34 research outputs found
Firedec: a two-channel finite-resolution image deconvolution algorithm
We present a two-channel deconvolution method that decomposes images into a
parametric point-source channel and a pixelized extended-source channel. Based
on the central idea of the deconvolution algorithm proposed by Magain, Courbin
& Sohy (1998), the method aims at improving the resolution of the data rather
than at completely removing the point spread function (PSF). Improvements over
the original method include a better regularization of the pixel channel of the
image, based on wavelet filtering and multiscale analysis, and a better
controlled separation of the point source vs. the extended source. In addition,
the method is able to simultaneously deconvolve many individual frames of the
same object taken with different instruments under different PSF conditions.
For this purpose, we introduce a general geometric transformation between
individual images. This transformation allows the combination of the images
without having to interpolate them. We illustrate the capability of our
algorithm using real and simulated images with complex diffraction-limited PSF.Comment: Accepted in A&A. An application of the technique to real data is
available in Cantale et al. http://arxiv.org/abs/1601.05192v
A fast empirical method for galaxy shape measurements in weak lensing surveys
We describe a simple and fast method to correct ellipticity measurements of
galaxies from the distortion by the instrumental and atmospheric point spread
function (PSF), in view of weak lensing shear measurements. The method performs
a classification of galaxies and associated PSFs according to measured shape
parameters, and corrects the measured galaxy ellipticites by querying a large
lookup table (LUT), built by supervised learning. We have applied this new
method to the GREAT10 image analysis challenge, and present in this paper a
refined solution that obtains the competitive quality factor of Q = 104,
without any shear power spectrum denoising or training. Of particular interest
is the efficiency of the method, with a processing time below 3 ms per galaxy
on an ordinary CPU.Comment: 8 pages, 6 figures. Metric values updated according to the final
GREAT10 analysis software (Kitching et al. 2012, MNRAS 423, 3163-3208), no
qualitative changes. Associated code available at
http://lastro.epfl.ch/megalu
COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses XIII: Time delays and 9-yr optical monitoring of the lensed quasar RX J1131-1231
We present the results from nine years of optically monitoring the
gravitationally lensed z=0.658 quasar RX J1131-1231. The R-band light curves of
the four individual images of the quasar were obtained using deconvolution
photometry for a total of 707 epochs. Several sharp quasar variability features
strongly constrain the time delays between the quasar images. Using three
different numerical techniques, we measure these delays for all possible pairs
of quasar images while always processing the four light curves simultaneously.
For all three methods, the delays between the three close images A, B, and C
are compatible with being 0, while we measure the delay of image D to be 91
days, with a fractional uncertainty of 1.5% (1 sigma), including systematic
errors. Our analysis of random and systematic errors accounts in a realistic
way for the observed quasar variability, fluctuating microlensing magnification
over a broad range of temporal scales, noise properties, and seasonal gaps.
Finally, we find that our time-delay measurement methods yield compatible
results when applied to subsets of the data.Comment: 11 pages, 9 figures, minor additions to the text only, techniques and
results remain unchanged, A&A in pres
Image analysis for cosmology: results from the GREAT10 Galaxy Challenge
In this paper, we present results from the weak-lensing shape measurement GRavitational lEnsing Accuracy Testing 2010 (GREAT10) Galaxy Challenge. This marks an order of magnitude step change in the level of scrutiny employed in weak-lensing shape measurement analysis. We provide descriptions of each method tested and include 10 evaluation metrics over 24 simulation branches. GREAT10 was the first shape measurement challenge to include variable fields; both the shear field and the point spread function (PSF) vary across the images in a realistic manner. The variable fields enable a variety of metrics that are inaccessible to constant shear simulations, including a direct measure of the impact of shape measurement inaccuracies, and the impact of PSF size and ellipticity, on the shear power spectrum. To assess the impact of shape measurement bias for cosmic shear, we present a general pseudo-Câ„“ formalism that propagates spatially varying systematics in cosmic shear through to power spectrum estimates. We also show how one-point estimators of bias can be extracted from variable shear simulations. The GREAT10 Galaxy Challenge received 95 submissions and saw a factor of 3 improvement in the accuracy achieved by other shape measurement methods. The best methods achieve sub-per cent average biases. We find a strong dependence on accuracy as a function of signal-to-noise ratio, and indications of a weak dependence on galaxy type and size. Some requirements for the most ambitious cosmic shear experiments are met above a signal-to-noise ratio of 20. These results have the caveat that the simulated PSF was a ground-based PSF. Our results are a snapshot of the accuracy of current shape measurement methods and are a benchmark upon which improvement can be brought. This provides a foundation for a better understanding of the strengths and limitations of shape measurement method
VLT adaptive optics search for luminous substructures in the lens galaxy towards SDSS J0924+0219
Anomalous flux ratios between quasar images are suspected to be caused by
substructures in lens galaxies. We present new deep and high resolution H and
Ks imaging of the strongly lensed quasar SDSS J0924+0219 obtained using the ESO
VLT with adaptive optics and the Laser Guide Star system. SDSS J0924+0219 is
particularly interesting as the observed flux ratio between the quasar images
vastly disagree with the predictions from smooth mass models. With our adaptive
optics observations we find a luminous object, Object L, located ~0.3" to the
North of the lens galaxy, but we show that it can not be responsible for the
anomalous flux ratios. Object L as well as a luminous extension of the lens
galaxy to the South are seen in the archival HST/ACS image in the F814W filter.
This suggests that Object L is part of a bar in the lens galaxy, as also
supported by the presence of a significant disk component in the light profile
of the lens galaxy. Finally, we do not find evidence for any other luminous
substructure that may explain the quasar images flux ratios. However, owe to
the persistence of the flux ratio anomaly over time (~ 7 years) a combination
of microlensing and milli-lensing is the favorite explanation for the
observations.Comment: Accepted for publication in A&A, 6 pages, 4 figure
Cosmological distance indicators
We review three distance measurement techniques beyond the local universe:
(1) gravitational lens time delays, (2) baryon acoustic oscillation (BAO), and
(3) HI intensity mapping. We describe the principles and theory behind each
method, the ingredients needed for measuring such distances, the current
observational results, and future prospects. Time delays from strongly lensed
quasars currently provide constraints on with < 4% uncertainty, and with
1% within reach from ongoing surveys and efforts. Recent exciting discoveries
of strongly lensed supernovae hold great promise for time-delay cosmography.
BAO features have been detected in redshift surveys up to z <~ 0.8 with
galaxies and z ~ 2 with Ly- forest, providing precise distance
measurements and with < 2% uncertainty in flat CDM. Future BAO
surveys will probe the distance scale with percent-level precision. HI
intensity mapping has great potential to map BAO distances at z ~ 0.8 and
beyond with precisions of a few percent. The next years ahead will be exciting
as various cosmological probes reach 1% uncertainty in determining , to
assess the current tension in measurements that could indicate new
physics.Comment: Review article accepted for publication in Space Science Reviews
(Springer), 45 pages, 10 figures. Chapter of a special collection resulting
from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination in
the Space Ag