42 research outputs found
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
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
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 âquick fixâ 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 âtoo difficultâ 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 â an essential first step â 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
Theory of Parabolic Arcs in Interstellar Scintillation Spectra
Our theory relates the secondary spectrum, the 2D power spectrum of the radio
dynamic spectrum, to the scattered pulsar image in a thin scattering screen
geometry. Recently discovered parabolic arcs in secondary spectra are generic
features for media that scatter radiation at angles much larger than the rms
scattering angle. Each point in the secondary spectrum maps particular values
of differential arrival-time delay and fringe rate (or differential Doppler
frequency) between pairs of components in the scattered image. Arcs correspond
to a parabolic relation between these quantities through their common
dependence on the angle of arrival of scattered components. Arcs appear even
without consideration of the dispersive nature of the plasma. Arcs are more
prominent in media with negligible inner scale and with shallow wavenumber
spectra, such as the Kolmogorov spectrum, and when the scattered image is
elongated along the velocity direction. The arc phenomenon can be used,
therefore, to constrain the inner scale and the anisotropy of scattering
irregularities for directions to nearby pulsars. Arcs are truncated by finite
source size and thus provide sub micro arc sec resolution for probing emission
regions in pulsars and compact active galactic nuclei. Multiple arcs sometimes
seen signify two or more discrete scattering screens along the propagation
path, and small arclets oriented oppositely to the main arc persisting for long
durations indicate the occurrence of long-term multiple images from the
scattering screen.Comment: 22 pages, 11 figures, submitted to the Astrophysical Journa
Understanding the Radio Variability of Sgr A*
We determine the characteristics of the 7mm to 20cm wavelength radio
variability in Sgr A* on time scales from days to three decades. The amplitude
of the intensity modulation is between 30 and 39% at all wavelengths. Analysis
of uniformly sampled data with proper accounting of the sampling errors
associated with the lightcurves shows that Sgr A* exhibits no 57- or 106-day
quasi-periodic oscillations, contrary to previous claims. The cause of the
variability is investigated by examining a number of plausible scintillation
models, enabling those variations which could be attributed to interstellar
scintillation to be isolated from those that must be intrinsic to the source.
Thin-screen scattering models do not account for the variability amplitude on
most time scales. However, models in which the scattering region is extended
out to a radius of 50-500pc from the Galactic Center account well for the broad
characteristics of the variability on >4-day time scales. The ~ 10% variability
on <4-day time scales at 0.7-3cm appears to be intrinsic to the source. The
degree of scintillation variability expected at millimeter wavelengths depends
sensitively on the intrinsic source size; the variations, if due to
scintillation, would require an intrinsic source size smaller than that
expected.Comment: Ap.J. accepted, high-resolution version at
http://www.aoc.nrao.edu/~jmacquar/SgrA.pd
Interstellar Scintillation Observations of 146 Extragalactic Radio Sources
From 1979--1996 the Green Bank Interferometer was used by the Naval Research
Laboratory to monitor the flux density from 146 compact radio sources at
frequencies near 2 and 8 GHz. We filter the ``light curves'' to separate
intrinsic variations on times of a year or more from more rapid interstellar
scintilation (ISS) on times of 5--50 d. Whereas the intrinsic variation at 2
GHz is similar to that at 8 GHz (though diminished in amplitude), the ISS
variation is much stronger at 2 than at 8 GHz. We characterize the ISS
variation by an rms amplitude and a timescale and examine the statistics of
these parameters for the 121 sources with significant ISS at 2 GHz. We model
the scintillations using the NE2001 Galactic electron model assuming the
sources are brightness-limited.
We find the observed rms amplitude to be in general agreement with the model,
provided that the compact components of the sources have about 50% of their
flux density in a component with maximum brightness temperatures
--K. Thus our results are consistent with cm-wavelength VLBI
studies of compact AGNs, in that the maximum brightness temperatures found are
consistent with the inverse synchrotron limit at K, boosted
in jet configurations by Doppler factors up to about 20. The average of the
observed 2 GHz ISS timescales is in reasonable agreement with the model at
Galactic latitudes above about 10\de. At lower latitudes the observed
timescales are too fast, suggesting that the transverse plasma velocity
increases more than expected beyond about 1 kpc.Comment: 32 pages, 16 figures. Submitted to Ap
Speckle noise and dynamic range in coronagraphic images
This paper is concerned with the theoretical properties of high contrast
coronagraphic images in the context of exoplanet searches. We derive and
analyze the statistical properties of the residual starlight in coronagraphic
images, and describe the effect of a coronagraph on the speckle and photon
noise. Current observations with coronagraphic instruments have shown that the
main limitations to high contrast imaging are due to residual quasi-static
speckles. We tackle this problem in this paper, and propose a generalization of
our statistical model to include the description of static, quasi-static and
fast residual atmospheric speckles. The results provide insight into the
effects on the dynamic range of wavefront control, coronagraphy, active speckle
reduction, and differential speckle calibration. The study is focused on
ground-based imaging with extreme adaptive optics, but the approach is general
enough to be applicable to space, with different parameters.Comment: 31 pages, 18 figure
Precise Wavefront Correction with an Unbalanced Nulling Interferometer for Exo-Planet Imaging Coronagraphs
Very high dynamical range coronagraphs targeting direct exo-planet detection
(10^9 - 10^10 contrast) at small angular separation (few lambda/D units)
usually require an input wavefront quality on the order of ten thousandths of
wavelength RMS. We propose a novel method based on a pre-optics setup that
behaves partly as a low-efficiency coronagraph, and partly as a
high-sensitivity wavefront aberration compensator (phase and amplitude). The
combination of the two effects results in a highly accurate corrected
wavefront. First, an (intensity-) unbalanced nulling interferometer (UNI)
performs a rejection of part of the wavefront electric field. Then the
recombined output wavefront has its input aberrations magnified. Because of the
unbalanced recombination scheme, aberrations can be free of phase singular
points (zeros) and can therefore be compensated by a downstream phase and
amplitude correction (PAC) adaptive optics system, using two deformable
mirrors. In the image plane, the central star's peak intensity and the noise
level of its speckled halo are reduced by the UNI-PAC combination: the
output-corrected wavefront aberrations can be interpreted as an improved
compensation of the initial (eventually already corrected) incident wavefront
aberrations. The important conclusion is that not all the elements in the
optical setup using UNI-PAC need to reach the lambda/10000 rms surface error
quality.Comment: Accepted for publication in A&
Size of the Vela Pulsar's Radio Emission Region: 500 km
We use interstellar scattering of the Vela pulsar to determine the size of
its emission region. From interferometric phase variations on short baselines,
we find that radio-wave scattering broadens the source by 3.4+/-0.3
milliarcseconds along the major axis at position angle 81+/-3 degrees. The
ratio of minor axis to major axis is 0.51+/-0.03. Comparison of angular and
temporal broadening indicates that the scattering material lies in the Vela-X
supernova remnant surrounding the pulsar. From the modulation of the pulsar's
scintillation on very short baselines, we infer a size of 500 km for the
pulsar's emission region. We suggest that radio-wave refraction within the
pulsar's magnetosphere may plausibly explain this size.Comment: 14 pages, includes 2 figures. Also available at:
http://charm.physics.ucsb.edu:80/people/cgwinn/cgwinn_group/cgwinn_group.htm
Size of the Vela Pulsar's Emission Region at 13 cm Wavelength
We present measurements of the size of the Vela pulsar in 3 gates across the
pulse, from observations of the distribution of intensity. We calculate the
effects on this distribution of noise in the observing system, and measure and
remove it using observations of a strong continuum source. We also calculate
and remove the expected effects of averaging in time and frequency. We find
that effects of variations in pulsar flux density and instrumental gain,
self-noise, and one-bit digitization are undetectably small. Effects of
normalization of the correlation are detectable, but do not affect the fitted
size. The size of the pulsar declines from 440 +/- 90 km (FWHM of best-fitting
Gaussian distribution) to less than 200 km across the pulse. We discuss
implications of this size for theories of pulsar emission.Comment: 51 pages, 10 figures. To appear in ApJ. Also available at
http://www.physics.ucsb.edu/~cgwinn/pulsar/size_14.p