30 research outputs found
Detection of faint companions through stochastic speckle discrimination
We propose a new post-processing technique for the detection of faint
companions from a sequence of adaptive optics corrected short exposures. The
algorithm exploits the difference in shape between the on-axis and off-axis
irradiance distributions and it does not require the signal to be above the
noise level. We show that the method is particularly useful in dealing with
static speckles. Its application to real and simulated data gives excellent
results in the low-signal regime where it outperforms the standard approach of
computing signal-to-noise ratio on one long exposure. We also show that
accurate noise estimation in adaptive optics images of close companions is
rendered impossible due to the presence of static speckles. This new method
provides means of reliable estimation of the confidence intervals for the
detection hypothesis.Comment: accepted for publication in Ap
Reference-less detection, astrometry, and photometry of faint companions with adaptive optics
We propose a complete framework for the detection, astrometry, and photometry
of faint companions from a sequence of adaptive optics corrected short
exposures. The algorithms exploit the difference in statistics between the
on-axis and off-axis intensity. Using moderate-Strehl ratio data obtained with
the natural guide star adaptive optics system on the Lick Observatory's 3-m
Shane Telescope, we compare these methods to the standard approach of PSF
fitting. We give detection limits for the Lick system, as well as a first guide
to expected accuracy of differential photometry and astrometry with the new
techniques. The proposed approach to detection offers a new way of determining
dynamic range, while the new algorithms for differential photometry and
astrometry yield accurate results for very faint and close-in companions where
PSF fitting fails. All three proposed algorithms are self-calibrating, i.e.
they do not require observation of a calibration star thus improving the
observing efficiency.Comment: Astrophysical Journal 698 (2009) 28-4
Speckle statistics in adaptive optics images at visible wavelengths
Residual speckles in adaptive optics (AO) images represent a well-known
limitation on the achievement of the contrast needed for faint source
detection. Speckles in AO imagery can be the result of either residual
atmospheric aberrations, not corrected by the AO, or slowly evolving
aberrations induced by the optical system. We take advantage of the high
temporal cadence (1 ms) of the data acquired by the System for Coronagraphy
with High-order Adaptive Optics from R to K bands-VIS forerunner experiment at
the Large Binocular Telescope to characterize the AO residual speckles at
visible wavelengths. An accurate knowledge of the speckle pattern and its
dynamics is of paramount importance for the application of methods aimed at
their mitigation. By means of both an automatic identification software and
information theory, we study the main statistical properties of AO residuals
and their dynamics. We therefore provide a speckle characterization that can be
incorporated into numerical simulations to increase their realism and to
optimize the performances of both real-time and postprocessing techniques aimed
at the reduction of the speckle noise
Temporal variability and statistics of the Strehl ratio in adaptive-optics images
We have investigated the temporal variability and statistics of the
"instantaneous" Strehl ratio. The observations were carried out with the 3.63-m
AEOS telescope equipped with a high-order adaptive optics system. In this paper
Strehl ratio is defined as the peak intensity of a single short exposure. We
have also studied the behaviour of the phase variance computed on the
reconstructed wavefronts. We tested the Marechal approximation and used it to
explain the observed negative skewness of the Strehl ratio distribution. The
estimate of the phase variance is shown to fit a three-parameter Gamma
distribution model. We show that simple scaling of the reconstructed wavefronts
has a large impact on the shape of the Strehl ratio distribution.Comment: submitted to PAS
Diffraction-limited imaging at IR wavelengths using aperture masks and fully filled apertures
The performance of a phase recovery algorithm developed for speckle data collected using a pupil-plane mask has been investigated for use at near-infrared wavelengths. The method, based on the radio-astronomical self-calibration technique, has been tested alongside a state-of-the-art implementation of the Knox-Thompson scheme using both simulated and real specklegrams. Results indicate that the new procedure is as effective as the Knox-Thompson based image reconstruction scheme and is applicable to a wide range of astrophysically interesting sources
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
LBTO's long march to full operation: step 2
Step 1 (Veillet et al.1), after a review of the development of the Large Binocular Telescope Observatory (LBTO from the early concepts of the early 80s to mid-2014, outlined a six-year plan (LBT2020) aimed at optimizing LBTO's scientific production while mitigating the consequences of the inevitable setbacks brought on by the considerable complexity of the telescope and the very diverse nature of the LBTO partnership. Step 2 is now focusing on the first two years of implementation of this plan, presenting the encountered obstacles, technical, cultural and political, and how they were overcome. Weather and another incident with one of the Adaptive Secondaries slowed down commissioning activities. All the facility instruments should have been commissioned and offered in binocular mode in early or mid-2016. It will happen instead by the end of 2016. On a brighter side, the first scientific publications using the LBT as a 23-m telescope through interferometry were published in 2015 and the overall number of publications has been raising at a good pace. Three second generation instruments were selected, scheduled to come on the telescope in the next three to five years. They will all use the excellent performance of the LBT Adaptive Optics (AO), which will be even better thanks to an upgrade of the AO to be completed in 2018. Less progress than hoped was made to move the current observing mode of the telescope to a whole LBT-wide queue. In two years from now, we should have a fully operational telescope, including a laser-based Ground Layer AO (GLAO) system, hopefully fully running in queue, with new instruments in development, new services offered to the users, and a stronger scientific production
Reference-less detection, astrometry, and photometry of faint companions with adaptive optics
We propose a complete framework for the detection, astrometry, and photometry of faint companions from a sequence of adaptive optics (AO) corrected short exposures. The algorithms exploit the difference in statistics between the on-axis and off-axis intensity of the AO point-spread function (PSF) to differentiate real sources from speckles. We validate the new approach and illustrate its performance using moderate Strehl ratio data obtained with the natural guide star AO system on the Lick Observatory\u27s 3 m Shane Telescope. We obtain almost a 2 mag gain in achievable contrast by using our detection method compared to 5 sigma detectability in long exposures. We also present a first guide to expected accuracy of differential photometry and astrometry with the new techniques. Our approach performs better than PSF-fitting in general and especially so for close companions, which are located within the uncompensated seeing (speckle) halo. All three proposed algorithms are self-calibrating, i.e., they do not require observation of a calibration star. One of the advantages of this approach is improved observing efficiency