167 research outputs found
Study of a MEMS-based Shack-Hartmann wavefront sensor with adjustable pupil sampling for astronomical adaptive optics
We introduce a Shack-Hartmann wavefront sensor for adaptive optics that enables dynamic control of the spatial sampling of an incoming wavefront using a segmented mirror microelectrical mechanical systems (MEMS) device. Unlike a conventional lenslet array, subapertures are defined by either segments or groups of segments of a mirror array, with the ability to change spatial pupil sampling arbitrarily by redefining the segment grouping. Control over the spatial sampling of the wavefront allows for the minimization of wavefront reconstruction error for different intensities of guide source and different atmospheric conditions, which in turn maximizes an adaptive optics system's delivered Strehl ratio. Requirements for the MEMS devices needed in this Shack-Hartmann wavefront sensor are also presented
Design considerations for low-light level low-Fresnel number optical systems
Low-Fresnel number optical systems exhibit significant diffraction effects that cause a shift in the peaks
of on-axis irradiance away from the geometric focal point. This is currently interpreted as a change of the
focal length of an optical system, leading optical system designers to compensate for the effect by assuming
the image plane is coincident with the peak of on-axis irradiance. While this may be an appropriate
interpretation for certain applications, I show that despite the shift in peak irradiance away from the
geometrical focal point, a change in a system’s optical power will not increase the on-axis irradiance at
that distance. This is important for low-light level applications where it is necessary to mitigate diffraction
induced transmission losses. I also show that low-Fresnel number systems have increased tolerance
on system power at the geometrical focal point and as a result are inherently achromatic
Twelve-thousand laser-AO observations: first results from the Robo-AO large surveys
Robo-AO is the first AO system which can feasibly perform surveys of thousands of targets. The system has been operating in a fully robotic mode on the Palomar 1.5m telescope for almost two years. Robo-AO has completed nearly 12,000 high-angular-resolution observations in almost 20 separate science programs including exoplanet characterization, field star binarity, young star binarity and solar system observations. We summarize the Robo-AO surveys and the observations completed to date. We also describe the data-reduction pipeline we developed for Robo-AO—the first fully-automated AO data-reduction, point-spread-function subtraction and companion-search pipeline
Laser-only adaptive optics achieves significant image quality gains compared to seeing-limited observations over the entire sky
Adaptive optics laser guide star systems perform atmospheric correction of
stellar wavefronts in two parts: stellar tip-tilt and high-spatial-order
laser-correction. The requirement of a sufficiently bright guide star in the
field-of-view to correct tip-tilt limits sky coverage. Here we show an
improvement to effective seeing without the need for nearby bright stars,
enabling full sky coverage by performing only laser-assisted wavefront
correction. We used Robo-AO, the first robotic AO system, to comprehensively
demonstrate this laser-only correction. We analyze observations from four years
of efficient robotic operation covering 15,000 targets and 42,000 observations,
each realizing different seeing conditions. Using an autoguider (or a
post-processing software equivalent) and the laser to improve effective seeing
independent of the brightness of a target, Robo-AO observations show a 39+/-19%
improvement to effective FWHM, without any tip-tilt correction. We also
demonstrate that 50% encircled-energy performance without tip-tilt correction
remains comparable to diffraction-limited, standard Robo-AO performance.
Faint-target science programs primarily limited by 50% encircled-energy (e.g.
those employing integral field spectrographs placed behind the AO system) may
see significant benefits to sky coverage from employing laser-only AO.Comment: Accepted for publication in The Astronomical Journal. 7 pages, 6
figure
Multiplicity of the Galactic Senior Citizens: A high-resolution search for cool subdwarf companions
Cool subdwarfs are the oldest members of the low mass stellar population.
Mostly present in the galactic halo, subdwarfs are characterized by their low
metallicity. Measuring their binary fraction and comparing it to solar
metallicity stars could give key insights into the star formation process early
in the history of the Milky Way. However, because of their low luminosity and
relative rarity in the solar neighborhood, binarity surveys of cool subdwarfs
have suffered from small sample sizes and incompleteness. Previous surveys have
suggested that the binary fraction of red subdwarfs is much lower than for
their main sequence cousins. Using the highly efficient RoboAO system, we
present the largest yet high-resolution survey of subdwarfs, sensitive to
angular separations, down to 0.15 arcsec, and contrast ratios, up to 6
magnitude difference, invisible in past surveys. Of 344 target cool subdwarfs,
40 are in multiple systems, 16 newly discovered, for a binary fraction of 11.6
percent and 1.8 percent error. We also discovered 6 triple star systems for a
triplet fraction of 1.7 percent and 0.7 percent error. Comparisons to similar
surveys of solar metallicity dwarf stars gives a 3 sigma disparity in
luminosity between companion stars, with subdwarfs displaying a shortage of low
contrast companions.Comment: 13 pages, 10 figures, submitted to Ap
On-sky wide field adaptive optics correction using multiple laser guide stars at the MMT
We describe results from the first astronomical adaptive optics system to use
multiple laser guide stars, located at the 6.5-m MMT telescope in Arizona. Its
initial operational mode, ground-layer adaptive optics (GLAO), provides uniform
stellar wavefront correction within the 2 arc minute diameter laser beacon
constellation, reducing the stellar image widths by as much as 53%, from 0.70
to 0.33 arc seconds at lambda = 2.14 microns. GLAO is achieved by applying a
correction to the telescope's adaptive secondary mirror that is an average of
wavefront measurements from five laser beacons supplemented with image motion
from a faint stellar source. Optimization of the adaptive optics system in
subsequent commissioning runs will further improve correction performance where
it is predicted to deliver 0.1 to 0.2 arc second resolution in the
near-infrared during a majority of seeing conditions.Comment: 13 pages, 1 table, 7 figures. Accepted for publication in
Astrophysical Journal. Expected March 200
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