128 research outputs found
The Copula: A Tool for Simulating Speckle Dynamics
Use of a copula for generating a sequence of correlated speckle patterns is introduced. The chief characteristic of this algorithm is that it generates a continuous speckle sequence with a specified evolution of the correlation and does so with just two arrays of random numbers. Thus, physically realistic temporally varying speckle patterns with proper first- and second-order statistics are easily realized. We illustrate use of the algorithm for generating sequences with prescribed Gaussian, exponential, and equal-interval correlations and demonstrate how correlation times can be specified independently. This approach to generating sequences of random realizations with prescribed correlations should prove useful in modeling such phenomena as dynamic light scatter, flow-dependent laser speckle contrast, and propagation of spatial coherence
Enhancement of the Spitzer Infrared Array Camera Distortion Correction for Parallax Measurements
The Spitzer Space Telescope Infrared Array (IRAC) offers a rare opportunity to measure distances and determine physical properties of the faintest and coldest brown dwarfs. The current distortion correction is a 3rd order polynomial represented by TAN-SIP parameters within the headers. The current correction, good to 100 mas, was derived from deep imaging, using marginally resolved galaxies in some cases, and has remained stable throughout both the cryogenic and warm mission. Using recent Spitzer calibration observations mapped to HST/ACS calibration observations of 47 Tuc with an absolute accuracy good to 1 mas, we are working towards a possible 5th order polynomial correction that theoretically could allow measurements to within 20 mas. Extensive testing, using observations of 47 Tuc, NGC 6791 and NGC 2264, are underway, after which the new parameters will be used to update all the 3.6 and 4.5um data taken within warm and cryogenic missions. We anticipate if achievable, this new accuracy could be combined with other ongoing enhancements (Ingalls et al, 9143-52) that will permit measurements of parallaxes out to about 50 pc, increasing the volume surveyed by a factor of 100, and enabling new capabilities such as luminosity measurements of the population of young brown dwarfs in the beta Pictoris moving group
Spitzer Space Telescope Mid-IR Light Curves of Neptune
We have used the Spitzer Space Telescope in 2016 February to obtain high cadence, high signal-to-noise, 17 hr duration light curves of Neptune at 3.6 and 4.5 μm. The light curve duration was chosen to correspond to the rotation period of Neptune. Both light curves are slowly varying with time, with full amplitudes of 1.1 mag at 3.6 μm and 0.6 mag at 4.5 μm. We have also extracted sparsely sampled 18 hr light curves of Neptune at W1 (3.4 μm) and W2 (4.6 μm) from the Wide-feld Infrared Survey Explorer (WISE)/NEOWISE archive at six epochs in 2010–2015. These light curves all show similar shapes and amplitudes compared to the Spitzer light curves but with considerable variation from epoch to epoch. These amplitudes are much larger than those observed with Kepler/K2 in the visible (amplitude ~0.02 mag) or at 845 nm with the Hubble Space Telescope (HST) in 2015 and at 763 nm in 2016 (amplitude ~0.2 mag). We interpret the Spitzer and WISE light curves as arising entirely from reflected solar photons, from higher levels in Neptune's atmosphere than for K2. Methane gas is the dominant opacity source in Neptune's atmosphere, and methane absorption bands are present in the HST 763 and 845 nm, WISE W1, and Spitzer 3.6 μm filters
The Science Advantage of a Redder Filter for WFIRST
WFIRST will be capable of providing Hubble-quality imaging performance over several thousand square degrees of the sky. The wide-area, high spatial resolution survey data from WFIRST will be unsurpassed for many decades into the future. With the current baseline design, the WFIRST filter complement will extend from the bluest wavelength allowed by the optical design to a reddest filter (F184W) that has a red cutoff at 2.0 microns. In this white paper, we outline some of the science advantages for adding a K_s filter with a 2.15 micron central wavelength in order to extend the wavelength coverage for WFIRST as far to the red as the possible given the thermal performance of the observatory and the sensitivity of the detectors
Enhancement of the Spitzer Infrared Array Camera (IRAC) distortion correction for parallax measurements
The Spitzer Space Telescope Infrared Array (IRAC) offers a rare opportunity to measure distances and determine physical properties of the faintest and coldest brown dwarfs. The current distortion correction is a 3rd order polynomial represented by TAN-SIP parameters within the headers. The current correction, good to 100 mas, was derived from deep imaging, using marginally resolved galaxies in some cases, and has remained stable throughout both the cryogenic and warm mission. Using recent Spitzer calibration observations mapped to HST/ACS calibration observations of 47 Tuc with an absolute accuracy good to 1 mas, we are working towards a possible 5th order polynomial correction that theoretically could allow measurements to within 20 mas. Extensive testing, using observations of 47 Tuc, NGC 6791 and NGC 2264, are underway, after which the new parameters will be used to update all the 3.6 and 4.5um data taken within warm and cryogenic missions. We anticipate if achievable, this new accuracy could be combined with other ongoing enhancements (Ingalls et al, 9143-52) that will permit measurements of parallaxes out to about 50 pc, increasing the volume surveyed by a factor of 100, and enabling new capabilities such as luminosity measurements of the population of young brown dwarfs in the beta Pictoris moving group
The Science Case for an Extended Spitzer Mission
Although the final observations of the Spitzer Warm Mission are currently
scheduled for March 2019, it can continue operations through the end of the
decade with no loss of photometric precision. As we will show, there is a
strong science case for extending the current Warm Mission to December 2020.
Spitzer has already made major impacts in the fields of exoplanets (including
microlensing events), characterizing near Earth objects, enhancing our
knowledge of nearby stars and brown dwarfs, understanding the properties and
structure of our Milky Way galaxy, and deep wide-field extragalactic surveys to
study galaxy birth and evolution. By extending Spitzer through 2020, it can
continue to make ground-breaking discoveries in those fields, and provide
crucial support to the NASA flagship missions JWST and WFIRST, as well as the
upcoming TESS mission, and it will complement ground-based observations by LSST
and the new large telescopes of the next decade. This scientific program
addresses NASA's Science Mission Directive's objectives in astrophysics, which
include discovering how the universe works, exploring how it began and evolved,
and searching for life on planets around other stars.Comment: 75 pages. See page 3 for Table of Contents and page 4 for Executive
Summar
Engineering of Niobium Surfaces Through Accelerated Neutral Atom Beam Technology For Quantum Applications
A major roadblock to scalable quantum computing is phase decoherence and
energy relaxation caused by qubits interacting with defect-related two-level
systems (TLS). Native oxides present on the surfaces of superconducting metals
used in quantum devices are acknowledged to be a source of TLS that decrease
qubit coherence times. Reducing microwave loss by surface engineering (i.e.,
replacing uncontrolled native oxide of superconducting metals with a thin,
stable surface with predictable characteristics) can be a key enabler for
pushing performance forward with devices of higher quality factor. In this
work, we present a novel approach to replace the native oxide of niobium
(typically formed in an uncontrolled fashion when its pristine surface is
exposed to air) with an engineered oxide, using a room-temperature process that
leverages Accelerated Neutral Atom Beam (ANAB) technology at 300 mm wafer
scale. This ANAB beam is composed of a mixture of argon and oxygen, with
tunable energy per atom, which is rastered across the wafer surface. The
ANAB-engineered Nb-oxide thickness was found to vary from 2 nm to 6 nm
depending on ANAB process parameters. Modeling of variable-energy XPS data
confirm thickness and compositional control of the Nb surface oxide by the ANAB
process. These results correlate well with those from transmission electron
microscopy and X-ray reflectometry. Since ANAB is broadly applicable to
material surfaces, the present study indicates its promise for modification of
the surfaces of superconducting quantum circuits to achieve longer coherence
times.Comment: 22 pages, 7 figures, will be submitted to Superconductor Science and
Technology Special Focus Issue Journa
Doppler optical coherence tomography imaging of local fluid flow and shear stress within microporous scaffolds
Establishing a relationship between perfusion rate and fluid shear stress in a 3-dimensional cell culture environment is an ongoing and challenging task faced by tissue engineers. In this study, we explore Doppler optical coherence tomography (DOCT) as a potential imaging tool for in situ monitoring of local fluid flow profiles inside porous chitosan scaffolds. From the measured fluid flow profiles, the fluid shear stresses are evaluated. We examine the localized fluid flow and shear stress within low and high porosity chitosan scaffolds, which are subjected to a constant input flow rate of 0.5 ml·min(-1). The DOCT results show that the behaviour of the fluid flow and shear stress in micropores is strongly dependent on the micropore interconnectivity, porosity, and size of pores within the scaffold. For low porosity and high porosity chitosan scaffolds examined, the measured local fluid flow and shear stress varied from micropore to micropore with a mean shear stress of 0.49±0.3 dyn·cm(-2) and 0.38±0.2 dyn·cm(-2), respectively. In addition, we show that the scaffold’s porosity and interconnectivity can be quantified by combining analyses of the 3-dimensional structural and flow images obtained from DOCT
The First Detection of Photometric Variability in a Y Dwarf: WISE J140518.39+553421.3
We present the first detection of the photometric variability in a spectroscopically confirmed Y dwarf. The Infrared Array Camera on board the Spitzer Space Telescope was used to obtain time series photometry of WISE J140518.39+553421.3 at 3.6 and 4.5 μm over a 24-hr period at two different epochs separated by 149 days. Variability is evident at 4.5 μm in the first epoch and at 3.6 and 4.5 μm in the second epoch, which suggests that the underlying cause or causes of this variability change on the timescales of months. The second-epoch [3.6] and [4.5] light curves are nearly sinusoidal in form, in phase, have periods of roughly 8.5 hr, and have semi-amplitudes of 3.5%. We find that a simple geometric spot model with a single bright spot reproduces these observations well. We also compare our measured semi-amplitudes of the second-epoch light curves to predictions of the static, one-dimensional, partly cloudy, and hot spot models of Morley and collaborators, and find that neither set of models can reproduce the observed [3.6] and [4.5] semi-amplitudes simultaneously. Therefore, more advanced two-dimensional or three-dimensional models that include time-dependent phenomena like vertical mixing, cloud formation, and thermal relaxation are sorely needed in order to properly interpret our observations
- …