374 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
Alice: The Rosetta Ultraviolet Imaging Spectrograph
We describe the design, performance and scientific objectives of the
NASA-funded ALICE instrument aboard the ESA Rosetta asteroid flyby/comet
rendezvous mission. ALICE is a lightweight, low-power, and low-cost imaging
spectrograph optimized for cometary far-ultraviolet (FUV) spectroscopy. It will
be the first UV spectrograph to study a comet at close range. It is designed to
obtain spatially-resolved spectra of Rosetta mission targets in the 700-2050 A
spectral band with a spectral resolution between 8 A and 12 A for extended
sources that fill its ~0.05 deg x 6.0 deg field-of-view. ALICE employs an
off-axis telescope feeding a 0.15-m normal incidence Rowland circle
spectrograph with a concave holographic reflection grating. The imaging
microchannel plate detector utilizes dual solar-blind opaque photocathodes (KBr
and CsI) and employs a 2 D delay-line readout array. The instrument is
controlled by an internal microprocessor. During the prime Rosetta mission,
ALICE will characterize comet 67P/Churyumov-Gerasimenko's coma, its nucleus,
and the nucleus/coma coupling; during cruise to the comet, ALICE will make
observations of the mission's two asteroid flyby targets and of Mars, its
moons, and of Earth's moon. ALICE has already successfully completed the
in-flight commissioning phase and is operating normally in flight. It has been
characterized in flight with stellar flux calibrations, observations of the
Moon during the first Earth fly-by, and observations of comet Linear T7 in 2004
and comet 9P/Tempel 1 during the 2005 Deep Impact comet-collision observing
campaignComment: 11 pages, 7 figure
Possible detection of two giant extrasolar planets orbiting the eclipsing polar UZ Fornacis
We present new high-speed, multi-observatory, multi-instrument photometry of
the eclipsing polar UZ For in order to measure precise mid-eclipse times with
the aim of detecting any orbital period variations. When combined with
published eclipse times and archival data spanning ~27 years, we detect
departures from a linear and quadratic trend of ~60 s. The departures are
strongly suggestive of two cyclic variations of 16(3) and 5.25(25) years. The
two favoured mechanisms to drive the periodicities are either two giant
extrasolar planets as companions to the binary (with minimum masses of
6.3(1.5)M(Jupiter) and 7.7(1.2)M(Jupiter)) or a magnetic cycle mechanism (e.g.
Applegate's mechanism) of the secondary star. Applegate's mechanism would
require the entire radiant energy output of the secondary and would therefore
seem to be the least likely of the two, barring any further refinements in the
effect of magnetic fieilds (e.g. those of Lanza et al.). The two planet model
can provide realistic solutions but it does not quite capture all of the
eclipse times measurements. A highly eccentric orbit for the outer planet would
fit the data nicely, but we find that such a solution would be unstable. It is
also possible that the periodicities are driven by some combination of both
mechanisms. Further observations of this system are encouraged.Comment: 10 pages, 4 figures, 2 table
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
Microchannel Plates for the UVCS and SUMER Instruments on the SOHO Satellite
The microchannel plates for the detectors in the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) and UVCS (Ultraviolet Coronograph Spectrometer) instruments aboard the Solar Orbiting Heliospheric Observatory (SOHO) mission to be launched in late 1995 are described. A low resistance Z stack of microchannel plates (MCP's) is employed in a detector format of 27 mm x 10 mm using a multilayer cross delay line anode (XDL) with 1024 x 360 digitized pixels. The MCP stacks provide gains of greater than 2 x 10(exp 7) with good pulse height distributions (as low as 25% FWHM) under uniform flood illumination. Background rates of approx. 0.6 event cm(exp -2) sec(exp -1) are obtained for this configuration. Local counting rates up to about 800 events/pixel/sec have been achieved with little drop of the MCP gain. MCP preconditioning results are discussed, showing that some MCP stacks fail to have gain decreases when subjected to a high flux UV scrub. Also, although the bare MCP quantum efficiencies are close to those expected (10%), we found that the long wavelength response of KBr photocathodes could be substantially enhanced by the MCP scrubbing process. Flat field images are characterized by a low level of MCP fixed pattern noise and are stable. Preliminary calibration results for the instruments are shown
Large-Amplitude Ultraviolet Variations in the RR Lyrae Star ROTSE-I J143753.84+345924.8
The NASA Galaxy Evolution Explorer (GALEX) satellite has obtained
simultaneous near and far ultraviolet light curves of the ROTSE-I Catalog RR
Lyrae ab-type variable star J143753.84+345924.8. A series of 38 GALEX Deep
Imaging Survey observations well distributed in phase within the star's
0.56432d period shows an AB=4.9mag variation in the far UV (1350-1750A) band
and an AB=1.8mag variation in the near UV (1750-2750A) band, compared with only
a 0.8mag variation in the broad, unfiltered ROTSE-I (4500-10000A) band. These
GALEX UV observations are the first to reveal a large RR Lyrae amplitude
variation at wavelengths below 1800A. We compare the GALEX and ROTSE-I
observations to predictions made by recent Kurucz stellar atmosphere models. We
use published physical parameters for the comparable period (0.57433d),
well-observed RR Lyrae star WY Antliae to compute predicted FUV, NUV, and
ROTSE-I light curves for J143753.84+345924.8. The observed light curves agree
with the Kurucz predictions for [Fe/H]=-1.25 to within AB=0.2mag in the GALEX
NUV and ROTSE-I bands, and within 0.5mag in the FUV. At all metallicities
between solar and one hundredth solar, the Kurucz models predict 6-8mag of
variation at wavelengths between 1000-1700A. Other variable stars with similar
temperature variations, such as Cepheids, should also have large-amplitude FUV
light curves, observable during the ongoing GALEX imaging surveys.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http:/www.galex.caltech.edu/PUBLICATIONS after
November 22, 200
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
- …