonlinear trends with an integrated or stationary noise component
This paper proposes a new test for the presence of a nonlinear deterministic trend approximated by a Fourier expansion in a univariate time series for which there is no prior knowledge as to whether the noise component is stationary or contains an autoregressive unit root. Our approach builds on the work of Perron and Yabu (2009a) and is based on a Feasible Generalized Least Squares procedure that uses a super-efficient estimator of the sum of the autoregressive coefficients α when α = 1. The resulting Wald test statistic asymptotically follows a chi-square distribution in both the I(0) and I(1) cases. To improve the finite sample properties of the test, we use a bias-corrected version of the OLS estimator of α proposed by Roy and Fuller (2001). We show that our procedure is substantially more powerful than currently available alternatives. We illustrate the usefulness of our method via an application to modelling the trend of global and hemispheric temperatures
Temporal variability and statistics of the Strehl ratio in adaptive-optics images
We have investigated the temporal variability and statistics of the
"instantaneous" Strehl ratio. The observations were carried out with the 3.63-m
AEOS telescope equipped with a high-order adaptive optics system. In this paper
Strehl ratio is defined as the peak intensity of a single short exposure. We
have also studied the behaviour of the phase variance computed on the
reconstructed wavefronts. We tested the Marechal approximation and used it to
explain the observed negative skewness of the Strehl ratio distribution. The
estimate of the phase variance is shown to fit a three-parameter Gamma
distribution model. We show that simple scaling of the reconstructed wavefronts
has a large impact on the shape of the Strehl ratio distribution.Comment: submitted to PAS
Astrometry and Photometry with Coronagraphs
We propose a solution to the problem of astrometric and photometric
calibration of coronagraphic images with a simple optical device which, in
theory, is easy to use. Our design uses the Fraunhofer approximation of Fourier
optics. Placing a periodic grid of wires (we use a square grid) with known
width and spacing in a pupil plane in front of the occulting coronagraphic
focal plane mask produces fiducial images of the obscured star at known
locations relative to the star. We also derive the intensity of these fiducial
images in the coronagraphic image. These calibrator images can be used for
precise relative astrometry, to establish companionship of other objects in the
field of view through measurement of common proper motion or common parallax,
to determine orbits, and to observe disk structure around the star
quantitatively. The calibrator spots also have known brightness, selectable by
the coronagraph designer, permitting accurate relative photometry in the
coronagraphic image. This technique, which enables precision exoplanetary
science, is relevant to future coronagraphic instruments, and is particularly
useful for `extreme' adaptive optics and space-based coronagraphy.Comment: To appear in ApJ August 2006, 27 preprint style pages 4 figure
Optimal Dark Hole Generation via Two Deformable Mirrors with Stroke Minimization
The past decade has seen a significant growth in research targeted at space
based observatories for imaging exo-solar planets. The challenge is in
designing an imaging system for high-contrast. Even with a perfect coronagraph
that modifies the point spread function to achieve high-contrast, wavefront
sensing and control is needed to correct the errors in the optics and generate
a "dark hole". The high-contrast imaging laboratory at Princeton University is
equipped with two Boston Micromachines Kilo-DMs. We review here an algorithm
designed to achieve high-contrast on both sides of the image plane while
minimizing the stroke necessary from each deformable mirror (DM). This
algorithm uses the first DM to correct for amplitude aberrations and the second
DM to create a flat wavefront in the pupil plane. We then show the first
results obtained at Princeton with this correction algorithm, and we
demonstrate a symmetric dark hole in monochromatic light
- …