56 research outputs found
Speckle Statistics in Adaptively Corrected Images
(abridged) Imaging observations are generally affected by a fluctuating
background of speckles, a particular problem when detecting faint stellar
companions at small angular separations. Knowing the distribution of the
speckle intensities at a given location in the image plane is important for
understanding the noise limits of companion detection. The speckle noise limit
in a long-exposure image is characterized by the intensity variance and the
speckle lifetime. In this paper we address the former quantity through the
distribution function of speckle intensity. Previous theoretical work has
predicted a form for this distribution function at a single location in the
image plane. We developed a fast readout mode to take short exposures of
stellar images corrected by adaptive optics at the ground-based UCO/Lick
Observatory, with integration times of 5 ms and a time between successive
frames of 14.5 ms ( m). These observations temporally
oversample and spatially Nyquist sample the observed speckle patterns. We show,
for various locations in the image plane, the observed distribution of speckle
intensities is consistent with the predicted form. Additionally, we demonstrate
a method by which and can be mapped over the image plane. As the
quantity is proportional to the PSF of the telescope free of random
atmospheric aberrations, this method can be used for PSF calibration and
reconstruction.Comment: 7 pages, 4 figures, ApJ accepte
Deep Near-Infrared Imaging of a Field in the Outer Disk of M82 with the ALTAIR Adaptive Optics System on Gemini North
Deep H and K' images, recorded with the ALTAIR adaptive optics system and
NIRI imager on Gemini North, are used to probe the red stellar content in a
field with a projected distance of 1 kpc above the disk plane of the starburst
galaxy M82. The data have an angular resolution of 0.08 arcsec FWHM, and
individual AGB and RGB stars are resolved. The AGB extends to at least 1.7 mag
in K above the RGB-tip, which occurs at K = 21.7. The relative numbers of
bright AGB stars and RGB stars are consistent with stellar evolution models,
and one of the brightest AGB stars has an H-K color and K brightness that is
consistent with it being a C star. The brightnesses of the AGB stars suggest
that they formed during intermediate epochs, possibily after the last major
interaction with M81. Therefore, star formation in M82 during intermediate
epochs may not have been restricted to the plane of the disk.Comment: 16 pages of text plus 7 postscript figures; to appear in the PAS
The AU Microscopii Debris Disk: Multiwavelength Imaging and Modeling
(abridged) Debris disks around main sequence stars are produced by the
erosion and evaporation of unseen parent bodies. AU Microscopii (GJ 803) is a
compelling object to study in the context of disk evolution across different
spectral types, as it is an M dwarf whose near edge-on disk may be directly
compared to that of its A5V sibling beta Pic. We resolve the disk from 8-60 AU
in the near-IR JHK' bands at high resolution with the Keck II telescope and
adaptive optics, and develop a novel data reduction technique for the removal
of the stellar point spread function. The point source detection sensitivity in
the disk midplane is more than a magnitude less sensitive than regions away
from the disk for some radii. We measure a blue color across the near-IR bands,
and confirm the presence of substructure in the inner disk. Some of the
structural features exhibit wavelength-dependent positions. The disk
architecture and characteristics of grain composition are inferred through
modeling. We approach the modeling of the dust distribution in a manner that
complements previous work. Using a Monte Carlo radiative transfer code, we
compare a relatively simple model of the distribution of porous grains to a
broad data set, simultaneously fitting to midplane surface brightness profiles
and the spectral energy distribution. Our model confirms that the large-scale
architecture of the disk is consistent with detailed models of steady-state
grain dynamics. Here, a belt of parent bodies from 35-40 AU is responsible for
producing dust that is then swept outward by the stellar wind and radiation
pressures. We infer the presence of very small grains in the outer region, down
to sizes of ~0.05 micron. These sizes are consistent with stellar mass-loss
rates Mdot_* << 10^2 Mdot_sun.Comment: ApJ accepted, 56 pages, preprint style. Version in emulateapj with
high-resolution figures available at http://tinyurl.com/y6ent
Characterizing the Adaptive Optics Off-Axis Point-Spread Function. II. Methods for Use in Laser Guide Star Observations
Most current astronomical adaptive optics (AO) systems rely on the
availability of a bright star to measure the distortion of the incoming
wavefront. Replacing the guide star with an artificial laser beacon alleviates
this dependency on bright stars and therefore increases sky coverage, but it
does not eliminate another serious problem for AO observations. This is the
issue of PSF variation with time and field position near the guide star. In
fact, because a natural guide star is still necessary for correction of the
low-order phase error, characterization of laser guide star (LGS) AO PSF
spatial variation is more complicated than for a natural guide star alone. We
discuss six methods for characterizing LGS AO PSF variation that can
potentially improve the determination of the PSF away from the laser spot, that
is, off-axis. Calibration images of dense star fields are used to determine the
change in PSF variation with field position. This is augmented by AO system
telemetry and simple computer simulations to determine a more accurate off-axis
PSF. We report on tests of the methods using the laser AO system on the Lick
Observatory Shane Telescope. [Abstract truncated.]Comment: 31 pages, 5 figures, accepted by PAS
L'-band AGPM vector vortex coronagraph's first light on LBTI/LMIRCam
We present the first observations obtained with the L'-band AGPM vortex
coronagraph recently installed on LBTI/LMIRCam. The AGPM (Annular Groove Phase
Mask) is a vector vortex coronagraph made from diamond subwavelength gratings.
It is designed to improve the sensitivity and dynamic range of high-resolution
imaging at very small inner working angles, down to 0.09 arcseconds in the case
of LBTI/LMIRCam in the L' band. During the first hours on sky, we observed the
young A5V star HR\,8799 with the goal to demonstrate the AGPM performance and
assess its relevance for the ongoing LBTI planet survey (LEECH). Preliminary
analyses of the data reveal the four known planets clearly at high SNR and
provide unprecedented sensitivity limits in the inner planetary system (down to
the diffraction limit of 0.09 arcseconds).Comment: 9 pages, 4 figures, SPIE proceeding
Characterizing the Adaptive Optics Off-Axis Point-Spread Function - I: A Semi-Empirical Method for Use in Natural-Guide-Star Observations
Even though the technology of adaptive optics (AO) is rapidly maturing,
calibration of the resulting images remains a major challenge. The AO
point-spread function (PSF) changes quickly both in time and position on the
sky. In a typical observation the star used for guiding will be separated from
the scientific target by 10" to 30". This is sufficient separation to render
images of the guide star by themselves nearly useless in characterizing the PSF
at the off-axis target position. A semi-empirical technique is described that
improves the determination of the AO off-axis PSF. The method uses calibration
images of dense star fields to determine the change in PSF with field position.
It then uses this information to correct contemporaneous images of the guide
star to produce a PSF that is more accurate for both the target position and
the time of a scientific observation. We report on tests of the method using
natural-guide-star AO systems on the Canada-France-Hawaii Telescope and Lick
Observatory Shane Telescope, augmented by simple atmospheric computer
simulations. At 25" off-axis, predicting the PSF full width at half maximum
using only information about the guide star results in an error of 60%. Using
an image of a dense star field lowers this error to 33%, and our method, which
also folds in information about the on-axis PSF, further decreases the error to
19%.Comment: 29 pages, 9 figures, accepted for publication in the PAS
TMT adaptive optics systems control architecture
Achieving the science goals of TMT will require AO subsystems of unprecedented power and sophistication, including a Real Time Controller (RTC) subsystem that will implement wavefront reconstruction and control algorithms for up to four different laser guide star (LGS) AO systems. The requirements for the RTC represent a significant advance over the current generation of astronomical AO control systems, both in terms of the wavefront reconstruction algorithms to be employed and the new hardware approaches that will be required. Additionally, the number of active components included in the AO systems and the complexity of their interactions will require a highly automated AO Sequencer that will work in concert with the TMT Telescope and Instrument Sequencers. In this paper, we will describe the control and software requirements for the whole AO system, and in particular for the RTC and the AO Sequencer. We will describe the challenges involved in developing these systems and will present a conceptual design
The Core of NGC 6240 from Keck Adaptive Optics and HST NICMOS Observations
We present results of near infrared imaging of the disk-galaxy-merger NGC
6240 using adaptive optics on the Keck II Telescope and reprocessed archival
data from NICMOS on the Hubble Space Telescope. Both the North and South nuclei
of NGC 6240 are clearly elongated, with considerable sub-structure within each
nucleus. In K' band there are at least two point-sources within the North
nucleus; we tentatively identify the south-western point-source within the
North nucleus as the position of one of the two AGNs. Within the South nucleus,
the northern sub-nucleus is more highly reddened. Based upon the nuclear
separation measured at 5 GHz, we suggest that the AGN in the South nucleus is
still enshrouded in dust at K' band, and is located slightly to the north of
the brightest point in K' band. Within the South nucleus there is strong H2 1-0
S(1) line emission from the northern sub-nucleus, contrary to the conclusions
of previous seeing-limited observations. Narrowband H2 emission-line images
show that a streamer or ribbon of excited molecular hydrogen connects the North
and South nuclei. We suggest that this linear feature corresponds to a bridge
of gas connecting the two nuclei, as seen in computer simulations of mergers.
Many point-like regions are seen around the two nuclei. These are most
prominent at 1.1 microns with NICMOS, and in K'-band with Keck adaptive optics.
We suggest that these point-sources represent young star clusters formed in the
course of the merger.Comment: 50 pages, 13 figures. To be published in the Astrophysical Journal,
March 10, 200
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