175,898 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
Fast Spectral Variability from Cygnus X-1
We have developed an algorithm that, starting from the observed properties of
the X-ray spectrum and fast variability of an X-ray binary allows the
production of synthetic data reproducing observables such as power density
spectra and time lags, as well as their energy dependence. This allows to
reconstruct the variability of parameters of the energy spectrum and to reduce
substantially the effects of Poisson noise, allowing to study fast spectral
variations. We have applied the algorithm to Rossi X-ray Timing Explorer data
of the black-hole binary Cygnus X-1, fitting the energy spectrum with a
simplified power law model. We recovered the distribution of the power law
spectral indices on time-scales as low as 62 ms as being limited between 1.6
and 1.8. The index is positively correlated with the flux even on such
time-scales.Comment: 14 pages, 19 figures, accepted by MNRA
Multiple field-of-view MCAO for a Large Solar Telescope: LOST simulations
In the framework of a 4m class Solar Telescope we studied the performance of
the MCAO using the LOST simulation package. In particular, in this work we
focus on two different methods to reduce the time delay error which is
particularly critical in solar adaptive optics: a) the optimization of the
wavefront reconstruction by reordering the modal base on the basis of the
Mutual Information and b) the possibility of forecasting the wavefront
correction through different approaches. We evaluate these techniques
underlining pros and cons of their usage in different control conditions by
analyzing the results of the simulations and make some preliminary tests on
real data.Comment: 10 pages, 5 figures to be published in Adaptive Optics Systems II
(Proceedings Volume) Proceedings of SPI
Statistical Time Series Models of Pilot Control with Applications to Instrument Discrimination
A general description of the methodology used in obtaining the transfer function models and verification of model fidelity, frequency domain plots of the modeled transfer functions, numerical results obtained from an analysis of poles and zeroes obtained from z plane to s-plane conversions of the transfer functions, and the results of a study on the sequential introduction of other variables, both exogenous and endogenous into the loop are contained
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