184 research outputs found
Electrode level Monte Carlo model of radiation damage effects on astronomical CCDs
Current optical space telescopes rely upon silicon Charge Coupled Devices
(CCDs) to detect and image the incoming photons. The performance of a CCD
detector depends on its ability to transfer electrons through the silicon
efficiently, so that the signal from every pixel may be read out through a
single amplifier. This process of electron transfer is highly susceptible to
the effects of solar proton damage (or non-ionizing radiation damage). This is
because charged particles passing through the CCD displace silicon atoms,
introducing energy levels into the semi-conductor bandgap which act as
localized electron traps. The reduction in Charge Transfer Efficiency (CTE)
leads to signal loss and image smearing. The European Space Agency's
astrometric Gaia mission will make extensive use of CCDs to create the most
complete and accurate stereoscopic map to date of the Milky Way. In the context
of the Gaia mission CTE is referred to with the complementary quantity Charge
Transfer Inefficiency (CTI = 1-CTE). CTI is an extremely important issue that
threatens Gaia's performances. We present here a detailed Monte Carlo model
which has been developed to simulate the operation of a damaged CCD at the
pixel electrode level. This model implements a new approach to both the charge
density distribution within a pixel and the charge capture and release
probabilities, which allows the reproduction of CTI effects on a variety of
measurements for a large signal level range in particular for signals of the
order of a few electrons. A running version of the model as well as a brief
documentation and a few examples are readily available at
http://www.strw.leidenuniv.nl/~prodhomme/cemga.php as part of the CEMGA java
package (CTI Effects Models for Gaia).Comment: Accepted by MNRAS on 13 February 2011. 15 pages, 7 figures and 5
table
Factors influencing the temperature sensitivity of PMMA based optical fiber Bragg gratings
The Bragg wavelength of a PMMA based fiber grating is determined by the effective core index and the grating pitch, which, in temperature sensing, depend on the thermo-optic and thermal expansion coefficients of PMMA. These two coefficients are a function of surrounding temperature and humidity. Amorphous polymers including PMMA exhibit a certain degree of anisotropic thermal expansion. The anisotropic nature of expansion mainly depends on the polymer processing history. The expansion coefficient is believed to be lower in the direction of the molecular orientation than in the direction perpendicular to the draw direction. Such anisotropic behavior of polymers can be expected in drawn PMMA based optical fiber, and will lead to a reduced thermal expansion coefficient and larger temperature sensitivity than would be the case were the fiber to be isotropic. Extensive work has been carried out to identify these factors. The temperature responses of gratings have been measured at different relative humidity. Gratings fabricated on annealed and non-annealed PMMA optical fibers are used to compare the sensitivity performance as annealing is considered to be able to mitigate the anisotropic effect in PMMA optical fiber. Furthermore an experiment has been designed to eliminate the thermal expansion contribution to the grating wavelength change, leading to increased temperature sensitivity and improved response linearity
The impact of CCD radiation damage on Gaia astrometry: I. Image location estimation in the presence of radiation damage
The Gaia mission has been designed to perform absolute astrometric
measurements with unprecedented accuracy; the end-of-mission parallax standard
error is required to be 30 micro-arcseconds for a G2V type star of magnitude
15. These requirements set a stringent constraint on the accuracy of the
estimation of the location of the stellar image on the CCD for each
observation: e.g., 0.3 milli-arseconds (mas) or 0.005 pixels for the same V=15
G2V star. However the Gaia CCDs will suffer from charge transfer inefficiency
(CTI) caused by radiation damage that will degrade the stellar image quality
and may degrade the astrometric performance of Gaia if not properly addressed.
For the first time at this level of detail, the potential impact of radiation
damage on the performance of Gaia is investigated. In this first paper we focus
on the evaluation of the CTI impact on the image location accuracy. We show
that CTI decreases the stellar image signal-to-noise ratio and irreversibly
degrades the image location estimation precision. As a consequence the location
estimation standard errors increase by up to 6% for a radiation damage level
equivalent to the end-of-mission. In addition the CTI-induced image distortion
introduces a systematic bias in the image location estimation (up to 0.05
pixels or 3 mas in the Gaia operating conditions). We present a novel approach
to CTI mitigation that enables, without correction of the raw data, the
unbiased estimation of the image location and flux from damaged observations.
Its implementation reduces the maximum measured location bias for the faintest
magnitude to 0.005 pixels (~4e-4 pixels at magnitude 15). In a second paper we
will investigate how the CTI effects affect the final astrometric accuracy of
Gaia by propagating residual errors through the astrometric solution.Comment: 23 figures, 6 tables, accepted for publication in the Monthly Notices
of the Royal Astronomical Society the 4th of October 201
Comparing simulations and test data of a radiation damaged CCD for the Euclid mission
The radiation damage effects from the harsh radiative environment outside the Earth's atmosphere can be a cause for concern for most space missions. With the science goals becoming ever more demanding, the requirements on the precision of the instruments on board these missions also increases, and it is therefore important to investigate how the radiation induced damage affects the Charge-Coupled Devices (CCDs) that most of these instruments rely on.
The primary goal of the Euclid mission is to study the nature of dark matter and dark energy using weak lensing and baryonic acoustic oscillation techniques. The weak lensing technique depends on very precise shape measurements of distant galaxies obtained by a large CCD array. It is anticipated that over the 6 year nominal lifetime of mission, the CCDs will be degraded to an extent that these measurements will not be possible unless the radiation damage effects are corrected. We have therefore created a Monte Carlo model that simulates the physical processes taking place when transferring signal through a radiation damaged CCD. The software is based on Shockley-Read-Hall theory, and is made to mimic the physical properties in the CCD as close as possible. The code runs on a single electrode level and takes charge cloud size and density, three dimensional trap position, and multi-level clocking into account. A key element of the model is that it takes device specific simulations of electron density as a direct input, thereby avoiding to make any analytical assumptions about the size and density of the charge cloud. This paper illustrates how test data and simulated data can be compared in order to further our understanding of the positions and properties of the individual radiation-induced traps
HCMV pUL135 remodels the actin cytoskeleton to impair immune recognition of infected cells
Immune evasion genes help human cytomegalovirus (HCMV) establish lifelong persistence. Without immune pressure, laboratory-adapted HCMV strains have undergone genetic alterations. Among these, the deletion of the UL/b’ domain is associated with loss of virulence. In a screen of UL/b’, we identified pUL135 as a protein responsible for the characteristic cytopathic effect of clinical HCMV strains that also protected from natural killer (NK) and T cell attack. pUL135 interacted directly with abl interactor 1 (ABI1) and ABI2 to recruit the WAVE2 regulatory complex to the plasma membrane, remodel the actin cytoskeleton and dramatically reduce the efficiency of immune synapse (IS) formation. An intimate association between F-actin filaments in target cells and the IS was dispelled by pUL135 expression. Thus, F-actin in target cells plays a critical role in synaptogenesis, and this can be exploited by pathogens to protect against cytotoxic immune effector cells. An independent interaction between pUL135 and talin disrupted cell contacts with the extracellular matrix
VO2-based radiative thermal transistor with a semi-transparent base
International audienc
Simplified charge transfer inefficiency correction in CCDs by trap-pumping
A major concern when using Charge-Coupled Devices in hostile radiation environments is radiation induced Charge Transfer Inefficiency. The displacement damage from non-ionising radiation incident on the detector creates defects within the silicon lattice, these defects can capture and hold charge for a period of time dependent on the operating temperature and the type of defect, or “trap species”. The location and type of defect can be determined to a high degree of precision using the trap-pumping technique, whereby background charges are input and then shuffled forwards and backwards between pixels many times and repeated using different transfer timings to promote resonant charge-pumping at particular defect sites. Where the charge transfer timings used in the trap-pumping process are equivalent to the nominal CCD readout modes, a simple “trap-map” of the defects that will most likely contribute to charge transfer inefficiency in the CCD array can be quickly generated. This paper describes a concept for how such a “trap-map” can be used to correct images subject to non-ionising radiation damage and provides initial results from an analytical algorithm and our recommendations for future developments
Comparing simulations and test data of a radiation damaged charge-coupled device for the Euclid mission
The visible imager instrument on board the Euclid mission is a weak-lensing experiment that depends on very precise shape measurements of distant galaxies obtained by a large charge-coupled device (CCD) array. Due to the harsh radiative environment outside the Earth’s atmosphere, it is anticipated that the CCDs over the mission lifetime will be degraded to an extent that these measurements will be possible only through the correction of radiation damage effects. We have therefore created a Monte Carlo model that simulates the physical processes taking place when transferring signals through a radiation-damaged CCD. The software is based on Shockley–Read–Hall theory and is made to mimic the physical properties in the CCD as closely as possible. The code runs on a single electrode level and takes the three-dimensional trap position, potential structure of the pixel, and multilevel clocking into account. A key element of the model is that it also takes device specific simulations of electron density as a direct input, thereby avoiding making any analytical assumptions about the size and density of the charge cloud. This paper illustrates how test data and simulated data can be compared in order to further our understanding of the positions and properties of the individual radiation-induced traps
The astrometric core solution for the Gaia mission. Overview of models, algorithms and software implementation
The Gaia satellite will observe about one billion stars and other point-like
sources. The astrometric core solution will determine the astrometric
parameters (position, parallax, and proper motion) for a subset of these
sources, using a global solution approach which must also include a large
number of parameters for the satellite attitude and optical instrument. The
accurate and efficient implementation of this solution is an extremely
demanding task, but crucial for the outcome of the mission. We provide a
comprehensive overview of the mathematical and physical models applicable to
this solution, as well as its numerical and algorithmic framework. The
astrometric core solution is a simultaneous least-squares estimation of about
half a billion parameters, including the astrometric parameters for some 100
million well-behaved so-called primary sources. The global nature of the
solution requires an iterative approach, which can be broken down into a small
number of distinct processing blocks (source, attitude, calibration and global
updating) and auxiliary processes (including the frame rotator and selection of
primary sources). We describe each of these processes in some detail, formulate
the underlying models, from which the observation equations are derived, and
outline the adopted numerical solution methods with due consideration of
robustness and the structure of the resulting system of equations. Appendices
provide brief introductions to some important mathematical tools (quaternions
and B-splines for the attitude representation, and a modified Cholesky
algorithm for positive semidefinite problems) and discuss some complications
expected in the real mission data.Comment: 48 pages, 19 figures. Accepted for publication in Astronomy &
Astrophysic
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