5,867 research outputs found
Image quality and high contrast improvements on VLT/NACO
NACO is the famous and versatile diffraction limited NIR imager and
spectrograph with which ESO celebrated 10 years of Adaptive Optics at the VLT.
Since two years a substantial effort has been put in to understanding and
fixing issues that directly affect the image quality and the high contrast
performances of the instrument. Experiments to compensate the non-common-path
aberrations and recover the highest possible Strehl ratios have been carried
out successfully and a plan is hereafter described to perform such measurements
regularly. The drift associated to pupil tracking since 2007 was fixed in
October 2011. NACO is therefore even better suited for high contrast imaging
and can be used with coronagraphic masks in the image plane. Some contrast
measurements are shown and discussed. The work accomplished on NACO will serve
as reference for the next generation instruments on the VLT, especially those
working at the diffraction limit and making use of angular differential imaging
(i.e. SPHERE, VISIR, possibly ERIS).Comment: 14 pages, 5 figures, SPIE 2012 Astronomical Instrumentation
Proceedin
Phase Errors in Diffraction-Limited Imaging: Contrast Limits for Sparse Aperture Masking
Bispectrum phase, closure phase and their generalisation to kernel-phase are
all independent of pupil-plane phase errors to first-order. This property, when
used with Sparse Aperture Masking (SAM) behind adaptive optics, has been used
recently in high-contrast observations at or inside the formal diffraction
limit of large telescopes. Finding the limitations to these techniques requires
an understanding of spatial and temporal third-order phase effects, as well as
effects such as time-variable dispersion when coupled with the non-zero
bandwidths in real observations. In this paper, formulae describing many of
these errors are developed, so that a comparison can be made to fundamental
noise processes of photon- and background-noise. I show that the current
generation of aperture-masking observations of young solar-type stars, taken
carefully in excellent observing conditions, are consistent with being limited
by temporal phase noise and photon noise. This has relevance for plans to
combine pupil-remapping with spatial filtering. Finally, I describe calibration
strategies for kernel-phase, including the optimised calibrator weighting as
used for LkCa 15, and the restricted kernel-phase POISE technique that avoids
explicit dependence on calibrators.Comment: 11 pages, 5 figures, resubmitted to MNRAS after responding to
referee's comment
Adaptive optics in high-contrast imaging
The development of adaptive optics (AO) played a major role in modern
astronomy over the last three decades. By compensating for the atmospheric
turbulence, these systems enable to reach the diffraction limit on large
telescopes. In this review, we will focus on high contrast applications of
adaptive optics, namely, imaging the close vicinity of bright stellar objects
and revealing regions otherwise hidden within the turbulent halo of the
atmosphere to look for objects with a contrast ratio lower than 10^-4 with
respect to the central star. Such high-contrast AO-corrected observations have
led to fundamental results in our current understanding of planetary formation
and evolution as well as stellar evolution. AO systems equipped three
generations of instruments, from the first pioneering experiments in the
nineties, to the first wave of instruments on 8m-class telescopes in the years
2000, and finally to the extreme AO systems that have recently started
operations. Along with high-contrast techniques, AO enables to reveal the
circumstellar environment: massive protoplanetary disks featuring spiral arms,
gaps or other asymmetries hinting at on-going planet formation, young giant
planets shining in thermal emission, or tenuous debris disks and micron-sized
dust leftover from collisions in massive asteroid-belt analogs. After
introducing the science case and technical requirements, we will review the
architecture of standard and extreme AO systems, before presenting a few
selected science highlights obtained with recent AO instruments.Comment: 24 pages, 14 figure
The unlikely rise of masking interferometry: leading the way with 19th century technology
The exquisite precision delivered by interferometric techniques is rapidly
being applied to more and more branches of optical astronomy. One particularly
successful strategy to obtain structures at the scale of the diffraction limit
is Aperture Masking Interferometry, which is presently experience a golden age
with implementations at a host of large telescopes around the world. This
startlingly durable technique, which turns 144 years old this year, presently
sets the standard for the recovery of faint companions within a few resolution
elements from the core of a stellar point spread function. This invited review
will give a historical introduction and overview of the modern status of the
technique, the science being delivered, and prospects for new advances and
applications.Comment: This is an invited review for SPIE Amsterdam in 2012. It presents a
brief history of masking interferometry, and some thoughts on future
progress. 11 pages, 4 figs, lots of ref
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
On-sky speckle nulling demonstration at small angular separation with SCExAO
This paper presents the first on-sky demonstration of speckle nulling, which
was achieved at the Subaru Telescope in the context of the Subaru Coronagraphic
Extreme Adaptive Optics (SCExAO) Project. Despite the absence of a high-order
high-bandwidth closed-loop AO system, observations conducted with SCExAO show
that even in poor-to-moderate observing conditions, speckle nulling can be used
to suppress static and slow speckles even in the presence of a brighter dynamic
speckle halo, suggesting that more advanced high-contrast imaging algorithms
developed in the laboratory can be applied to ground-based systems.Comment: 5 figures, accepted for publication by PAS
SCExAO as a precursor to an ELT exoplanet direct imaging instrument
The Subaru Coronagraphic Extreme AO (SCExAO) instrument consists of a high
performance Phase Induced Amplitude Apodisation (PIAA) coronagraph combined
with an extreme Adaptive Optics (AO) system operating in the near-infrared (H
band). The extreme AO system driven by the 2000 element deformable mirror will
allow for Strehl ratios >90% to be achieved in the H-band when it goes closed
loop. This makes the SCExAO instrument a powerful platform for high contrast
imaging down to angular separations of the order of 1lambda/D and an ideal
testbed for exploring coronagraphic techniques for ELTs. In this paper we
report on the recent progress in regards to the development of the instrument,
which includes the addition of a visible bench that makes use of the light at
shorter wavelengths not currently utilized by SCExAO and closing the loop on
the tip/tilt wavefront sensor. We will also discuss several exciting guest
instruments which will expand the capabilities of SCExAO over the next few
years; namely CHARIS which is a integral field spectrograph as well as
VAMPIRES, a visible aperture masking experiment based on polarimetric analysis
of circumstellar disks. In addition we will elucidate the unique role extreme
AO systems will play in enabling high precision radial velocity spectroscopy
for the detection of small companions.Comment: 7 pages, 2 figures Proceedings of AO4ELTs3 conference, paper 13396,
Florence, Italy, May 201
The VAMPIRES instrument: Imaging the innermost regions of protoplanetary disks with polarimetric interferometry
Direct imaging of protoplanetary disks promises to provide key insight into
the complex sequence of processes by which planets are formed. However imaging
the innermost region of such disks (a zone critical to planet formation) is
challenging for traditional observational techniques (such as near-IR imaging
and coronagraphy) due to the relatively long wavelengths involved and the area
occulted by the coronagraphic mask. Here we introduce a new instrument --
VAMPIRES -- which combines non-redundant aperture-masking interferometry with
differential polarimetry to directly image this previously inaccessible
innermost region. By using the polarisation of light scattered by dust in the
disk to provide precise differential calibration of interferometric
visibilities and closure phases, VAMPIRES allows direct imaging at and beyond
the telescope diffraction limit. Integrated into the SCExAO system at the
Subaru telescope, VAMPIRES operates at visible wavelengths (where polarisation
is high) while allowing simultaneous infrared observations conducted by HICIAO.
Here we describe the instrumental design and unique observing technique and
present the results of the first on-sky commissioning observations, validating
the excellent visibility and closure phase precision which are then used to
project expected science performance metrics
A Demonstration of Wavefront Sensing and Mirror Phasing from the Image Domain
In astronomy and microscopy, distortions in the wavefront affect the dynamic
range of a high contrast imaging system. These aberrations are either imposed
by a turbulent medium such as the atmosphere, by static or thermal aberrations
in the optical path, or by imperfectly phased subapertures in a segmented
mirror. Active and adaptive optics (AO), consisting of a wavefront sensor and a
deformable mirror, are employed to address this problem. Nevertheless, the
non-common-path between the wavefront sensor and the science camera leads to
persistent quasi-static speckles that are difficult to calibrate and which
impose a floor on the image contrast. In this paper we present the first
experimental demonstration of a novel wavefront sensor requiring only a minor
asymmetric obscuration of the pupil, using the science camera itself to detect
high order wavefront errors from the speckle pattern produced. We apply this to
correct errors imposed on a deformable microelectromechanical (MEMS) segmented
mirror in a closed loop, restoring a high quality point spread function (PSF)
and residual wavefront errors of order nm using 1600 nm light, from a
starting point of nm in piston and mrad in tip-tilt. We
recommend this as a method for measuring the non-common-path error in
AO-equipped ground based telescopes, as well as as an approach to phasing
difficult segmented mirrors such as on the \emph{James Webb Space Telescope}
primary and as a future direction for extreme adaptive optics.Comment: 9 pages, 6 figure
The Palomar Kernel Phase Experiment: Testing Kernel Phase Interferometry for Ground-based Astronomical Observations
At present, the principal limitation on the resolution and contrast of
astronomical imaging instruments comes from aberrations in the optical path,
which may be imposed by the Earth's turbulent atmosphere or by variations in
the alignment and shape of the telescope optics. These errors can be corrected
physically, with active and adaptive optics, and in post-processing of the
resulting image. A recently-developed adaptive optics post-processing
technique, called kernel phase interferometry, uses linear combinations of
phases that are self-calibrating with respect to small errors, with the goal of
constructing observables that are robust against the residual optical
aberrations in otherwise well-corrected imaging systems. Here we present a
direct comparison between kernel phase and the more established competing
techniques, aperture masking interferometry, point spread function (PSF)
fitting and bispectral analysis. We resolve the alpha Ophiuchi binary system
near periastron, using the Palomar 200-Inch Telescope. This is the first case
in which kernel phase has been used with a full aperture to resolve a system
close to the diffraction limit with ground-based extreme adaptive optics
observations. Excellent agreement in astrometric quantities is found between
kernel phase and masking, and kernel phase significantly outperforms PSF
fitting and bispectral analysis, demonstrating its viability as an alternative
to conventional non-redundant masking under appropriate conditions.Comment: Accepted to MNRA
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