Modeling and removal of optical ghosts in the PROBA-3/ASPIICS externally occulted solar coronagraph

Context: ASPIICS is a novel externally occulted solar coronagraph, which will be launched onboard the PROBA-3 mission of the European Space Agency. The external occulter will be placed on the first satellite approximately 150 m ahead of the second satellite that will carry an optical instrument. During 6 hours per orbit, the satellites will fly in a precise formation, constituting a giant externally occulted coronagraph. Large distance between the external occulter and the primary objective will allow observations of the white-light solar corona starting from extremely low heights 1.1RSun. Aims: To analyze influence of optical ghost images formed inside the telescope and develop an algorithm for their removal. Methods: We implement the optical layout of ASPIICS in Zemax and study the ghost behaviour in sequential and non-sequential regimes. We identify sources of the ghost contributions and analyze their geometrical behaviour. Finally we develop a mathematical model and software to calculate ghost images for any given input image. Results: We show that ghost light can be important in the outer part of the field of view, where the coronal signal is weak, since the energy of bright inner corona is redistributed to the outer corona. However the model allows to remove the ghost contribution. Due to a large distance between the external occulter and the primary objective, the primary objective does not produce a significant ghost. The use of the Lyot spot in ASPIICS is not necessary.Comment: 14 pages, 13 figure

Predicting adsorption of water/organic mixtures using molecular simulation

The use of Monte Carlo simulation to predict the adsorption of mixtures of polar and nonpolar species on activated carbon was investigated using water and ethane on BPL carbon as a prototype system. The structure of the adsorbent was modeled by an array of slit-shaped pores, characterized by a pore-size distribution. The chemical heterogeneity of the carbon was taken into account by including oxygen-containing sites on the surface of the pores. The pore-size distribution was obtained from pure-ethane adsorption on the same carbon sample, while the concentration and distribution of surface sites were determined by analyzing pure-water adsorption. Model predictions agree well with experimental multicomponent data

Molecular simulation of phase coexistence in adsorption in porous solids

In this work a recently proposed method, the gauge-cell Gibbs ensemble Monte Carlo, is extended to deal with polar substances. The behaviour of water, a hydrogen bonding, weakly adsorbing fluid, is compared with that of methane, a strongly adsorbing, non-polar fluid, in the vicinity of the phase transition. The mechanisms of condensation for the two species are seen to be significantly different in nature. A systematic study of the effect of the pore width on the phase equilibrium of water is also performed. Our results show that the narrowing of the pore shifts the equilibrium transition pressure to lower values and reduces the extent of vapour metastability, but exerts little influence on the stability of the liquid phase

Characterisation of the surface chemistry of activated carbon by molecular simulation of water adsorption

We propose a model for activated carbon incorporating both structural and chemical heterogeneity. Structural heterogeneity is represented by an array of slit-shaped pores, characterised by a pore size distribution. This distribution was calculated from experimental data on pure-component adsorption of ethane, using Grand Canonical Monte Carlo simulations to describe adsorption at the individual pore level. Chemical heterogeneity is represented in our model by including oxygen-containing surface sites. The results from molecular simulation in the model carbon are compared to the experimental pure-water adsorption isotherm on the same sample. From such a comparison, one is able to draw conclusions regarding the, concentration and distribution of surface sites in pores of different, width

Simulation study of the effect of the chemical heterogeneity of activated carbon on water adsorption

In this paper we present results from the molecular simulation of water adsorption in slit-shaped activated carbon pores. We calculate adsorption isotherms by grand canonical Monte Carlo (GCMC) simulation, Henry's constants by Monte Carlo integration, and vapor-liquid equilibrium data by the gauge-cell Monte Carlo method, to investigate the chemical heterogeneity of activated carbon adsorbents. Several types of polar oxygen-containing sites are placed on the surface of the carbon with different densities and local distributions, in order to determine the individual effects of each of these factors on the adsorption of water. Our results confirm the role of surface sites in the enhancement of water adsorption, Furthermore, we show that the local distribution of these sites has a strong effect on low-pressure adsorption, while the overall site density affects mainly the vapor-liquid phase transition. The type of oxygen-containing group is shown not to be of critical importance, since more complex groups can effectively be represented by simpler sites. This study forms the basis for the development of a model for activated carbon that is able to represent the chemical heterogeneity of this type of material

Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements VI. Ni II

We present the first detailed ab initio quantum mechanical calculations for total and state-specific recombination rate coefficients for e + Ni III --> Ni II. These rates are obtained using a unified treatment for total electron-ion recombination that treats the nonresonant radiative recombination and the resonant dielectronic recombination in a self-consistent unified manner in the close coupling approximation. Large-scale calculations are carried out using a 49-state wavefunction expansion from core configurations 3d^8, 3d^74s, and 3d^64p that permits the inclusion of prominent dipole allowed core transitions. These extensive calculations for the recombination rates of Ni II required hundreds of CPU hours on the Cray T90. The total recombination rate coefficients are provided for a wide range of temperature. The state-specific recombination rates for 532 bound states of doublet and quartet symmetries, and the corresponding photoionization cross sections for leaving the core in the ground state, are presented. Present total recombination rate coefficients differ considerably from the currently used data in astrophysical models.Comment: ApJ Suppl. (submitted), 4 figure

Reducing sequence risk using trend following and the CAPE ratio

The risk of experiencing bad investment outcomes at the wrong time, or sequence risk, is a poorly understood, but crucial aspect of the risk faced by investors, in particular those in the decumulation phase of their savings journey, typically over the period of retirement financed by a defined contributions pension scheme. Using US equity return data from 1872-2014 we show how this risk can be significantly reduced by applying trend-following investment strategies. We also demonstrate that knowledge of a valuation ratio such as the CAPE ratio at the beginning of a decumulation period is useful for enhancing sustainable investment income

Size Matters: Tail Risk, Momentum and Trend Following in International Equity Portfolios

We investigate the relationship between size and momentum across a wide range of international equity markets. A distinction is made between relative momentum where assets are ranked according to their performance against each other, and absolute momentum (or trend following) where assets are categorized according to whether they have recently exhibited positive, nominal return characteristics. We find only limited evidence for the outperformance of relative momentum portfolios. Trend following, however, is observed to be a very effective strategy over the study period delivering superior risk-adjusted returns across a range of size categories in both developed and emerging markets while not reversing the performance superiority of smaller firms. We also find, contrary to popular perception, that it is the mid cap-sector that dominates in emerging markets and suggest that this sector should be considered as the equivalent to developed economy small-cap investing

Experiments with Choirs – Practice and Pitfalls

This paper presents work in progress with choirs in their normal rehearsal venues to discover why they do not maintain pitch regularly when singing a cappella music. Following an extensive survey of choral practitioners which gathered evidence of pitch drift in a cappella choral singing, a series of experiments was undertaken with amateur choirs around the United Kingdom. For each choir, data was collected over twenty rehearsals by a member of the choir using equipment supplied by the Open University. The data collected included recordings of a specially composed work, acoustic and environmental measurements, singers' attendance and a summary of each rehearsal by the conductor. In addition, singers were asked to take part in a listening test which measured their ability to discriminate pitch differences between two notes. Preliminary findings shown that pitch drift varies from week to week, and the drift is in the flat direction despite the singers who responded to the survey being able to better discriminate small pitch differences in the flat direction than the sharp