47 research outputs found
Extended object reconstruction in adaptive-optics imaging: the multiresolution approach
We propose the application of multiresolution transforms, such as wavelets
(WT) and curvelets (CT), to the reconstruction of images of extended objects
that have been acquired with adaptive optics (AO) systems. Such multichannel
approaches normally make use of probabilistic tools in order to distinguish
significant structures from noise and reconstruction residuals. Furthermore, we
aim to check the historical assumption that image-reconstruction algorithms
using static PSFs are not suitable for AO imaging. We convolve an image of
Saturn taken with the Hubble Space Telescope (HST) with AO PSFs from the 5-m
Hale telescope at the Palomar Observatory and add both shot and readout noise.
Subsequently, we apply different approaches to the blurred and noisy data in
order to recover the original object. The approaches include multi-frame blind
deconvolution (with the algorithm IDAC), myopic deconvolution with
regularization (with MISTRAL) and wavelets- or curvelets-based static PSF
deconvolution (AWMLE and ACMLE algorithms). We used the mean squared error
(MSE) and the structural similarity index (SSIM) to compare the results. We
discuss the strengths and weaknesses of the two metrics. We found that CT
produces better results than WT, as measured in terms of MSE and SSIM.
Multichannel deconvolution with a static PSF produces results which are
generally better than the results obtained with the myopic/blind approaches
(for the images we tested) thus showing that the ability of a method to
suppress the noise and to track the underlying iterative process is just as
critical as the capability of the myopic/blind approaches to update the PSF.Comment: In revision in Astronomy & Astrophysics. 19 pages, 13 figure
Fainter and closer: finding planets by symmetry breaking
Imaging of planets is very difficult, due to the glare from their nearby,
much brighter suns. Static and slowly-evolving aberrations are the limiting
factors, even after application of adaptive optics. The residual speckle
pattern is highly symmetrical due to diffraction from the telescope's aperture.
We suggest to break this symmetry and thus to locate planets hidden beneath it.
An eccentric pupil mask is rotated to modulate the residual light pattern not
removed by other means. This modulation is then exploited to reveal the
planet's constant signal. In well-corrected ground-based observations we can
reach planets six stellar magnitudes fainter than their sun, and only 2-3 times
the diffraction limit from it. At ten times the diffraction limit, we detect
planets 16 magnitudes fainter. The stellar background drops by five magnitudes.Comment: Accepted for publication in Optics Expres
Reference-less detection, astrometry, and photometry of faint companions with adaptive optics
We propose a complete framework for the detection, astrometry, and photometry
of faint companions from a sequence of adaptive optics corrected short
exposures. The algorithms exploit the difference in statistics between the
on-axis and off-axis intensity. Using moderate-Strehl ratio data obtained with
the natural guide star adaptive optics system on the Lick Observatory's 3-m
Shane Telescope, we compare these methods to the standard approach of PSF
fitting. We give detection limits for the Lick system, as well as a first guide
to expected accuracy of differential photometry and astrometry with the new
techniques. The proposed approach to detection offers a new way of determining
dynamic range, while the new algorithms for differential photometry and
astrometry yield accurate results for very faint and close-in companions where
PSF fitting fails. All three proposed algorithms are self-calibrating, i.e.
they do not require observation of a calibration star thus improving the
observing efficiency.Comment: Astrophysical Journal 698 (2009) 28-4
Detection of faint companions through stochastic speckle discrimination
We propose a new post-processing technique for the detection of faint
companions from a sequence of adaptive optics corrected short exposures. The
algorithm exploits the difference in shape between the on-axis and off-axis
irradiance distributions and it does not require the signal to be above the
noise level. We show that the method is particularly useful in dealing with
static speckles. Its application to real and simulated data gives excellent
results in the low-signal regime where it outperforms the standard approach of
computing signal-to-noise ratio on one long exposure. We also show that
accurate noise estimation in adaptive optics images of close companions is
rendered impossible due to the presence of static speckles. This new method
provides means of reliable estimation of the confidence intervals for the
detection hypothesis.Comment: accepted for publication in Ap
Lucky imaging and speckle discrimination for the detection of faint companions with adaptive optics
We have analyzed the application of frame selection ("lucky imaging") to adaptive optics (AO), short-exposure observations of faint companions. We have used the instantaneous Strehl ratio as an image quality metric. The probability density function (PDF) of this quantity can be used to determine the outcome of frame selection in terms of optimizing the Strehl ratio and the peak-signal-to-noise-ratio of the shift-and-add image. In the presence of static speckles, frame selection can lead to both: improvement in resolution--as quantified by the Strehl ratio, as well as faint signal detectability--given by the peak-signal-to-noise-ratio. This theoretical prediction is confirmed with real data from AO observations using Lick Observatory's 3m Shane telescope, and the Palomar Observatory's 5m Hale telescope. In addition, we propose a novel statistics-based technique for the detection of faint companions from a sequence of AO-corrected exposures. The algorithm, which we call stochastic speckle discrimination, utilizes the "statistical signature" of the centre of the point spread function (PSF) to discriminate between faint companions and static speckles. The technique yields excellent results even for signals invisible in the shift-and-add images
Differential Tilt Variance Effects of Turbulence in Imagery: Comparing Simulation with Theory
Differential tilt variance is a useful metric for interpreting the distorting effects of turbulence in incoherent imaging systems. In this paper, we compare the theoretical model of differential tilt variance to simulations. Simulation is based on a Monte Carlo wave optics approach with split step propagation. Results show that the simulation closely matches theory. The results also show that care must be taken when selecting a method to estimate tilts
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
Review of small-angle coronagraphic techniques in the wake of ground-based second-generation adaptive optics systems
Small-angle coronagraphy is technically and scientifically appealing because
it enables the use of smaller telescopes, allows covering wider wavelength
ranges, and potentially increases the yield and completeness of circumstellar
environment - exoplanets and disks - detection and characterization campaigns.
However, opening up this new parameter space is challenging. Here we will
review the four posts of high contrast imaging and their intricate interactions
at very small angles (within the first 4 resolution elements from the star).
The four posts are: choice of coronagraph, optimized wavefront control,
observing strategy, and post-processing methods. After detailing each of the
four foundations, we will present the lessons learned from the 10+ years of
operations of zeroth and first-generation adaptive optics systems. We will then
tentatively show how informative the current integration of second-generation
adaptive optics system is, and which lessons can already be drawn from this
fresh experience. Then, we will review the current state of the art, by
presenting world record contrasts obtained in the framework of technological
demonstrations for space-based exoplanet imaging and characterization mission
concepts. Finally, we will conclude by emphasizing the importance of the
cross-breeding between techniques developed for both ground-based and
space-based projects, which is relevant for future high contrast imaging
instruments and facilities in space or on the ground.Comment: 21 pages, 7 figure