24 research outputs found
Combined calculi for photon orbital and spin angular momenta
Context. Wavelength, photon spin angular momentum (PSAM), and photon orbital
angular momentum (POAM), completely describe the state of a photon or an
electric field (an ensemble of photons). Wavelength relates directly to energy
and linear momentum, the corresponding kinetic quantities. PSAMand POAM,
themselves kinetic quantities, are colloquially known as polarization and
optical vortices, respectively. Astrophysical sources emit photons that carry
this information. Aims. PSAM characteristics of an electric field (intensity)
are compactly described by the Jones (Stokes/Mueller) calculus. Similarly, I
created calculi to represent POAM characteristics of electric fields and
intensities in an astrophysical context. Adding wavelength dependence to all of
these calculi is trivial. The next logical steps are to 1) form photon total
angular momentum (PTAM = POAM + PSAM) calculi; 2) prove their validity using
operators and expectation values; and 3) show that instrumental PSAM can affect
measured POAM values for certain types electric fields. Methods. I derive the
PTAM calculi of electric fields and intensities by combining the POAM and PSAM
calculi. I show how these quantities propagate from celestial sphere to image
plane. I also form the PTAMoperator (the sum of the POAMand PSAMoperators),
with and without instrumental PSAM, and calculate the corresponding expectation
values. Results. Apart from the vector, matrix, dot product, and direct product
symbols, the PTAM and POAM calculi appear superficially identical. I provide
tables with all possible forms of PTAM calculi. I prove that PTAM expectation
values are correct for instruments with and without instrumental PSAM. I also
show that POAM measurements of "unfactored" PTAM electric fields passing
through non-zero instrumental circular PSAM can be biased. Conclusions. The
combined PTAM calculi provide insight into how to mathematically model PTAM
sources and calibrate POAMand PSAM- induced POAM measurement errors
The 2012 Interferometric Imaging Beauty Contest
We present the results of the fifth Interferometric Imaging Beauty Contest.
The contest consists in blind imaging of test data sets derived from model
sources and distributed in the OIFITS format. Two scenarios of imaging with
CHARA/MIRC-6T were offered for reconstruction: imaging a T Tauri disc and
imaging a spotted red supergiant. There were eight different teams competing
this time: Monnier with the software package MACIM; Hofmann, Schertl and
Weigelt with IRS; Thi\'ebaut and Soulez with MiRA ; Young with BSMEM; Mary and
Vannier with MIROIRS; Millour and Vannier with independent BSMEM and MiRA
entries; Rengaswamy with an original method; and Elias with the radio-astronomy
package CASA. The contest model images, the data delivered to the contestants
and the rules are described as well as the results of the image reconstruction
obtained by each method. These results are discussed as well as the strengths
and limitations of each algorithm
World Health Organization cardiovascular disease risk charts: revised models to estimate risk in 21 global regions
BACKGROUND: To help adapt cardiovascular disease risk prediction approaches to low-income and middle-income countries, WHO has convened an effort to develop, evaluate, and illustrate revised risk models. Here, we report the derivation, validation, and illustration of the revised WHO cardiovascular disease risk prediction charts that have been adapted to the circumstances of 21 global regions. METHODS: In this model revision initiative, we derived 10-year risk prediction models for fatal and non-fatal cardiovascular disease (ie, myocardial infarction and stroke) using individual participant data from the Emerging Risk Factors Collaboration. Models included information on age, smoking status, systolic blood pressure, history of diabetes, and total cholesterol. For derivation, we included participants aged 40-80 years without a known baseline history of cardiovascular disease, who were followed up until the first myocardial infarction, fatal coronary heart disease, or stroke event. We recalibrated models using age-specific and sex-specific incidences and risk factor values available from 21 global regions. For external validation, we analysed individual participant data from studies distinct from those used in model derivation. We illustrated models by analysing data on a further 123â743 individuals from surveys in 79 countries collected with the WHO STEPwise Approach to Surveillance. FINDINGS: Our risk model derivation involved 376â177 individuals from 85 cohorts, and 19â333 incident cardiovascular events recorded during 10 years of follow-up. The derived risk prediction models discriminated well in external validation cohorts (19 cohorts, 1â096â061 individuals, 25â950 cardiovascular disease events), with Harrell's C indices ranging from 0·685 (95% CI 0·629-0·741) to 0·833 (0·783-0·882). For a given risk factor profile, we found substantial variation across global regions in the estimated 10-year predicted risk. For example, estimated cardiovascular disease risk for a 60-year-old male smoker without diabetes and with systolic blood pressure of 140 mm Hg and total cholesterol of 5 mmol/L ranged from 11% in Andean Latin America to 30% in central Asia. When applied to data from 79 countries (mostly low-income and middle-income countries), the proportion of individuals aged 40-64 years estimated to be at greater than 20% risk ranged from less than 1% in Uganda to more than 16% in Egypt. INTERPRETATION: We have derived, calibrated, and validated new WHO risk prediction models to estimate cardiovascular disease risk in 21 Global Burden of Disease regions. The widespread use of these models could enhance the accuracy, practicability, and sustainability of efforts to reduce the burden of cardiovascular disease worldwide. FUNDING: World Health Organization, British Heart Foundation (BHF), BHF Cambridge Centre for Research Excellence, UK Medical Research Council, and National Institute for Health Research
Imaging Stars by Performing Full-Stokes Optical Interferometric Polarimetry
Optical interferometry and polarimetry have separately provided new insights into stellar astronomy, especially in the
fields of fundamental parameters and atmospheric models. We present: scientific justifications for âfull-Stokesâ optical
interferometric polarimetry (OIP); updated instrument requirements; preliminary beam combiner designs; polarimeter
design; end-to-end OIP data reduction; and realistic reimaged full-Stokes models of Be stars with a suitable number of
telescopes plus noise sources. All of this work represents preliminary research to construct an OIP beam combiner
The Penn Polarimeters
This report describes the inception, development and extensive use over 30 years of elliptical polarimeters at the University
of Pennsylvania. The initial Mark I polarimeter design utilized oriented retarder plates and a calcite Foster-Clarke
prism as the analyzer. The Mark I polarimeter was used on the Kitt Peak 0.9 m in 1969-70 to accomplish a survey of approximately
70 objects before the device was relocated to the 0.72 m reflector at the Flower and Cook Observatory. Successive
generations of automation and improvements included the early-80âs optical redesign to utilize a photoelastic
modulated wave plate and an Ithaco lock-in amplifierâthe photoelastic modulating polarimeter. The final design in 2000
concluded with a fully remote operable device. The legacy of the polarimetric programs includes studies of close binaries,
pulsating hot stars, and luminous late-type variables
Imaging Stars by Performing Full-Stokes Optical Interferometric Polarimetry
Optical interferometry and polarimetry have separately provided new insights into stellar astronomy, especially in the
fields of fundamental parameters and atmospheric models. We present: scientific justifications for âfull-Stokesâ optical
interferometric polarimetry (OIP); updated instrument requirements; preliminary beam combiner designs; polarimeter
design; end-to-end OIP data reduction; and realistic reimaged full-Stokes models of Be stars with a suitable number of
telescopes plus noise sources. All of this work represents preliminary research to construct an OIP beam combiner