720 research outputs found
A high resolution, high frame rate detector based on a microchannel plate read out with the Medipix2 counting CMOS pixel chip.
The future of ground-based optical astronomy lies with advancements in adaptive optics (AO) to overcome the limitations that the atmosphere places on high resolution imaging. A key technology for AO systems on future very large telescopes are the wavefront sensors (WFS) which detect the optical phase error and send corrections to deformable mirrors. Telescopes with >30 m diameters will require WFS detectors that have large pixel formats (512x512), low noise (<3 e-/pixel) and very high frame rates (~1 kHz). These requirements have led to the idea of a bare CMOS active pixel device (the Medipix2 chip) functioning in counting mode as an anode with noiseless readout for a microchannel plate (MCP) detector and at 1 kHz continuous frame rate. First measurement results obtained with this novel detector are presented both for UV photons and beta particles
Stacked Dual-Wavelength Near-Infrared Organic Photodetectors
Organic near-infrared (NIR) detectors have potential applications in biomedicine, agriculture, and manufacturing industries to identify and quantify materials contactless, in real time and at a low cost. Recently, tunable narrow-band NIR sensors based on charge-transfer state absorption of bulk-heterojunctions embedded into Fabry-PĂ©rot micro-cavities have been demonstrated. In this work, this type of sensor is further miniaturized by stacking two sub-cavities on top of each other. The resulting three-terminal device detects and distinguishes photons at two specific wavelengths. By varying the thickness of each sub-cavity, the detection ranges of the two sub-sensors are tuned independently between 790 and 1180, and 1020 and 1435 nm, respectively, with full-width-at-half-maxima ranging between 35 and 61 nm. Transfer matrix modeling is employed to select and optimize device architectures with a suppressed cross-talk in the coupled resonator system formed by the sub-cavities, and thus to allow for two distinct resonances. These stacked photodetectors pave the way for highly integrated, bi-signal spectroscopy tunable over a broad NIR range. To demonstrate the application potential, the stacked dual sensor is used to determine the ethanol concentration in a water solution
GALEX UV Spectroscopy and Deep Imaging of LIRGs in the ELAIS S1 field
The ELAIS S1 field was observed by GALEX in both its Wide Spectroscopic and
Deep Imaging Survey modes. This field was previously observed by the Infrared
Space Observatory and we made use of the catalogue of multi-wavelength data
published by the ELAIS consortium to select galaxies common to the two samples.
Among the 959 objects with GALEX spectroscopy, 88 are present in the ELAIS
catalog and 19 are galaxies with an optical spectroscopic redshift. The
distribution of redshifts covers the range . The selected galaxies
have bolometric IR luminosities (deduced from the flux using ISOCAM) which means that we cover a wide range of galaxies from
normal to Ultra Luminous IR Galaxies. The mean () UV luminosity (not
corrected for extinction) amounts to
L_\sun for the low-z () sample. The UV slope (assuming
) correlates with the GALEX FUV-NUV color if
the sample is restricted to galaxies below . Taking advantage of the
UV and IR data, we estimate the dust attenuation from the IR/UV ratio and
compare it to the UV slope . We find that it is not possible to uniquely
estimate the dust attenuation from for our sample of galaxies. These
galaxies are highly extinguished with a median value .
Once the dust correction applied, the UV- and IR-based SFRs correlate. For the
closest galaxy with the best quality spectrum, we see a feature consistent with
being produced by a bump near 220nm in the attenuation curve.Comment: This paper has been published as part of the GALEX ApJL Special Issue
(ApJ 619, L63
Star formation in the nearby universe: the ultraviolet and infrared points of view
This work presents the main ultraviolet (UV) and far-infrared (FIR)
properties of two samples of nearby galaxies selected from the GALEX (\AA, hereafter NUV) and IRAS (m) surveys respectively.
They are built in order to get detection at both wavelengths for most of the
galaxies. Star formation rate (SFR) estimators based on the UV and FIR
emissions are compared. Systematic differences are found between the SFR
estimators for individual galaxies based on the NUV fluxes corrected for dust
attenuation and on the total IR luminosity. A combined estimator based on NUV
and IR luminosities seems to be the best proxy over the whole range of values
of SFR. Although both samples present similar average values of the birthrate
parameter b, their star-formation-related properties are substantially
different: NUV-selected galaxies tend to show larger values of for lower
masses, SFRs and dust attenuations, supporting previous scenarios for the star
formation history (SFH). Conversely, about 20% of the FIR-selected galaxies
show high values of , SFR and NUV attenuation. These galaxies, most of them
being LIRGs and ULIRGs, break down the downsizing picture for the SFH, however
their relative contribution per unit volume is small in the local Universe.
Finally, the cosmic SFR density of the local Universe is estimated in a
consistent way from the NUV and IR luminosities.Comment: 43 pages, 13 figures, accepted for publication in Astrophysical
Journal Supplement Serie
UV and FIR selected star-forming galaxies at z=0: differences and overlaps
We study two samples of local galaxies, one is UV (GALEX) selected and the
other FIR (IRAS) selected, to address the question whether UV and FIR surveys
see the two sides ('bright' and 'dark') of the star formation of the same
population of galaxies or two different populations of star forming galaxies.
No significant difference between the L () luminosity
functions of the UV and FIR samples is found. Also, after the correction for
the `Malmquist bias' (bias for flux limited samples), the FIR-to-UV ratio v.s.
L relations of the two samples are consistent with each other. In the
range of 9 \la \log(L_{tot}/L_\sun) \la 12, both can be approximated by a
simple linear relation of \log (L_{60}/L_{FUV})=\log(L_{tot}/L_\sun)-9.66.
These are consistent with the hypothesis that the two samples represent the
same population of star forming galaxies, and their well documented differences
in L and in FIR-to-UV ratio are due only to the selection effect. A
comparison between the UV luminosity functions shows marginal evidence for a
population of faint UV galaxies missing in the FIR selected sample. The
contribution from these 'FIR-quiet' galaxies to the overall UV population is
insignificant, given that the K-band luminosity functions (i.e. the stellar
mass functions) of the two samples do not show any significant difference.Comment: 21 pages, 7 figures. Accepted by Ap
Tema Con Variazioni: Quantum Channel Capacity
Channel capacity describes the size of the nearly ideal channels, which can
be obtained from many uses of a given channel, using an optimal error
correcting code. In this paper we collect and compare minor and major
variations in the mathematically precise statements of this idea which have
been put forward in the literature. We show that all the variations considered
lead to equivalent capacity definitions. In particular, it makes no difference
whether one requires mean or maximal errors to go to zero, and it makes no
difference whether errors are required to vanish for any sequence of block
sizes compatible with the rate, or only for one infinite sequence.Comment: 32 pages, uses iopart.cl
A noiseless kilohertz frame rate imaging detector based on microchannel plates read out with the Medipix2 CMOS pixel chip
A new hybrid optical imaging detector is described that is being developed for the next generation adaptive optics (AO) wavefront sensors (WFS) for ground-based telescopes. The detector consists of a photocathode and proximity focused microchannel plates (MCPs) read out by the Medipix2 CMOS pixel ASIC. Each pixel of the Medipix2 device measures 55x55 um2 and comprises pre-amplifier, a window discriminator and a 14-bit counter. The 256x256 Medipix2 array can be read out noiselessly in 287 us. The readout can be electronically shuttered down to a temporal window of a few us. The Medipix2 is buttable on 3 sides to produce 512x(n*256) pixel devices. Measurements with ultraviolet light yield a spatial resolution of the detector at the Nyquist limit. Sub-pixel resolution can be achieved using centroiding algorithms. For the AO application, very high continuous frame rates of the order of 1 kHz are required for a matrix of 512x512 pixels. The design concepts of a parallel readout board are presented that will allow this fast data throughput. The development status of the optical WFS tube is also explained
Numerical Comparison of Cusum and Shiryaev-Roberts Procedures for Detecting Changes in Distributions
The CUSUM procedure is known to be optimal for detecting a change in
distribution under a minimax scenario, whereas the Shiryaev-Roberts procedure
is optimal for detecting a change that occurs at a distant time horizon. As a
simpler alternative to the conventional Monte Carlo approach, we propose a
numerical method for the systematic comparison of the two detection schemes in
both settings, i.e., minimax and for detecting changes that occur in the
distant future. Our goal is accomplished by deriving a set of exact integral
equations for the performance metrics, which are then solved numerically. We
present detailed numerical results for the problem of detecting a change in the
mean of a Gaussian sequence, which show that the difference between the two
procedures is significant only when detecting small changes.Comment: 21 pages, 8 figures, to appear in Communications in Statistics -
Theory and Method
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