72 research outputs found
The Anisoplanatic Point Spread Function in Adaptive Optics
The effects of anisoplanatism on the adaptive optics point spread function
are investigated. A model is derived that combines observations of the guide
star with an analytic formulation of anisoplanatism to generate predictions for
the adaptive optics point spread function at arbitrary locations within the
field of view. The analytic formulation captures the dependencies of
anisoplanatism on aperture diameter, observing wavelength, angular offset,
zenith angle and turbulence profile. The predictions of this model are compared
to narrowband 2.12 um and 1.65 um images of a 21 arcsec binary (mV=7.3, 7.6)
acquired with the Palomar Adaptive Optics System on the Hale 5 meter telescope.
Contemporaneous measurements of the turbulence profile made with a DIMM/MASS
unit are used together with images of the primary to predict the point spread
function of the binary companion. Predicted companion Strehl ratios are shown
to match measurements to within a few percent, whereas predictions based on the
isoplanatic angle approximation are highly discrepant. The predicted companion
point spread functions are shown to agree with observations to 10%. These
predictions are used to measure the differential photometry between binary
members to an accuracy of 1 part in 10^{3}, and the differential astrometry to
an accuracy of 1 mas. Errors in the differential astrometry are shown to be
dominated by differential atmospheric tilt jitter. These results are compared
to other techniques that have been employed for photometry, astrometry, and
high contrast imaging.Comment: 26 pages, 7 figure
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
High-order aberration compensation with Multi-frame Blind Deconvolution and Phase Diversity image restoration techniques
Context. For accurately measuring intensities and determining magnetic field
strengths of small-scale solar (magnetic) structure, knowledge of and
compensation for the point spread function is crucial. For images recorded with
the Swedish 1-meter Solar Telescope, restoration with Multi-Frame Blind
Deconvolution and Joint Phase Diverse Speckle methods lead to remarkable
improvements in image quality but granulation contrasts that are too low,
indicating additional stray light. Aims. We propose a method to compensate for
stray light from high-order atmospheric aberrations not included in MFBD and
JPDS processing. Methods. To compensate for uncorrected aberrations, a
reformulation of the image restoration process is proposed that allows the
average effect of hundreds of high-order modes to be compensated for by relying
on Kolmogorov statistics for these modes. The applicability of the method
requires simultaneous measurements of Fried's parameter r0. The method is
tested with simulations as well as real data and extended to include
compensation for conventional stray light. Results. We find that only part of
the reduction of granulation contrast in SST images is due to uncompensated
high-order aberrations. The remainder is still unaccounted for and attributed
to stray light from the atmosphere, the telescope with its re-imaging system
and to various high-altitude seeing effects. Conclusions. We conclude that
statistical compensation of high-order modes is a viable method to reduce the
loss of contrast occurring when a limited number of aberrations is explicitly
compensated for with MFBD and JPDS processing. We show that good such
compensation is possible with only 10 recorded frames. The main limitation of
the method is that already MFBD and JPDS processing introduces high-order
compensation that, if not taken into account, can lead to over-compensation.Comment: in press in Astronomy & Astrophysic
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
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
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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