200 research outputs found
The Asymmetric Pupil Fourier Wavefront Sensor
This paper introduces a novel wavefront sensing approach that relies on the
Fourier analysis of a single conventional direct image. In the high Strehl
ratio regime, the relation between the phase measured in the Fourier plane and
the wavefront errors in the pupil can be linearized, as was shown in a previous
work that introduced the notion of generalized closure-phase, or kernel-phase.
The technique, to be usable as presented requires two conditions to be met: (1)
the wavefront errors must be kept small (of the order of one radian or less)
and (2) the pupil must include some asymmetry, that can be introduced with a
mask, for the problem to become solvable. Simulations show that this asymmetric
pupil Fourier wavefront sensing or APF-WFS technique can improve the Strehl
ratio from 50 to over 90 % in just a few iterations, with excellent photon
noise sensitivity properties, suggesting that on-sky close loop APF-WFS is
possible with an extreme adaptive optics system.Comment: 5 figures, accepted for publication by PAS
Kernel-phases for high-contrast detection beyond the resolution limit
The detection of high contrast companions at small angular separation appears
feasible in conventional direct images using the self-calibration properties of
interferometric observable quantities. In the high-Strehl regime, available
from space borne observatories and using AO in the mid-infrared, quantities
comparable to the closure-phase that are used with great success in
non-redundant masking inteferometry, can be extracted from direct images, even
taken with a redundant aperture. These new phase-noise immune observable
quantities, called Kernel-phases, are determined a-priori from the knowledge of
the geometry of the pupil only. Re-analysis of HST/NICMOS archive and other
ground based AO images, using this new Kernel-phase algorithm, demonstrates the
power of the method, and its ability to detect companions at the resolution
limit and beyond.Comment: 7 pages, 4 figures, 2011 SPIE conference proceeding
Closed-loop focal plane wavefront control with the SCExAO instrument
This article describes the implementation of a focal plane based wavefront
control loop on the high-contrast imaging instrument SCExAO (Subaru
Coronagraphic Extreme Adaptive Optics). The sensor relies on the Fourier
analysis of conventional focal-plane images acquired after an asymmetric mask
is introduced in the pupil of the instrument. This absolute sensor is used here
in a closed-loop to compensate the non-common path errors that normally affects
any imaging system relying on an upstream adaptive optics system.This specific
implementation was used to control low order modes corresponding to eight
zernike modes (from focus to spherical). This loop was successfully run on-sky
at the Subaru Telescope and is used to offset the SCExAO deformable mirror
shape used as a zero-point by the high-order wavefront sensor. The paper
precises the range of errors this wavefront sensing approach can operate within
and explores the impact of saturation of the data and how it can be bypassed,
at a cost in performance. Beyond this application, because of its low hardware
impact, APF-WFS can easily be ported in a wide variety of wavefront sensing
contexts, for ground- as well space-borne telescopes, and for telescope pupils
that can be continuous, segmented or even sparse. The technique is powerful
because it measures the wavefront where it really matters, at the level of the
science detector.Comment: 9 pages, 14 figures, accepted for publication by A&
Mapping the Shores of the Brown Dwarf Desert. I. Upper Scorpius
We present the results of a survey for stellar and substellar companions to 82 young stars in the nearby OB association Upper Scorpius. This survey used nonredundant aperture mask interferometry to achieve typical contrast
limits of ΔK ~5-6 at the diffraction limit, revealing 12 new binary companions that lay below the detection limits
of traditional high-resolution imaging; we also summarize a complementary snapshot imaging survey that discovered
seven directly resolved companions. The overall frequency of binary companions (~35 +5 -4% at separations of
6-435 AU) appears to be equivalent to field stars of similar mass, but companions could be more common among
lower mass stars than for the field. The companion mass function has statistically significant differences compared to several suggested mass functions for the field, and we suggest an alternate lognormal parameterization of the mass function. Our survey limits encompass the entire brown dwarf mass range, but we only detected a single companion that might be a brown dwarf; this deficit resembles the so-called brown dwarf desert that has been observed by radial velocity planet searches. Finally, our survey’s deep detection limits extend into the top of the planetary mass function, reaching 8-12 MJup for half of our sample. We have not identified any planetary companions at high confidence (≳99.5%), but we have identified four candidate companions at lower confidence (≳97.5%) that merit additional follow-up to confirm or disprove their existence
High Performance Lyot and PIAA Coronagraphy for Arbitrarily shaped Telescope Apertures
Two high performance coronagraphic approaches compatible with segmented and
obstructed telescope pupils are described. Both concepts use entrance pupil
amplitude apodization and a combined phase and amplitude focal plane mask to
achieve full coronagraphic extinction of an on-axis point source. While the
first concept, named Apodized Pupil Complex Mask Lyot Coronagraph (APCMLC),
relies on a transmission mask to perform the pupil apodization, the second
concept, named Phase-Induced Amplitude Apodization complex mask coronagraph
(PIAACMC), uses beam remapping for lossless apodization. Both concepts
theoretically offer complete coronagraphic extinction (infinite contrast) of a
point source in monochromatic light, with high throughput and sub-lambda/D
inner working angle, regardless of aperture shape. The PIAACMC offers nearly
100% throughput and approaches the fundamental coronagraph performance limit
imposed by first principles. The steps toward designing the coronagraphs for
arbitrary apertures are described for monochromatic light. Designs for the
APCMLC and the higher performance PIAACMC are shown for several monolith and
segmented apertures, such as the apertures of the Subaru Telescope, Giant
Magellan Telescope (GMT), Thirty Meter Telescope (TMT), the European Extremely
Large Telescope (E-ELT) and the Large Binocular Telescope (LBT). Performance in
broadband light is also quantified, suggesting that the monochromatic designs
are suitable for use in up to 20% wide spectral bands for ground-based
telescopes.Comment: 19 pages, 12 figures, accepted for publication in Ap
The Role of Multiplicity in Disk Evolution and Planet Formation
The past decade has seen a revolution in our understanding of protoplanetary
disk evolution and planet formation in single star systems. However, the
majority of solar-type stars form in binary systems, so the impact of binary
companions on protoplanetary disks is an important element in our understanding
of planet formation. We have compiled a combined multiplicity/disk census of
Taurus-Auriga, plus a restricted sample of close binaries in other regions, in
order to explore the role of multiplicity in disk evolution. Our results imply
that the tidal influence of a close (<40 AU) binary companion significantly
hastens the process of protoplanetary disk dispersal, as ~2/3 of all close
binaries promptly disperse their disks within <1 Myr after formation. However,
prompt disk dispersal only occurs for a small fraction of wide binaries and
single stars, with ~80%-90% retaining their disks for at least ~2--3 Myr (but
rarely for more than ~5 Myr). Our new constraints on the disk clearing
timescale have significant implications for giant planet formation; most single
stars have 3--5 Myr within which to form giant planets, whereas most close
binary systems would have to form giant planets within <1 Myr. If core
accretion is the primary mode for giant planet formation, then gas giants in
close binaries should be rare. Conversely, since almost all single stars have a
similar period of time within which to form gas giants, their relative rarity
in RV surveys indicates either that the giant planet formation timescale is
very well-matched to the disk dispersal timescale or that features beyond the
disk lifetime set the likelihood of giant planet formation.Comment: Accepted to ApJ; 15 pages, 3 figures, 3 tables in emulateapj forma
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
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