235 research outputs found
Focal Plane Wavefront Sensing using Residual Adaptive Optics Speckles
Optical imperfections, misalignments, aberrations, and even dust can
significantly limit sensitivity in high-contrast imaging systems such as
coronagraphs. An upstream deformable mirror (DM) in the pupil can be used to
correct or compensate for these flaws, either to enhance Strehl ratio or
suppress residual coronagraphic halo. Measurement of the phase and amplitude of
the starlight halo at the science camera is essential for determining the DM
shape that compensates for any non-common-path (NCP) wavefront errors. Using DM
displacement ripples to create a series of probe and anti-halo speckles in the
focal plane has been proposed for space-based coronagraphs and successfully
demonstrated in the lab. We present the theory and first on-sky demonstration
of a technique to measure the complex halo using the rapidly-changing residual
atmospheric speckles at the 6.5m MMT telescope using the Clio mid-IR camera.
The AO system's wavefront sensor (WFS) measurements are used to estimate the
residual wavefront, allowing us to approximately compute the rapidly-evolving
phase and amplitude of speckle halo. When combined with relatively-short,
synchronized science camera images, the complex speckle estimates can be used
to interferometrically analyze the images, leading to an estimate of the static
diffraction halo with NCP effects included. In an operational system, this
information could be collected continuously and used to iteratively correct
quasi-static NCP errors or suppress imperfect coronagraphic halos.Comment: Astrophysical Journal (accepted). 26 pages, 21 figure
Modeling giant extrasolar ring systems in eclipse and the case of J1407b: sculpting by exomoons?
The light curve of 1SWASP J140747.93-394542.6, a 16 Myr old star in the
Sco-Cen OB association, underwent a complex series of deep eclipses that lasted
56 days, centered on April 2007. This light curve is interpreted as the transit
of a giant ring system that is filling up a fraction of the Hill sphere of an
unseen secondary companion, J1407b. We fit the light curve with a model of an
azimuthally symmetric ring system, including spatial scales down to the
temporal limit set by the star's diameter and relative velocity. The best ring
model has 37 rings and extends out to a radius of 0.6 AU (90 million km), and
the rings have an estimated total mass on the order of . The ring
system has one clearly defined gap at 0.4 AU (61 million km), which we
hypothesize is being cleared out by a exosatellite orbiting
around J1407b. This eclipse and model implies that we are seeing a
circumplanetary disk undergoing a dynamic transition to an
exosatellite-sculpted ring structure and is one of the first seen outside our
Solar system.Comment: 13 pages, 6 figures, 3 tables. Accepted for publication in ApJ. Data
and computer code for model at: http://github.com/mkenworthy/exoring
Infrared Variability of the Gliese 569B System
Gliese 569B is a multiple brown dwarf system whose exact nature has been the
subject of several investigations over the past few years. Interpretation has
partially relied on infra-red photometry and spectroscopy of the resolved
components of the system. We present seeing limited Ks photometry over four
nights, searching for variability in this young low mass substellar system. Our
photometry is consistent with other reported photometry, and we report the
tentative detection of several periodic signals consistent with rotational
modulation due to spots on their surfaces. The five significant periods range
from 2.90 hours to 12.8 hours with peak to peak variabilities from 28 mmag to
62 mmag in the Ks band.
If both components are rotating with the shortest periods, then their
rotation axes are not parallel with each other, and the rotation axis of the Bb
component is not perpendicular to the Ba-Bb orbital plane. If Bb has one of the
longer rotational periods, then the Bb rotation axis is consistent with being
parallel to the orbital axis of the Ba-Bb system.Comment: 22 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Impact of time-variant turbulence behavior on prediction for adaptive optics systems
For high contrast imaging systems, the time delay is one of the major
limiting factors for the performance of the extreme adaptive optics (AO)
sub-system and, in turn, the final contrast. The time delay is due to the
finite time needed to measure the incoming disturbance and then apply the
correction. By predicting the behavior of the atmospheric disturbance over the
time delay we can in principle achieve a better AO performance. Atmospheric
turbulence parameters which determine the wavefront phase fluctuations have
time-varying behavior. We present a stochastic model for wind speed and model
time-variant atmospheric turbulence effects using varying wind speed. We test a
low-order, data-driven predictor, the linear minimum mean square error
predictor, for a near-infrared AO system under varying conditions. Our results
show varying wind can have a significant impact on the performance of wavefront
prediction, preventing it from reaching optimal performance. The impact depends
on the strength of the wind fluctuations with the greatest loss in expected
performance being for high wind speeds.Comment: 10 pages, 8 figures; Accepted to JOSA A March 201
Congestion offsets: Transforming cities by letting buses compete
Cities around the world have been unable to keep up with demand for road space, and as a result suffer from congestion. This paper argues that the consequence of this congestion has been not just poorly performing roads, but a structurally inefficient urban transport solution, due to the impact of congestion on bus services. As a way of controlling congestion, road pricing has been rejected by most cities because it is difficult to sell to the public and difficult to deploy. An examination of both the strengths and weaknesses of road pricing is used as a basis from which to develop an alternative method of congestion control, namely congestion offsets. This approach treats roads as a commons, and does not argue for the creation of a market for road space, as road pricing does. It is argued that congestion offsets would allow congestion to be controlled in a simple and fair manner, thereby enabling buses to emerge as a real competitor to cars in urban areas. The historical absence of congestion control in cities has allowed massive distortions to build up; cities have become awash with both cars and bitumen. Congestion offsets would allow these distortions to be slowly unwound, thus helping to transform cities more into how, it is argued, they should have always been
The Vector-APP: a Broadband Apodizing Phase Plate that yields Complementary PSFs
The apodizing phase plate (APP) is a solid-state pupil optic that clears out
a D-shaped area next to the core of the ensuing PSF. To make the APP more
efficient for high-contrast imaging, its bandwidth should be as large as
possible, and the location of the D-shaped area should be easily swapped to the
other side of the PSF. We present the design of a broadband APP that yields two
PSFs that have the opposite sides cleared out. Both properties are enabled by a
half-wave liquid crystal layer, for which the local fast axis orientation over
the pupil is forced to follow the required phase structure. For each of the two
circular polarization states, the required phase apodization is thus obtained,
and, moreover, the PSFs after a quarter-wave plate and a polarizing
beam-splitter are complementary due to the antisymmetric nature of the phase
apodization. The device can be achromatized in the same way as half-wave plates
of the Pancharatnam type or by layering self-aligning twisted liquid crystals
to form a monolithic film called a multi-twist retarder. As the VAPP introduces
a known phase diversity between the two PSFs, they may be used directly for
wavefront sensing. By applying an additional quarter-wave plate in front, the
device also acts as a regular polarizing beam-splitter, which therefore
furnishes high-contrast polarimetric imaging. If the PSF core is not saturated,
the polarimetric dual-beam correction can also be applied to polarized
circumstellar structure. The prototype results show the viability of the
vector-APP concept.Comment: Proc. SPIE 8450-2
Focal-plane wavefront sensing with high-order adaptive optics systems
We investigate methods to calibrate the non-common path aberrations at an
adaptive optics system having a wavefront-correcting device working at an
extremely high resolution (larger than 150x150). We use focal-plane images
collected successively, the corresponding phase-diversity information and
numerically efficient algorithms to calculate the required wavefront updates.
The wavefront correction is applied iteratively until the algorithms converge.
Different approaches are studied. In addition of the standard Gerchberg-Saxton
algorithm, we test the extension of the Fast & Furious algorithm that uses
three images and creates an estimate of the pupil amplitudes. We also test
recently proposed phase-retrieval methods based on convex optimisation. The
results indicate that in the framework we consider, the calibration task is
easiest with algorithms similar to the Fast & Furious.Comment: 11 pages, 7 figures, published in SPIE proceeding
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