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High fidelity imaging of geosynchronous satellites with the MROI
Interferometry currently provides the only practicable way to image satellites in Geosynchronous Earth Orbit (GEO) with sub-meter spatial resolution. The Magdalena Ridge Observatory Interferometer (MROI) is being funded by the US Air Force Research Laboratory to demonstrate the 9.5 magnitude sensitivity (at 2.2 µm wavelength) and baseline-bootstrapping capability that will be needed to realize a useful turn-key GEO imaging capability. This program will utilize the central three telescopes of the MROI and will aim to validate routine acquisition of fringe data on faint well-resolved targets. In parallel with this effort, the University of Cambridge are investigating the spatial resolution and imaging fidelity that can be achieved with different numbers of array elements. We present preliminary simulations of snapshot GEO satellite imaging with the MROI. Our results indicate that faithful imaging of the main satellite components can be obtained with as few as 7 unit telescopes, and that increasing the number of telescopes to 10 improves the effective spatial resolution from 0.75 meter to 0.5 meter and enables imaging of more complex targets.This is the author accepted manuscript. The final version is available from SPIE via http://dx.doi.org/10.1117/12.223247
Michelson Interferometry with the Keck I Telescope
We report the first use of Michelson interferometry on the Keck I telescope
for diffraction-limited imaging in the near infrared JHK and L bands. By using
an aperture mask located close to the f/25 secondary, the 10 m Keck primary
mirror was transformed into a separate-element, multiple aperture
interferometer. This has allowed diffraction-limited imaging of a large number
of bright astrophysical targets, including the geometrically complex dust
envelopes around a number of evolved stars. The successful restoration of these
images, with dynamic ranges in excess of 200:1, highlights the significant
capabilities of sparse aperture imaging as compared with more conventional
filled-pupil speckle imaging for the class of bright targets considered here.
In particular the enhancement of the signal-to-noise ratio of the Fourier data,
precipitated by the reduction in atmospheric noise, allows high fidelity
imaging of complex sources with small numbers of short-exposure images relative
to speckle. Multi-epoch measurements confirm the reliability of this imaging
technique and our whole dataset provides a powerful demonstration of the
capabilities of aperture masking methods when utilized with the current
generation of large-aperture telescopes. The relationship between these new
results and recent advances in interferometry and adaptive optics is briefly
discussed.Comment: Accepted into Publications of the Astronomical Society of the
Pacific. To appear in vol. 112. Paper contains 10 pages, 8 figure
Mid-infrared spectra of late-type stars: Long-term evolution
Recent ground-based mid-infrared spectra of 29 late-type stars, most with
substantial dust shells, are compared to ground-based spectra of these stars
from the 1960s and 1970s and to IRAS-LRS spectra obtained in 1983. The spectra
of about half the stars show no detectable changes, implying that their
distributions of circumstellar material and associated dust grain properties
have changed little over this time interval. However, many of the stars with
strong silicate features showed marked changes. In nearly all cases the
silicate peak has strengthened with respect to the underlying continuum,
although there is one case (VY~CMa) in which the silicate feature has almost
completely disappeared. This suggests that, in general, an oxygen-rich star
experiences long periods of gradual silicate feature strengthening, punctuated
by relatively rare periods when the feature weakens. We discuss various
mechanisms for producing the changes, favoring the slow evolution of the
intrinsic dust properties (i.e., the chemical composition or grain structure).
Although most IRAS spectra agree well with ground-based spectra, there are a
number of cases where they fall well outside the expected range of uncertainty.
In almost all such cases the slopes of the red and blue LRS spectra do not
match in their region of overlap.Comment: Accepted in ApJ, 20 pages, 5 figures, 1 tabl
Photon counting strategies with low light level CCDs
Low light level charge coupled devices (L3CCDs) have recently been developed,
incorporating on-chip gain. They may be operated to give an effective readout
noise much less than one electron by implementing an on-chip gain process
allowing the detection of individual photons. However, the gain mechanism is
stochastic and so introduces significant extra noise into the system. In this
paper we examine how best to process the output signal from an L3CCD so as to
minimize the contribution of stochastic noise, while still maintaining
photometric accuracy.
We achieve this by optimising a transfer function which translates the
digitised output signal levels from the L3CCD into a value approximating the
photon input as closely as possible by applying thresholding techniques. We
identify several thresholding strategies and quantify their impact on photon
counting accuracy and effective signal-to-noise.
We find that it is possible to eliminate the noise introduced by the gain
process at the lowest light levels. Reduced improvements are achieved as the
light level increases up to about twenty photons per pixel and above this there
is negligible improvement. Operating L3CCDs at very high speeds will keep the
photon flux low, giving the best improvements in signal-to-noise ratio.Comment: 7 pages, accepted by MNRA
An augmented reality interface for visualising and interacting with virtual content
In this paper, a novel AR interface is proposed that provides generic solutions to the tasks involved in augmenting simultaneously different types of virtual information and processing of tracking data for natural interaction. Participants within the system can experience a real-time mixture of 3D objects, static video, images, textual information and 3D sound with the real environment. The userfriendly AR interface can achieve maximum interaction using simple but effective forms of collaboration based on the combinations of humancomputer interaction techniques. To prove the feasibility of the interface, the use of indoor AR techniques are employed to construct innovative applications and demonstrate examples from heritage to learning systems. Finally, an initial evaluation of the AR interface including some initial results is presented
The performance of the MROI fast tip-tilt correction system
The fast tip-tilt (FTT) correction system for the Magdalena Ridge Observatory Interferometer (MROI) is being developed by the University of Cambridge. The design incorporates an EMCCD camera protected by a thermal enclosure, optical mounts with passive thermal compensation, and control software running under Xenomai real-time Linux. The complete FTT system is now undergoing laboratory testing prior to being installed on the first MROI unit telescope in the fall of 2014. We are following a twin-track approach to testing the closed-loop performance: tracking tip-tilt perturbations introduced by an actuated flat mirror in the laboratory, and undertaking end-to-end simulations that incorporate realistic higher-order atmospheric perturbations. We report test results that demonstrate (a) the high stability of the entire opto-mechanical system, realized with a completely passive design; and (b) the fast tip-tilt correction performance and limiting sensitivity. Our preliminary results in both areas are close to those needed to realise the ambitious stability and sensitivity goals of the MROI which aims to match the performance of current natural guide star adaptive optics systems.Previously funded by the Naval Research Laboratory (under Agreement No. N00173-01-2-C902), MROI is currently funded by the US Department of Transportation, the State of New Mexico and by New Mexico Tech
The last gasps of VY CMa: Aperture synthesis and adaptive optics imagery
We present new observations of the red supergiant VY CMa at 1.25 micron, 1.65
micron, 2.26 micron, 3.08 micron and 4.8 micron. Two complementary
observational techniques were utilized: non-redundant aperture masking on the
10-m Keck-I telescope yielding images of the innermost regions at unprecedented
resolution, and adaptive optics imaging on the ESO 3.6-m telescope at La Silla
attaining extremely high (~10^5) peak-to-noise dynamic range over a wide field.
For the first time the inner dust shell has been resolved in the near-infrared
to reveal a one-sided extension of circumstellar emission within 0.1" (~15
R_star) of the star. The line-of-sight optical depths of the circumstellar dust
shell at 1.65 micron, 2.26 micron, and 3.08 micron have been estimated to be
1.86 +/- 0.42, 0.85 +/- 0.20, and 0.44 +/- 0.11. These new results allow the
bolometric luminosity of VY~CMa to be estimated independent of the dust shell
geometry, yielding L_star ~ 2x10^5 L_sun. A variety of dust condensations,
including a large scattering plume and a bow-shaped dust feature, were observed
in the faint, extended nebula up to 4" from the central source. While the
origin of the nebulous plume remains uncertain, a geometrical model is
developed assuming the plume is produced by radially-driven dust grains forming
at a rotating flow insertion point with a rotational period between 1200-4200
years, which is perhaps the stellar rotational period or the orbital period of
an unseen companion.Comment: 25 pages total with 1 table and 5 figures. Accepted by Astrophysical
Journal (to appear in February 1999
Planet Formation Imager (PFI): Introduction and Technical Considerations
Complex non-linear and dynamic processes lie at the heart of the planet
formation process. Through numerical simulation and basic observational
constraints, the basics of planet formation are now coming into focus. High
resolution imaging at a range of wavelengths will give us a glimpse into the
past of our own solar system and enable a robust theoretical framework for
predicting planetary system architectures around a range of stars surrounded by
disks with a diversity of initial conditions. Only long-baseline interferometry
can provide the needed angular resolution and wavelength coverage to reach
these goals and from here we launch our planning efforts. The aim of the
"Planet Formation Imager" (PFI) project is to develop the roadmap for the
construction of a new near-/mid-infrared interferometric facility that will be
optimized to unmask all the major stages of planet formation, from initial dust
coagulation, gap formation, evolution of transition disks, mass accretion onto
planetary embryos, and eventual disk dispersal. PFI will be able to detect the
emission of the cooling, newly-formed planets themselves over the first 100
Myrs, opening up both spectral investigations and also providing a vibrant look
into the early dynamical histories of planetary architectures. Here we
introduce the Planet Formation Imager (PFI) Project
(www.planetformationimager.org) and give initial thoughts on possible facility
architectures and technical advances that will be needed to meet the
challenging top-level science requirements.Comment: SPIE Astronomical Telescopes and Instrumentation conference, June
2014, Paper ID 9146-35, 10 pages, 2 Figure
Diffraction-limited near-IR imaging at Keck reveals asymmetric, time-variable nebula around carbon star CIT 6
We present multi-epoch, diffraction-limited images of the nebula around the
carbon star CIT 6 at 2.2 microns and 3.1 microns from aperture masking on the
Keck-I telescope. The near-IR nebula is resolved into two main components, an
elongated, bright feature showing time-variable asymmetry and a fainter
component about 60 milliarcseconds away with a cooler color temperature. These
images were precisely registered (~35 milliarcseconds) with respect to recent
visible images from the Hubble Space Telescope (Trammell et al. 2000), which
showed a bipolar structure in scattered light. The dominant near-IR feature is
associated with the northern lobe of this scattering nebula, and the
multi-wavelength dataset can be understood in terms of a bipolar dust shell
around CIT 6. Variability of the near-IR morphology is qualitatively consistent
with previously observed changes in red polarization, caused by varying
illumination geometry due to non-uniform dust production. The blue emission
morphology and polarization properties can not be explained by the above model
alone, but require the presence of a wide binary companion in the vicinity of
the southern polar lobe. The physical mechanisms responsible for the breaking
of spherical symmetry around extreme carbon stars, such as CIT 6 and IRC+10216,
remain uncertain.Comment: 18 pages, 5 figures (one in color), to appear in the Astrophysical
Journa
Near and mid-IR sub-arcsecond structure of the dusty symbiotic star R Aqr
The results of a high-resolution interferometric campaign targeting the
symbiotic long-period variable (LPV) R~Aqr are reported. With both
near-infrared measurements on baselines out to 10m and mid-infrared data
extending to 32m, we have been able to measure the characteristic sizes of
regions from the photosphere of the LPV and its extended molecular atmosphere,
out to the cooler circumstellar dust shell. The near-infrared data were taken
using aperture masking interferometry on the Keck-I telescope and show R~Aqr to
be partially resolved for wavelengths out to 2.2 microns but with a marked
enlargement, possibly due to molecular opacity, at 3.1 microns. Mid-infrared
interferometric measurements were obtained with the U.C. Berkeley Infrared
Spatial Interferometer (ISI) operating at 11.15 microns from 1992 to 1999.
Although this dataset is somewhat heterogeneous with incomplete coverage of the
Fourier plane and sampling of the pulsation cycle, clear changes in the
mid-infrared brightness distribution were observed, both as a function of
position angle on the sky and as a function of pulsation phase. Spherically
symmetric radiative transfer calculations of uniform-outflow dust shell models
produce brightness distributions and spectra which partially explain the data,
however limitations to this approximation are noted. Evidence for significant
deviation from circular symmetry was found in the mid-infrared and more
tentatively at 3.08 microns in the near-infrared, however no clear detection of
binarity or of non-LPV elements in the symbiotic system is reported.Comment: Accepted to Astrophysical Journal. To appear in volume 534. 14 pages;
3 postscript figure
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