6,765 research outputs found
Camera distortion self-calibration using the plumb-line constraint and minimal Hough entropy
In this paper we present a simple and robust method for self-correction of
camera distortion using single images of scenes which contain straight lines.
Since the most common distortion can be modelled as radial distortion, we
illustrate the method using the Harris radial distortion model, but the method
is applicable to any distortion model. The method is based on transforming the
edgels of the distorted image to a 1-D angular Hough space, and optimizing the
distortion correction parameters which minimize the entropy of the
corresponding normalized histogram. Properly corrected imagery will have fewer
curved lines, and therefore less spread in Hough space. Since the method does
not rely on any image structure beyond the existence of edgels sharing some
common orientations and does not use edge fitting, it is applicable to a wide
variety of image types. For instance, it can be applied equally well to images
of texture with weak but dominant orientations, or images with strong vanishing
points. Finally, the method is performed on both synthetic and real data
revealing that it is particularly robust to noise.Comment: 9 pages, 5 figures Corrected errors in equation 1
A Multiwavelength Analysis of the Strong Lensing Cluster RCS 022434-0002.5 at z=0.778
We present the results of two (101 ks total) Chandra observations of the
z=0.778 optically selected lensing cluster RCS022434-0002.5, along with weak
lensing and dynamical analyses of this object. An X-ray spectrum extracted
within R(2500) (362 h(70)^(-1) kpc) results in an integrated cluster
temperature of 5.1 (+0.9,-0.5) keV. The surface brightness profile of
RCS022434-0002.5 indicates the presence of a slight excess of emission in the
core. A hardness ratio image of this object reveals that this central emission
is primarily produced by soft X-rays. Further investigation yields a cluster
cooling time of 3.3 times 10^9 years, which is less than half of the age of the
universe at this redshift given the current LCDM cosmology. A weak lensing
analysis is performed using HST images, and our weak lensing mass estimate is
found to be in good agreement with the X-ray determined mass of the cluster.
Spectroscopic analysis reveals that RCS022434-0002.5 has a velocity dispersion
of 900 +/- 180 km/s, consistent with its X-ray temperature. The core gas mass
fraction of RCS022434-0002.5 is, however, found to be three times lower than
expected universal values. The radial distribution of X-ray point sources
within R(200) of this cluster peaks at ~0.7 R(200), possibly indicating that
the cluster potential is influencing AGN activity at that radius. Correlations
between X-ray and radio (VLA) point source positions are also examined.Comment: 32 pages, 9 figures. Accepted for publication in The Astrophysical
Journa
Hubble Space Telescope weak lensing study of the z=0.83 cluster MS 1054-03
We have measured the weak gravitational lensing signal of MS 1054-03, a rich
and X-ray luminous cluster of galaxies at a redshift of z=0.83, using a
two-colour mosaic of deep WFPC2 images. The small corrections for the size of
the PSF and the high number density of background galaxies obtained in these
observations result in an accurate and well calibrated measurement of the
lensing induced distortion. The strength of the lensing signal depends on the
redshift distribution of the background galaxies. We used photometric redshift
distributions from the Northern and Southern Hubble Deep Fields to relate the
lensing signal to the mass. The predicted variations of the signal as a
function of apparent source magnitude and colour agrees well with the observed
lensing signal. We determine a mass of (1.2+-0.2)x10^15 Msun within an aperture
of radius 1 Mpc. Under the assumption of an isothermal mass distribution, the
corresponding velocity dispersion is 1311^{+83}_{-89} km/s. For the
mass-to-light ratio we find 269+-37 Msun/Lsun. The errors in the mass and
mass-to-light ratio include the contribution from the random intrinsic
ellipticities of the source galaxies, but not the (systematic) error due to the
uncertainty in the redshift distribution. However, the estimates for the mass
and mass-to-light ratio of MS 1054-03 agree well with other estimators,
suggesting that the mass calibration works well. The reconstruction of the
projected mass surface density shows a complex mass distribution, consistent
with the light distribution. The results indicate that MS 1054-03 is a young
system. The timescale for relaxation is estimated to be at least 1 Gyr.
Averaging the tangential shear around the cluster galaxies, we find that the
velocity dispersion of an Lstar galaxy is 203+-33 km/s.Comment: 21 pages, Latex, with 27 figures (3 figures bitmapped), ApJ, in
press. Version (with non-bitmapped figures) available at
http://www.astro.rug.nl/~hoekstra/papers.htm
Means of confusion: how pixel noise affects shear estimates for weak gravitational lensing
Weak-lensing shear estimates show a troublesome dependence on the apparent
brightness of the galaxies used to measure the ellipticity: In several studies,
the amplitude of the inferred shear falls sharply with decreasing source
significance. This dependence limits the overall ability of upcoming large
weak-lensing surveys to constrain cosmological parameters.
We seek to provide a concise overview of the impact of pixel noise on
weak-lensing measurements, covering the entire path from noisy images to shear
estimates. We show that there are at least three distinct layers, where pixel
noise not only obscures but biases the outcome of the measurements: 1) the
propagation of pixel noise to the non-linear observable ellipticity; 2) the
response of the shape-measurement methods to limited amount of information
extractable from noisy images; and 3) the reaction of shear estimation
statistics to the presence of noise and outliers in the measured ellipticities.
We identify and discuss several fundamental problems and show that each of
them is able to introduce biases in the range of a few tenths to a few percent
for galaxies with typical significance levels. Furthermore, all of these biases
do not only depend on the brightness of galaxies but also on their ellipticity,
with more elliptical galaxies often being harder to measure correctly. We also
discuss existing possibilities to mitigate and novel ideas to avoid the biases
induced by pixel noise. We present a new shear estimator that shows a more
robust performance for noisy ellipticity samples. Finally, we release the
open-source python code to predict and efficiently sample from the noisy
ellipticity distribution and the shear estimators used in this work at
https://github.com/pmelchior/epsnoiseComment: integrated the origin of the moment correlation (thanks to Alan
Heavens). source code at https://github.com/pmelchior/epsnois
Recommended from our members
Correction of image radial distortion based on division model
This paper presents an approach for estimating and then removing image radial distortion. It works on
a single image and does not require a special calibration. The approach is extremely useful in many applications,
particularly those where human-made environments contain abundant lines. A division model is applied, in which
a straight line in the distorted image is treated as a circular arc. Levenberg–Marquardt (LM) iterative nonlinear
least squares method is adopted to calculate the arc’s parameters. Then “Taubin fit” is applied to obtain the initial
guess of the arc’s parameters which works as the initial input to the LM iteration. This dramatically improves the
convergence rate in the LM process to obtain the required parameters for correcting image radial distortion.
Hough entropy, as a measure, has achieved the quantitative evaluation of the estimated distortion based on
the probability distribution in one-dimensional θ Hough space. The experimental results on both synthetic
and real images have demonstrated that the proposed method can robustly estimate and then remove
image radial distortion with high accurac
The Magnetic Distortion Calibration System of the LHCb RICH1 Detector
The LHCb RICH1 detector uses hybrid photon detectors (HPDs) as its optical
sensors. A calibration system has been constructed to provide corrections for
distortions that are primarily due to external magnetic fields. We describe
here the system design, construction, operation and performance.Comment: 9 pages, 14 figure
Variation of the Diameter of the Sun as Measured by the Solar Disk Sextant (SDS)
The balloon-borne Solar Disk Sextant (SDS) experiment has measured the
angular size of the Sun on seven occasions spanning the years 1992 to 2011. The
solar half-diameter -- observed in a 100-nm wide passband centred at 615 nm --
is found to vary over that period by up to 200 mas, while the typical estimated
uncertainty of each measure is 20 mas. The diameter variation is not in phase
with the solar activity cycle; thus, the measured diameter variation cannot be
explained as an observational artefact of surface activity. Other possible
instrument-related explanations for the observed variation are considered but
found unlikely, leading us to conclude that the variation is real. The SDS is
described here in detail, as is the complete analysis procedure necessary to
calibrate the instrument and allow comparison of diameter measures across
decades.Comment: 41 pages; appendix and 2 figures added plus some changes in text
based on referee's comments; to appear in MNRA
- …