116 research outputs found
Gerstenhaber brackets on Hochschild cohomology of general twisted tensor products
We present techniques for computing Gerstenhaber brackets on Hochschild
cohomology of general twisted tensor product algebras. These techniques involve
twisted tensor product resolutions and are based on recent results on
Gerstenhaber brackets expressed on arbitrary bimodule resolutions.Comment: 14 pages, small changes in the presentation, minor corrections,
additional referee corrections, to appear in J. Pure Appl. Algebr
Calibration and performance of the photon-counting detectors for the Ultraviolet Imaging Telescopes (UVIT) of the Astrosat observatory
We describe calibration data, and discuss performance of the photon-counting
flight detectors for the Ultraviolet Imaging Telescopes on the Astrosat
observatory. The paper describes dark current, flat field and light-spot images
for FUV, NUV, and Visible band detectors at more than one wavelength setting
for each. We also report on nominal gain and low-gain operations, full- and
sub-window read rates, and non-photon-counting modes of operation, all expected
to be used in flight. We derive corrections to the event centroids from the
CMOS readout arrays, for different centroid algorithms. We derive spatial
resolution values for each detector and plots of point-source signal saturation
for different flux levels. We also discuss ways to correct for saturation in
extended object images.Comment: 27 pages, including 4 tables. 14 Figures. To be published in PAS
Noiseless, kilohertz-frame-rate, imaging detector based on micro-channel plates readout with the Medipix2 CMOS pixel chip
A new hybrid imaging detector is described that is being developed for the next generation adaptive optics (AO) wavefront sensors. The detector consists of proximity focused microchannel plates (MCPs) read out by pixelated CMOS application specific integrated circuit (ASIC) chips developed at CERN ("Medipix2"). Each Medipix2 pixel has an amplifier, lower and upper charge discriminators, and a 14-bit chounter. The 256x256 array can be read out noiselessly (photon counting) in 286 us. The Medipix2 is buttable on 3 sides to produce 512x(n*256) pixel devices. The readout can be electronically shuttered down to a terporal window of a few microseconds with an accuracy of 10 ns. Good quantum efficiencies can be achieved from the x-ray (open faced with opaque photocathodes) to the optical (sealed tube with multialkali or GaAs photocathode)
Attenuation of Scattered Thermal Energy Atomic Oxygen
The attenuation of scattered thermal energy atomic oxygen is relevant to the potential damage that can occur within a spacecraft which sweeps through atomic oxygen in low Earth orbit (LEO). Although there can be significant oxidation and resulting degradation of polymers and some metals on the external surfaces of spacecraft, there are often openings on a spacecraft such as telescope apertures, vents, and microwave cavities that can allow atomic oxygen to enter and scatter internally to the spacecraft. Atomic oxygen that enters a spacecraft can thermally accommodate and scatter to ultimately react or recombine on surfaces. The atomic oxygen that does enter a spacecraft can be scavenged by use of high erosion yield polymers to reduce its reaction on critical surfaces and materials. Polyoxymethylene and polyethylene can be used as effective atomic oxygen scavenging polymers
On the Solar EUV Deposition in the Inner Comae of Comets with Large Gas Production Rates
In this letter we have made a comparative study of degradation of solar EUV
radiation and EUV-generated photoelectrons in the inner comae of comets having
different gas production rates, Q, with values 1x10^28, 7x10^29, 1x10^31, and
1x10^32 s^-1. We found that in higher-Q comets the radial profile of H2O+
photo-production rate depicts a double-peak structure and that the differences
in sunward and anti-sunward photoionization rates are pronounced. We show that
photoelectron impact ionization is an order of magnitude larger than
photoionization rate near the lower photoionization peak in comets with Q >~
1x10^31 s^-1. The present study reveals the importance of photoelectrons
relative to solar EUV as the ionization source in the inner coma of high-Q
comets
Photon counting arrays for AO wavefront sensors
Future wavefront sensors for AO on large telescopes will require a large number of pixels and must operate at high frame rates. Unfortunately for CCDs, there is a readout noise penalty for operating faster, and this noise can add up rather quickly when considering the number of pixels required for the extended shape of a sodium laser guide star observed with a large telescope. Imaging photon counting detectors have zero readout noise and many pixels, but have suffered in the past with low QE at the longer wavelengths (>500 nm). Recent developments in GaAs photocathode technology, CMOS ASIC readouts and FPGA processing electronics have resulted in noiseless WFS detector designs that are competitive with silicon array detectors, though at ~40% the QE of CCDs. We review noiseless array detectors and compare their centroiding performance with CCDs using the best available characteristics of each. We show that for sub-aperture binning of 6x6 and greater that noiseless detectors have a smaller centroid error at fluences of 60 photons or less, though the specific number is dependent on seeing conditions and the centroid algorithm used. We then present the status of a 256x256 noiseless MCP/Medipix2 hybrid detector being developed for AO
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
GALEX FUV Observations of Comet C/2004 Q2 (Machholz): The Ionization Lifetime of Carbon
We present a measurement of the lifetime of ground state atomic carbon,
C(^3P), against ionization processes in interplanetary space and compare it to
the lifetime expected from the dominant physical processes likely to occur in
this medium. Our measurement is based on analysis of a far ultraviolet (FUV)
image of comet C/2004 Q2 (Machholz) recorded by the Galaxy Evolution Explorer
(GALEX) on 2005 March 1. The bright CI 1561 A and 1657 A multiplets dominate
the GALEX FUV band. We used the image to create high S/N radial profiles that
extended beyond one million km from the comet nucleus. Our measurements yielded
a total carbon lifetime of 7.1 -- 9.6 x 10^5 s (scaled to 1 AU). Which compares
favorably to calculations assuming solar photoionization, solar wind proton
change exchange and solar wind electron impact ionization are the dominant
processes occurring in this medium and that comet Machholz was embedded in the
slow solar wind. The shape of the CI profiles inside 3x10^5 km suggests that
either the CO lifetime is shorter than previously thought and/or a
shorter-lived carbon-bearing parent molecule, such as CH_4 is providing the
majority of the carbon in this region of the coma of comet Machholz.Comment: 26 pages, 6 figures, accepted for publication in the Astrophysical
Journa
[v]at is going on? Local and global ideologies about Indian English
ABSTRACTThis article examines local and global language ideologies surrounding a particular phonetic feature in Indian English, the pronunciation of /v/ as [w]. By focusing on how local and global participants – both individuals and institutions – imagine language variation through disparate framings of “neutral” and “standard,” it highlights how processes of globalization and localization are interconnected, dialogic, and symbiotic. Compared are (i) sociolinguistic constructions of Indian cartoon characters, (ii) American “accent training” institutes, (iii) Indian call center and language improvement books, (iv) American speakers’ interpretations of merged IE speech, and, (v) IE speakers’ attitudes about IE, “neutral,” and ”standard” language. The relative social capital of these populations mediates both how each constructs its respective ideology about language variation, and how these ideologies dialogically interact with each other. (Language variation, language ideologies, dialogic, standard language)1</jats:p
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