2,886 research outputs found
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.
First tests of wavefront sensing with a constellation of laser guide beacons
Adaptive optics to correct current telescopes over wide fields, or future
very large telescopes over even narrow fields, will require real-time wavefront
measurements made with a constellation of laser beacons. Here we report the
first such measurements, made at the 6.5 m MMT with five Rayleigh beacons in a
2 arcmin pentagon. Each beacon is made with a pulsed beam at 532 nm, of 4 W at
the exit pupil of the projector. The return is range-gated from 20-29 km and
recorded at 53 Hz by a 36-element Shack-Hartmann sensor. Wavefronts derived
from the beacons are compared with simultaneous wavefronts obtained for
individual natural stars within or near the constellation. Observations were
made in seeing averaging 1.0 arcsec with 2/3 of the aberration measured to be
from a ground layer of mean height 380 m. Under these conditions, subtraction
of the simple instantaneous average of the five beacon wavefronts from the
stellar wavefronts yielded a 40% rms reduction in the measured modes of the
distortion over a 2 arcmin field. We discuss the use of multiple Rayleigh
beacons as an alternative to single sodium beacons on 8 m telescopes, and the
impact of the new work on the design of a multi-sodium beacon system for the 25
m Giant Magellan Telescope.Comment: 25 pages, 7 figures, uses aastex.cls, to be published in the
Astrophys.
Linking Demographic Effects of Habitat Fragmentation Across Landscapes to Continental Source-Sink Dynamics
Forest fragmentation may cause increased brood parasitism and nest predation of breeding birds. In North America, nest parasitism and predation are expected to increase closer to forest edges because the brood-parasitic Brown-headed Cowbird(Molothrus ater) and generalist nest predators often enter the forest from adjoining developed (largely agricultural) habitats. Yet the abundance of brood parasites and nest predators at the patch scale may be strongly constrained by the total area of developed habitat at landscape scales. The scale and extent of landscape effects are unclear, however, because past studies were mostly conducted within local landscapes rather than across independent landscapes. We report replicated studies from 30 independent landscapes across 17 states of the United States that show that nest parasitism is strongly affected by fragmentation at a 20 km radius scale, equivalent to the maximum foraging range of cowbirds. Nest predation is influenced by both edge and landscape effects, and increases with fragmentation at a 10 km radius scale. Predation is additive to parasitism mortality, and the two together yield decreased population growth potential with increasing forest fragmentation at a 10 km radius scale for 20 of 22 bird species. Mapping of population growth potential across continental landscapes displays broad impacts of fragmentation on population viability and allows geographic prioritization for conservation
Discovery of a 0.15" Binary Brown Dwarf 2MASSJ 1426316+155701 With Gemini/Hokupa'a Adaptive Optics
Use of the highly sensitive Hokupa'a curvature wavefront sensor has allowed
for the first time direct adaptive optics (AO) guiding on brown dwarfs and VLM
stars (SpT=M7-L2). An initial survey of 9 such objects discovered one 0.15"
binary (2MASSJ 1426316+155701). The companion is about half as bright as the
primary (Delta K = 0.61+/-0.05. These spectral types also match the
observed H-K colors of each star. Based the previously observed low space
motion and activity we assign an age of .
Utilizing this age range and the latest DUSTY models of the Lyon group we
assign a photometric distance of and masses of
and . We therefore estimate a system separation of
and a period of respectively. Hence, 2M1426 is among
the smallest separation brown dwarf binaries resolved to date
Modeling of pulsed laser guide stars for the Thirty Meter Telescope project
The Thirty Meter Telescope (TMT) has been designed to include an adaptive
optics system and associated laser guide star (LGS) facility to correct for the
image distortion due to Earth's atmospheric turbulence and achieve
diffraction-limited imaging. We have calculated the response of mesospheric
sodium atoms to a pulsed laser that has been proposed for use in the LGS
facility, including modeling of the atomic physics, the light-atom
interactions, and the effect of the geomagnetic field and atomic collisions.
This particular pulsed laser format is shown to provide comparable photon
return to a continuous-wave (cw) laser of the same average power; both the cw
and pulsed lasers have the potential to satisfy the TMT design requirements for
photon return flux.Comment: 16 pages, 20 figure
Adaptive Optics Nulling Interferometric Constraints on the Mid-Infrared Exozodiacal Dust Emission around Vega
We present the results of mid-infrared nulling interferometric observations
of the main-sequence star alpha Lyr (Vega) using the 6.5 m MMT with its
adaptive secondary mirror. From the observations at 10.6 microns, we find that
there is no resolved emission from the circumstellar environment (at
separations greater than 0.8 AU) above 2.1% (3 sigma limit) of the level of the
stellar photospheric emission. Thus, we are able to place an upper limit on the
density of dust in the inner system of 650 times that of our own solar system's
zodiacal cloud. This limit is roughly 2.8 times better than those determined
with photometric excess observations such as those by IRAS. Comparison with
far-infrared observations by IRAS shows that the density of warm dust in the
inner system (< 30 AU) is significantly lower than cold dust at larger
separations. We consider two scenarios for grain removal, the sublimation of
ice grains and the presence of a planetary mass "sweeper." We find that if
sublimation of ice grains is the only removal process, a large fraction (> 80%)
of the material in the outer system is ice.Comment: 11 pages, 1 figure, Accepted to The Astrophysical Journal Letter
Mid-Infrared Imaging of the Post-AGB Star AC Herculis with the MMT Adaptive Optics System
We utilized the MMT's unique deformable secondary adaptive optics system to
produce high-resolution (FWHM=0.3"), very high Strehl mid-infrared (9.8, 11.7 &
18 micron) images of the post-AGB star AC Her. The very high (98+/-2%) Strehls
achieved with Mid-IR AO led naturally to an ultra-stable PSF independent of
airmass, seeing, or location on the sky. We find no significant difference
between AC Her's morphology and our unresolved PSF calibration stars (mu UMa &
alpha Her) at 9.8, 11.7, & 18 microns. Our current observations do not confirm
any extended Mid-IR structure around AC Her. These observations are in conflict
with previously reported Keck (seeing-limited) 11.7 and 18 micron images which
suggested the presence of a resolved ~0.6" edge-on circumbinary disk. We
conclude that AC Her has no extended Mid-IR structure on scales greater than
0.2" (R<75 AU). These first results of Mid-IR AO science are very encouraging
for future high accuracy Mid-IR imaging with this technique.Comment: To appear in the November 20, 2003 issue of the Astrophysical Journal
Letters. The preprint has 7 pages and 3 figures (one in color; but prints OK
in B&W
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