110 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
First On-Sky High Contrast Imaging with an Apodizing Phase Plate
We present the first astronomical observations obtained with an Apodizing
Phase Plate (APP). The plate is designed to suppress the stellar diffraction
pattern by 5 magnitudes from 2-9 lambda/D over a 180 degree region. Stellar
images were obtained in the M' band (4.85 microns) at the MMTO 6.5m telescope,
with adaptive wavefront correction made with a deformable secondary mirror
designed for low thermal background observations. The measured PSF shows a halo
intensity of 0.1% of the stellar peak at 2 lambda/D (0.36 arcsec), tapering off
as r^{-5/3} out to radius 9 lambda/D. Such a profile is consistent with
residual errors predicted for servo lag in the AO system.
We project a 5 sigma contrast limit, set by residual atmospheric
fluctuations, of 10.2 magnitudes at 0.36 arcsec separation for a one hour
exposure. This can be realised if static and quasi-static aberrations are
removed by differential imaging, and is close to the sensitivity level set by
thermal background photon noise for target stars with M'>3. The advantage of
using the phase plate is the removal of speckle noise caused by the residuals
in the diffraction pattern that remain after PSF subtraction. The APP gives
higher sensitivity over the range 2-5 lambda/D compared to direct imaging
techniques.Comment: 22 pages, 5 figures, 1 table, ApJ accepte
Calibrating a high-resolution wavefront corrector with a static focal-plane camera
We present a method to calibrate a high-resolution wavefront-correcting
device with a single, static camera, located in the focal plane; no moving of
any component is needed. The method is based on a localized diversity and
differential optical transfer functions (dOTF) to compute both the phase and
amplitude in the pupil plane located upstream of the last imaging optics. An
experiment with a spatial light modulator shows that the calibration is
sufficient to robustly operate a focal-plane wavefront sensing algorithm
controlling a wavefront corrector with ~40 000 degrees of freedom. We estimate
that the locations of identical wavefront corrector elements are determined
with a spatial resolution of 0.3% compared to the pupil diameter.Comment: 12 pages, 12 figures, accepted for publication in Applied Optic
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
Polarization dOTF: on-sky focal plane wavefront sensing
The differential Optical Transfer Function (dOTF) is a focal plane wavefront
sensing method that uses a diversity in the pupil plane to generate two
different focal plane images. The difference of their Fourier transforms
recovers the complex amplitude of the pupil down to the spatial scale of the
diversity. We produce two simultaneous PSF images with diversity using a
polarizing filter at the edge of the telescope pupil, and a polarization camera
to simultaneously record the two images. Here we present the first on-sky
demonstration of polarization dOTF at the 1.0m South African Astronomical
Observatory telescope in Sutherland, and our attempt to validate it with
simultaneous Shack-Hartmann wavefront sensor images.Comment: 11 pages, 9 figures, Proc. SPIE Vol. 991
Pupil Plane Phase Apodization
Phase apodization coronagraphs are implemented in a pupil plane to create a
dark hole in the science camera focal plane. They are successfully created as
"Apodizing Phase Plates" (APPs) using classical optical manufacturing, and as
"vector-APPs" using liquid-crystal patterning with essentially achromatic
performance. This type of coronagraph currently delivers excellent broadband
contrast (10) at small angular separations (few ) at
ground-based telescopes, owing to their insensitivity to tip/tilt errors.Comment: Invited chapter, to be published in the "Handbook of Astronomical
Instrumentation", Vol. 3, eds. A. Moore and D. Burrows, WSPC (2018). 9 pages,
1 figur
Recommended from our members
Managing and Delivering Gypsy and Traveller Sites: negotiating conflict
The many symptoms of failure to provide sufficient sites to accommodate Gypsies and Travellers include
poor health, anxiety, and an increasing disconnect from the broader community with Gypsies and
Travellers, and poorer education outcomes for their children. Council officers and elected members
receive complaints about unauthorised encampments and have difficulty responding if there are no
appropriate alternatives.
This is a complex problem with no ‘quick fix’ solution. But it is possible to deliver well-managed Gypsy
and Traveller sites and, where that is achieved, encampments and associated problems also reduce. The
evidence also shows that it is more cost effective than expensive police and court action to deal with
unauthorised sites.
Whilst challenges were found in the research, this report aims to bring balance and make counter
arguments to the pervasive notion that all sites are problematic and that Gypsy and Traveller issues
belong in the ‘too difficult’ pile of things to do by local authorities. The overriding message is that sites
can be well-managed, sustainable and vital elements of a diverse community. Challenges can be
overcome. Inequalities in treatment of Gypsies and Travellers must be addressed, and one element of
that – an essential first step – is the provision of sufficient, appropriate, well-managed accommodation.
The report, Managing and Delivering Gypsy and Traveller sites: negotiating conflict by Jo Richardson
and Janie Codona MBE, looks in detail and with many case examples at what councils and housing
associations are doing to develop and manage Gypsy and Traveller sites. Because it is the most detailed
practical study in recent years, in which dozens of sites were visited and many interviews held with
residents and with responsible staff, it will be invaluable for those who are planning, providing or
managing sites. It is also a call to action for those councils who are ignoring the issue
Expected Performance of a Self-Coherent Camera
Residual wavefront errors in optical elements limit the performance of
coronagraphs. To improve their efficiency, different types of devices have been
proposed to correct or calibrate these errors. In this paper, we study one of
these techniques proposed by Baudoz et al. 2006 and called Self-Coherent Camera
(SCC). The principle of this instrument is based on the lack of coherence
between the stellar light and the planet that is searched for. After recalling
the principle of the SCC, we simulate its performance under realistic
conditions and compare it with the performance of differential imaging.Comment: 6 pages, 4 figure
An apodizing phase plate coronagraph for VLT/NACO
We describe a coronagraphic optic for use with CONICA at the VLT that
provides suppression of diffraction from 1.8 to 7 lambda/D at 4.05 microns, an
optimal wavelength for direct imaging of cool extrasolar planets. The optic is
designed to provide 10 magnitudes of contrast at 0.2 arcseconds, over a
D-shaped region in the image plane, without the need for any focal plane
occulting mask.Comment: 9 pages, 5 figures, to appear in Proc. SPIE Vol. 773
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