1,119 research outputs found
Nonlinearity and pixel shifting effects in HXRG infrared detectors
We study the nonlinearity (NL) in the conversion from charge to voltage in
infrared detectors (HXRG) for use in precision astronomy. We present laboratory
measurements of the NL function of a H2RG detector and discuss the accuracy to
which it would need to be calibrated in future space missions to perform
cosmological measurements through the weak gravitational lensing technique. In
addition, we present an analysis of archival data from the infrared H1RG
detector of the Wide Field Camera 3 in the Hubble Space Telescope that provides
evidence consistent with the existence of a sensor effect analogous to the
brighter-fatter effect found in Charge-Coupled Devices. We propose a model in
which this effect could be understood as shifts in the effective pixel
boundaries, and discuss prospects of laboratory measurements to fully
characterize this effect.Comment: Accepted for publication in the Journal of Instrumentation (JINST).
Part of "Precision Astronomy with Fully Depleted CCDs" (Dec 1-2, 2016),
Brookhaven National Laboratory, Upton, NY, US
Realidades y falacias de la reconstrucción del tejido social en población desplazada
La investigación recoge los resultados del trabajo orientado a la reconstrucción del tejido social con población en situación de desplazamiento asentada en el Municipio de Soacha, realizado desde el 2002. Con un diseño cualitativo, específicamente la investigación participativa,
se privilegió el uso de técnicas dialógicas integrando investigación, educación y acción. La problemática observada en la Unidad de Atención y Orientación a Población Desplazada (UAO), llevó a identificar como intencionalidad del proyecto el generar procesos autogestionarios en la comunidad de desplazados, a partir de una reflexión crítica de su realidad. Sin embargo, los procesos de reconstrucción del tejido social requieren de una política y acciones municipales concretas de absorción y repoblamiento, que brinden alternativas viables de estabilización socioeconómica a la población en situación de desplazamiento, puesto que dichos procesos se ven obstaculizados por las condiciones de vulnerabilidad en la que se encuentra la población
Characterization and correction of charge-induced pixel shifts in DECam
Interaction of charges in CCDs with the already accumulated charge
distribution causes both a flux dependence of the point-spread function (an
increase of observed size with flux, also known as the brighter/fatter effect)
and pixel-to-pixel correlations of the Poissonian noise in flat fields. We
describe these effects in the Dark Energy Camera (DECam) with charge dependent
shifts of effective pixel borders, i.e. the Antilogus et al. (2014) model,
which we fit to measurements of flat-field Poissonian noise correlations. The
latter fall off approximately as a power-law r^-2.5 with pixel separation r,
are isotropic except for an asymmetry in the direct neighbors along rows and
columns, are stable in time, and are weakly dependent on wavelength. They show
variations from chip to chip at the 20% level that correlate with the silicon
resistivity. The charge shifts predicted by the model cause biased shape
measurements, primarily due to their effect on bright stars, at levels
exceeding weak lensing science requirements. We measure the flux dependence of
star images and show that the effect can be mitigated by applying the reverse
charge shifts at the pixel level during image processing. Differences in
stellar size, however, remain significant due to residuals at larger distance
from the centroid.Comment: typo and formatting fixes, matches version published in JINS
The Splashback Feature around DES Galaxy Clusters: Galaxy Density and Weak Lensing Profiles
Splashback refers to the process of matter that is accreting onto a dark matter halo reaching its first orbital apocenter and turning around in its orbit. The clustercentric radius at which this process occurs, r_(sp), defines a halo boundary that is connected to the dynamics of the cluster. A rapid decline in the halo profile is expected near r_(sp). We measure the galaxy number density and weak lensing mass profiles around REDMAPPER galaxy clusters in the first-year Dark Energy Survey (DES) data. For a cluster sample with mean M_(200m) mass ≈2.5 × 10^(14) M⊙, we find strong evidence of a splashback-like steepening of the galaxy density profile and measure r_(sp) = 1.13 ± 0.07 h^(−1) Mpc, consistent with the earlier Sloan Digital Sky Survey measurements of More et al. and Baxter et al. Moreover, our weak lensing measurement demonstrates for the first time the existence of a splashback-like steepening of the matter profile of galaxy clusters. We measure r_(sp) = 1.34 ± 0.21 h^(−1) Mpc from the weak lensing data, in good agreement with our galaxy density measurements. For different cluster and galaxy samples, we find that, consistent with ΛCDM simulations, r_(sp) scales with R_(200m) and does not evolve with redshift over the redshift range of 0.3–0.6. We also find that potential systematic effects associated with the REDMAPPER algorithm may impact the location of r_(sp). We discuss the progress needed to understand the systematic uncertainties and fully exploit forthcoming data from DES and future surveys, emphasizing the importance of more realistic mock catalogs and independent cluster samples
Laboratory Measurement of the Brighter-fatter Effect in an H2RG Infrared Detector
The "brighter-fatter" (BF) effect is a phenomenon—originally discovered in charge coupled devices—in which the size of the detector point-spread function (PSF) increases with brightness. We present, for the first time, laboratory measurements demonstrating the existence of the effect in a Hawaii-2RG HgCdTe near-infrared (NIR) detector. We use JPL's Precision Projector Laboratory, a facility for emulating astronomical observations with UV/VIS/NIR detectors, to project about 17,000 point sources onto the detector to stimulate the effect. After calibrating the detector for nonlinearity with flat-fields, we find evidence that charge is nonlinearly shifted from bright pixels to neighboring pixels during exposures of point sources, consistent with the existence of a BF-type effect. NASAs Wide Field Infrared Survey Telescope (WFIRST) will use similar detectors to measure weak gravitational lensing from the shapes of hundreds of million of galaxies in the NIR. The WFIRST PSF size must be calibrated to ≈0.1% to avoid biased inferences of dark matter and dark energy parameters; therefore further study and calibration of the BF effect in realistic images will be crucial
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