420 research outputs found
Ground-based adaptive optics coronagraphic performance under closed-loop predictive control
The discovery of the exoplanet Proxima b highlights the potential for the
coming generation of giant segmented mirror telescopes (GSMTs) to characterize
terrestrial --- potentially habitable --- planets orbiting nearby stars with
direct imaging. This will require continued development and implementation of
optimized adaptive optics systems feeding coronagraphs on the GSMTs. Such
development should proceed with an understanding of the fundamental limits
imposed by atmospheric turbulence. Here we seek to address this question with a
semi-analytic framework for calculating the post-coronagraph contrast in a
closed-loop AO system. We do this starting with the temporal power spectra of
the Fourier basis calculated assuming frozen flow turbulence, and then apply
closed-loop transfer functions. We include the benefits of a simple predictive
controller, which we show could provide over a factor of 1400 gain in raw PSF
contrast at 1 on bright stars, and more than a factor of 30 gain on
an I = 7.5 mag star such as Proxima. More sophisticated predictive control can
be expected to improve this even further. Assuming a photon noise limited
observing technique such as High Dispersion Coronagraphy, these gains in raw
contrast will decrease integration times by the same large factors. Predictive
control of atmospheric turbulence should therefore be seen as one of the key
technologies which will enable ground-based telescopes to characterize
terrrestrial planets.Comment: Accepted to JATI
MagAO-X: project status and first laboratory results
MagAO-X is an entirely new extreme adaptive optics system for the Magellan Clay 6.5 m telescope, funded by the NSF MRI program starting in Sep 2016. The key science goal of MagAO-X is high-contrast imaging of accreting protoplanets at Hα. With 2040 actuators operating at up to 3630 Hz, MagAO-X will deliver high Strehls (> 70%), high resolution (19 mas), and high contrast (< 1 × 10^(-4)) at Hα (656 nm). We present an overview of the MagAO-X system, review the system design, and discuss the current project status
The Magellan Adaptive Secondary VisAO Camera: Diffraction- Limited Broadband Visible Imaging and 20mas Fiber Array IFS
The Magellan Adaptive Secondary AO system, scheduled for first light in the
fall of 2011, will be able to simultaneously perform diffraction limited AO
science in both the mid-IR, using the BLINC/MIRAC4 10\{mu}m camera, and in the
visible using our novel VisAO camera. The VisAO camera will be able to operate
as either an imager, using a CCD47 with 8.5 mas pixels, or as an IFS, using a
custom fiber array at the focal plane with 20 mas elements in its highest
resolution mode. In imaging mode, the VisAO camera will have a full suite of
filters, coronagraphic focal plane occulting spots, and SDI prism/filters. The
imaging mode should provide ~20% mean Strehl diffraction-limited images over
the band 0.5-1.0 \{mu}m. In IFS mode, the VisAO instrument will provide R~1,800
spectra over the band 0.6-1.05 \{mu}m. Our unprecedented 20 mas spatially
resolved visible spectra would be the highest spatial resolution achieved to
date, either from the ground or in space. We also present lab results from our
recently fabricated advanced triplet Atmospheric Dispersion Corrector (ADC) and
the design of our novel wide-field acquisition and active optics lens. The
advanced ADC is designed to perform 58% better than conventional doublet ADCs
and is one of the enabling technologies that will allow us to achieve broadband
(0.5-1.0\{mu}m) diffraction limited imaging and wavefront sensing in the
visible.Comment: Proceedings of the SPIE, 2010, Vol. 7736, 77362
Resolving the H-alpha-emitting Region in the Wind of Eta Carinae
The massive evolved star Eta Carinae is the most luminous star in the Milky
Way and has the highest steady wind mass-loss rate of any known star. Radiative
transfer models of the spectrum by Hillier et al. predict that H-alpha is
mostly emitted in regions of the wind at radii of 6 to 60 AU from the star (2.5
to 25 mas at 2.35 kpc). We present diffraction-limited images (FWHM ~25 mas)
with Magellan adaptive optics in two epochs, showing that Eta Carinae
consistently appears ~2.5 to 3 mas wider in H-alpha emission compared to the
adjacent 643 nm continuum. This implies that the H-alpha line-forming region
may have a characteristic emitting radius of 12 mas or ~30 AU, in very good
agreement with the Hillier stellar-wind model. This provides direct
confirmation that the physical wind parameters of that model are roughly
correct, including the mass-loss rate of 10^-3 M_sun/yr, plus the clumping
factor, and the terminal velocity. Comparison of the H-alpha images
(ellipticity and PA) to the continuum images reveals no significant asymmetries
at H-alpha. Hence, any asymmetry induced by a companion or by the primary's
rotation do not strongly influence the global H-alpha emission in the outer
wind.Comment: Published in ApJ
Laser Guide Star for Large Segmented-Aperture Space Telescopes, Part I: Implications for Terrestrial Exoplanet Detection and Observatory Stability
Precision wavefront control on future segmented-aperture space telescopes
presents significant challenges, particularly in the context of high-contrast
exoplanet direct imaging. We present a new wavefront control architecture that
translates the ground-based artificial guide star concept to space with a laser
source aboard a second spacecraft, formation flying within the telescope
field-of-view. We describe the motivating problem of mirror segment motion and
develop wavefront sensing requirements as a function of guide star magnitude
and segment motion power spectrum. Several sample cases with different values
for transmitter power, pointing jitter, and wavelength are presented to
illustrate the advantages and challenges of having a non-stellar-magnitude
noise limited wavefront sensor for space telescopes. These notional designs
allow increased control authority, potentially relaxing spacecraft stability
requirements by two orders of magnitude, and increasing terrestrial exoplanet
discovery space by allowing high-contrast observations of stars of arbitrary
brightness.Comment: Submitted to A
Effects of Limit Feeding a High Concentrate Diet on Feedlot Performance and Carcass Composition of Lambs
Limit feeding high concentrate diets offers several potential benefits to lamb feeders. Limit feeding is one alternative to ad libitum feeding where intake is restricted to an amount which will permit animals to attain some predetermined daily weight gain. This type of feeding system provides a means to reduce day-to-day variations in feed intake, simplifies bunk management, and allows for greater control over feed inventories (Zinn, 1 986) . Using limit feeding to program a particular rate of gain also allows greater marketing flexibility. Restricting intake has been shown to improve the efficiency at which feed is converted into live weight gain in sheep (Glimp et al. , 1989) and cattle (Plegge, 1986) . Reduced average daily gains have been shown to reduce the fat content of empty body weight gains (Turgeon et al. , 1986) . The objectives of this study were to determine the effect of limit feeding a high concentrate diet to achieve three different rates of gain on feedlot performance and body composition of growing lambs
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