428 research outputs found
Multi-conjugate adaptive optics with hybrid laser beacon systems
Results for a multi-conjugate adaptive optical (MCAO) system with multiple laser beacons at multiple altitudes are presented in this paper. The use of multi-conjugate deformable mirrors (DM's) increases the corrected field of view of an adaptive optical telescope system. This improves the imaging capability for extended astronomical objects such as planets, galaxies, and nebulae. Multiple laser beacons, as opposed to multiple natural guide stars, are needed to achieve a useful degree of sky coverage. The use of laser beacons at multiple altitudes in a hybrid laser beacon configuration has been shown in previous papers to reduce both focus and tilt anisoplanatism. In this study we combine all three of these aspects. The hybrid beacon scenarios used in this study consists of multiple high altitude sodium beacons at 90 km and/or multiple low altitude Rayleigh beacons at 10 to 20 km. We present results for an 8-m class telescope for 2 and 3 different DM conjugate altitudes. For each of these MCAO configurations the following parameters are varied: number of Rayleigh beacons, number of Rayleigh beacon wavefront sensor (WFS) subapertures, Rayleigh beacon altitudes for the Rayleigh/sodium configuration, number of natural beacons for tip/tilt correction, and number of natural beacon WFS subapertures. When the WFS subaperture for the natural beacon is greater than 1 x 1 it contributes to the higher order correction in addition to being used for tip/tilt correction. Results are compared in terms of Strehl Ratio for the J, H, and K band
The footprint of cometary dust analogues: II. Morphology as a tracer of tensile strength and application to dust collection by the Rosetta spacecraft
The structure of cometary dust is a tracer of growth processes in the
formation of planetesimals. Instrumentation on board the Rosetta mission to
comet 67P/Churyumov- Gerasimenko captured dust particles and analysed them in
situ. However, these deposits are a product of a collision within the
instrument. We conducted laboratory experiments with cometary dust analogues,
simulating the collection process by Rosetta instruments (specifically COSIMA,
MIDAS). In Paper I we reported that velocity is a key driver in determining the
appearance of deposits. Here in Paper II we use materials with different
monomer sizes, and study the effect of tensile strength on the appearance of
deposits. We find that mass transfer efficiency increases from 1 up to
10% with increasing monomer diameter from 0.3 m to 1.5 m (i.e.
tensile strength decreasing from 12 to 3 kPa), and velocities
increasing from 0.5 to 6 m/s. Also, the relative abundance of small fragments
after impact is higher for material with higher tensile strength. The
degeneracy between the effects of velocity and material strength may be lifted
by performing a closer study of the deposits. This experimental method makes it
possible to estimate the mass transfer efficiency in the COSIMA instrument.
Extrapolating these results implies that more than half of the dust collected
during the Rosetta mission has not been imaged. We analysed two COSIMA targets
containing deposits from single collisions. The collision that occurred closest
to perihelion passage led to more small fragments on the target.Comment: 13 pages, 11 figures, accepted for publication in MNRA
The effects of consuming a high protein diet (4.4 g/kg/d) on body composition in resistance-trained individuals
BACKGROUND:
The consumption of dietary protein is important for resistance-trained individuals. It has been posited that intakes of 1.4 to 2.0 g/kg/day are needed for physically active individuals. Thus, the purpose of this investigation was to determine the effects of a very high protein diet (4.4 g/kg/d) on body composition in resistance-trained men and women. METHODS:
Thirty healthy resistance-trained individuals participated in this study (mean ± SD; age: 24.1 ± 5.6 yr; height: 171.4 ± 8.8 cm; weight: 73.3 ± 11.5 kg). Subjects were randomly assigned to one of the following groups: Control (CON) or high protein (HP). The CON group was instructed to maintain the same training and dietary habits over the course of the 8 week study. The HP group was instructed to consume 4.4 grams of protein per kg body weight daily. They were also instructed to maintain the same training and dietary habits (e.g. maintain the same fat and carbohydrate intake). Body composition (Bod Pod®), training volume (i.e. volume load), and food intake were determined at baseline and over the 8 week treatment period. RESULTS:
The HP group consumed significantly more protein and calories pre vs post (p \u3c 0.05). Furthermore, the HP group consumed significantly more protein and calories than the CON (p \u3c 0.05). The HP group consumed on average 307 ± 69 grams of protein compared to 138 ± 42 in the CON. When expressed per unit body weight, the HP group consumed 4.4 ± 0.8 g/kg/d of protein versus 1.8 ± 0.4 g/kg/d in the CON. There were no changes in training volume for either group. Moreover, there were no significant changes over time or between groups for body weight, fat mass, fat free mass, or percent body fat. CONCLUSIONS:
Consuming 5.5 times the recommended daily allowance of protein has no effect on body composition in resistance-trained individuals who otherwise maintain the same training regimen. This is the first interventional study to demonstrate that consuming a hypercaloric high protein diet does not result in an increase in body fat
The footprint of cometary dust analogs: I. Laboratory experiments of low-velocity impacts and comparison with Rosetta data
Cometary dust provides a unique window on dust growth mechanisms during the
onset of planet formation. Measurements by the Rosetta spacecraft show that the
dust in the coma of comet 67P/Churyumov-Gerasimenko has a granular structure at
size scales from sub-um up to several hundreds of um, indicating hierarchical
growth took place across these size scales. However, these dust particles may
have been modified during their collection by the spacecraft instruments. Here
we present the results of laboratory experiments that simulate the impact of
dust on the collection surfaces of COSIMA and MIDAS, instruments onboard the
Rosetta spacecraft. We map the size and structure of the footprints left by the
dust particles as a function of their initial size (up to several hundred um)
and velocity (up to 6 m/s). We find that in most collisions, only part of the
dust particle is left on the target; velocity is the main driver of the
appearance of these deposits. A boundary between sticking/bouncing and
fragmentation as an outcome of the particle-target collision is found at v ~ 2
m/s. For velocities below this value, particles either stick and leave a single
deposit on the target plate, or bounce, leaving a shallow footprint of
monomers. At velocities > 2 m/s and sizes > 80 um, particles fragment upon
collision, transferring up to 50 per cent of their mass in a rubble-pile-like
deposit on the target plate. The amount of mass transferred increases with the
impact velocity. The morphologies of the deposits are qualitatively similar to
those found by the COSIMA instrument.Comment: 14 pages, 12 figures, accepted for publication in MNRA
Ground-layer wavefront reconstruction from multiple natural guide stars
Observational tests of ground layer wavefront recovery have been made in open
loop using a constellation of four natural guide stars at the 1.55 m Kuiper
telescope in Arizona. Such tests explore the effectiveness of wide-field seeing
improvement by correction of low-lying atmospheric turbulence with ground-layer
adaptive optics (GLAO). The wavefronts from the four stars were measured
simultaneously on a Shack-Hartmann wavefront sensor (WFS). The WFS placed a 5 x
5 array of square subapertures across the pupil of the telescope, allowing for
wavefront reconstruction up to the fifth radial Zernike order. We find that the
wavefront aberration in each star can be roughly halved by subtracting the
average of the wavefronts from the other three stars. Wavefront correction on
this basis leads to a reduction in width of the seeing-limited stellar image by
up to a factor of 3, with image sharpening effective from the visible to near
infrared wavelengths over a field of at least 2 arc minutes. We conclude that
GLAO correction will be a valuable tool that can increase resolution and
spectrographic throughput across a broad range of seeing-limited observations.Comment: 25 pages, 8 figures, to be published in Astrophys.
RCW36: characterizing the outcome of massive star formation
Massive stars play a dominant role in the process of clustered star
formation, with their feedback into the molecular cloud through ionizing
radiation, stellar winds and outflows. The formation process of massive stars
is poorly constrained because of their scarcity, the short formation timescale
and obscuration. By obtaining a census of the newly formed stellar population,
the star formation history of the young cluster and the role of the massive
stars within it can be unraveled. We aim to reconstruct the formation history
of the young stellar population of the massive star-forming region RCW 36. We
study several dozens of individual objects, both photometrically and
spectroscopically, look for signs of multiple generations of young stars and
investigate the role of the massive stars in this process. We obtain a census
of the physical parameters and evolutionary status of the young stellar
population. Using a combination of near-infrared photometry and spectroscopy we
estimate ages and masses of individual objects. We identify the population of
embedded young stellar objects (YSO) by their infrared colors and emission line
spectra. RCW 36 harbors a stellar population of massive and intermediate-mass
stars located around the center of the cluster. Class 0/I and II sources are
found throughout the cluster. The central population has a median age of 1.1
+/- 0.6 Myr. Of the stars which could be classified, the most massive ones are
situated in the center of the cluster. The central cluster is surrounded by
filamentary cloud structures; within these, some embedded and accreting YSOs
are found. Our age determination is consistent with the filamentary structures
having been shaped by the ionizing radiation and stellar winds of the central
massive stars. The formation of a new generation of stars is ongoing, as
demonstrated by the presence of embedded protostellar clumps, and two exposed
jets.Comment: 18 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
Prospects for measuring supermassive black hole masses with future extremely large telescopes
The next generation of giant-segmented mirror telescopes ( 20 m) will
enable us to observe galactic nuclei at much higher angular resolution and
sensitivity than ever before. These capabilities will introduce a revolutionary
shift in our understanding of the origin and evolution of supermassive black
holes by enabling more precise black hole mass measurements in a mass range
that is unreachable today. We present simulations and predictions of the
observations of nuclei that will be made with the Thirty Meter Telescope (TMT)
and the adaptive optics assisted integral-field spectrograph IRIS, which is
capable of diffraction-limited spectroscopy from band (0.9 m) to
band (2.2 m). These simulations, for the first time, use realistic values
for the sky, telescope, adaptive optics system, and instrument, to determine
the expected signal-to-noise ratio of a range of possible targets spanning
intermediate mass black holes of \msun to the most massive black
holes known today of . We find that IRIS will be able to
observe Milky Way-mass black holes out the distance of the Virgo cluster, and
will allow us to observe many more brightest cluster galaxies where the most
massive black holes are thought to reside. We also evaluate how well the
kinematic moments of the velocity distributions can be constrained at the
different spectral resolutions and plate scales designed for IRIS. We find that
a spectral resolution of will be necessary to measure the masses of
intermediate mass black holes. By simulating the observations of galaxies found
in SDSS DR7, we find that over massive black holes will be observable at
distances between with the estimated sensitivity and angular
resolution provided by access to -band (0.9 m) spectroscopy from IRIS
and the TMT adaptive optics system. (Abridged)Comment: 19 pages, 20 figures, accepted to A
Adaptive Optics for Astronomy
Adaptive Optics is a prime example of how progress in observational astronomy
can be driven by technological developments. At many observatories it is now
considered to be part of a standard instrumentation suite, enabling
ground-based telescopes to reach the diffraction limit and thus providing
spatial resolution superior to that achievable from space with current or
planned satellites. In this review we consider adaptive optics from the
astrophysical perspective. We show that adaptive optics has led to important
advances in our understanding of a multitude of astrophysical processes, and
describe how the requirements from science applications are now driving the
development of the next generation of novel adaptive optics techniques.Comment: to appear in ARA&A vol 50, 201
Design considerations of the AO module for the Gemini South multiconjugate adaptive optics system
The adaptive optics system for the Gemini South telescope, currently in the design phase, consists of several major subsystem. The largest subsystem, called the AO module, contains most of the optics and electronics and is mounted on one of the Cassegrain instrument ports. The initial system will be a conventional laser guide star AO system, but the plan is to eventually expand it to a multi-conjugate system. The system is being designed to readily add the components necessary to upgrade to a multi-conjugate system. This paper describes the design challenges encountered and solutions that were derived for the AO module design. The complexity of the multi-conjugate version is illustrated, including optical, mechanical, electronic and controls issues
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