Search

From Greek antiquity to the era of computer science, astronomical refraction has raised questions about the real angular position of celestial bodies as seen from Earth surface, the structure of its atmosphere, and the celestial navigation. After introducing the embryonic ideas and the first measurements, the article first presents the early founders of a mathematically structured study: Cassini, Newton, Taylor--the last two inventing appropriate mathematical tools. Then the 18th century geodesists (Bouguer, Lacaille, etc.) enrich the corpus of observations to be understood, followed by the "classics"--Simpson, Bradley, Kramp, Laplace. The progress in metrology allows precise studies of the air refractive index and new theoretical efforts with Biot's omnipresence. This gain in accuracy requires new calculations involving more sophisticated atmospheric models, as Radau and Ivory, among others, will do. Ivory pointing out a divergence in the numerical series being used, Kummer and Hausdorff find a new mathematical way. Numerical work on computers is eventually making useless all this mathematical virtuosity, although present work keeps relying on Biot's theoretical advances. [ABSTRACT FROM AUTHOR]

The general theoretical properties and various approximate formulas for the angle χS of astronomical refraction at S are presented, beginning with its fundamental expression by the refraction integral and immediately leading to the Simpson formula. Assuming a spherically symmetric atmosphere, many additional results show up, starting with Oriani's "theorem" and Bouguer's equation. From these, an usual form of the refraction integral is derived. A fundamental approximation follows to deduce the Laplace formula, then the series expansions that generalize it but are divergent for an isothermal atmosphere--because of the approximation made. Two results, free from any approximation, exist and are presented: Biot's and Biot-Sang-Meyer-Fraser-White's theorems. It leads to an approximate link with extinction, as found by Laplace, and an approximation of the refraction χSh for points on the astronomical horizon. We compute the chromatic effects on χS, χSh--and (in a document associated with this article) on various characteristics of optical ducts considered in air--and an original expression of the "local vertical angular distortion coefficient" on the horizon (in Annexe B). [ABSTRACT FROM AUTHOR]

We present a set of treatment techniques for GMOS/IFU data cubes, including correction of the differential atmospheric refraction; Butterworth spatial filtering, to remove high spatial-frequency noise; instrumental fingerprint removal; Richardson–Lucy deconvolution, to improve the spatial resolution of the observations. A comparison with HST images shows that the treatment techniques preserve the morphology of the spatial structures in the data cubes, without introducing distortions or ghost features. The treatment procedure applied to the data cube of the central region of the late-type galaxy NGC 2835 allowed the detection of a nuclear emitting region with emission-line ratios indicating a probable LINER, although the classifications of Transition Object or Seyfert galaxy cannot be discarded. Such a detection was not possible without the data treatment. This shows that the benefits provided by this treatment procedure are not merely cosmetic, but actually affect the quality of different types of analyses, like the determination of the fraction of galaxies, of different morphological types, with a central Seyfert/LINER/Transition object emission. [ABSTRACT FROM AUTHOR]

In this work we study in details the influence of pure astronomical refraction on solar metrological measurements made from ground-based full disc imagery and provide the tools for correcting the measurements and estimating the associated uncertainties. For a given standard atmospheric model, we first use both analytical and numerical methods in order to test the validity of the commonly or historically used approximations of the differential effect of refraction as a function of zenith distance. For a given refraction model, we provide the exact formulae for correcting solar radius measurements at any heliographic angle and for any zenith distance. Then, using solar images recorded in the near-infrared between 2011 and 2016, we show that these corrections can be applied up to 70° using the usual approximate formulae and can be extended up to 80° of zenith distance provided that a standard atmospheric model and a full numerical integration of the refraction integral are used. We also provide estimates of the absolute uncertainties associated with the differential refraction corrections and show that approximate formulae can be used up to 80° of zenith distance for computing these uncertainties. For a given instrumental setup and the knowledge of the uncertainties associated with local weather records, this can be used to fix the maximum zenith distance one can observe depending on the required astrometric accuracy. [ABSTRACT FROM AUTHOR]

To achieve accurate and completely autonomous navigation for spacecraft, inertial/celestial integrated navigation gets increasing attention. In this study, a missile-borne inertial/stellar refraction integrated navigation scheme is proposed. Position Dilution of Precision (PDOP) for stellar refraction is introduced and the corresponding equation is derived. Based on the condition when PDOP reaches the minimum value, an optimized observation scheme is proposed. To verify the feasibility of the proposed scheme, numerical simulation is conducted. The results of the Extended Kalman Filter (EKF) and Unscented Kalman Filter (UKF) are compared and impact factors of navigation accuracy are studied in the simulation. The simulation results indicated that the proposed observation scheme has an accurate positioning performance, and the results of EKF and UKF are similar. [ABSTRACT FROM AUTHOR]

Although the formalism of Lecavelier des Etangs et al. is extremely useful to understand what shapes transmission spectra of exoplanets, it does not include the effects of a sharp change in flux with altitude generally associated with surfaces and optically thick clouds. Recent advances in understanding the effects of refraction in exoplanet transmission spectra have, however, demonstrated that even clear thick atmospheres have such a sharp change in flux due to a refractive boundary. We derive a more widely applicable analytical formalism by including first-order effects from all these 'surfaces' to compute an exoplanet's effective radius, effective atmospheric thickness and spectral modulation for an atmosphere with a constant scaleheight. We show that the effective radius cannot be located below these 'surfaces' and that our formalism matches the formalism of Lecavelier des Etangs et al. in the case of a clear atmosphere. Our formalism explains why clouds and refraction reduce the contrast of spectral features, and why refraction decreases the Rayleigh scattering slope as wavelength increases, but also shows that these are common effects of all 'surfaces'. We introduce the concept of a 'surface' cross-section, the minimum mean cross-section that can be observed, as an index to characterize the location of 'surfaces' and provide a simple method to estimate their effects on the spectral modulation of homogeneous atmospheres. We finally devise a numerical recipe that extends our formalism to atmospheres with a non-constant scaleheight and arbitrary sources of opacity, a potentially necessary step to interpret observations. [ABSTRACT FROM AUTHOR]

The stellar refraction navigation is a type of autonomous celestial navigation method for satellites that uses high-accuracy star sensors to obtain the stellar refraction. The apparent height and the stellar refraction angle are two kinds of measurements for the satellite stellar refraction navigation system. A better navigation performance can be achieved by the stellar refraction angle rather than the refraction apparent height. However, the measurement model of the stellar refraction angle is an implicit function, which needs an implicit filter method. A new implicit unscented Kalman filter (UKF) method is proposed and applied to the satellite stellar refraction navigation system in this paper. The main difference between the classical UKF and the implicit UKF lies in the measurement update. First, since the implicit measurement model can be rearranged into an equation whose one side is zero, the vector of zeros is regarded as the equivalent measurement, and the difference between the predicted measurement and its true value is utilized for updating the predicted state and its covariance matrix. Second, the measurement noise and its covariance matrix are transformed based on the unscented transformation principle. In addition, the implicit UKF is compared with the implicit extended Kalman filter (IEKF), the iterative IEKF, and the UKF. Simulations show that the proposed implicit UKF method can obtain about 19.0% and 17.5% improvements in three-dimensional root mean square position estimation compared to the IEKF and the iterative IEKF, respectively. Although the navigation performance of the UKF is close to the implicit UKFs, the computational time of the implicit UKF is much shorter than the UKFs. In addition, the sensitivities of the four different Kalman filters to the star sensor's faults and the atmosphere perturbation are compared, respectively. [ABSTRACT FROM PUBLISHER]

We study the shadow of the rotating black hole with quintessential energy (i) in vacuum, (ii) in the presence of plasma with radial power-law density. For the vacuum case, the quintessential field parameter of the rotating black hole significantly changes the shape of the shadow. With increasing quintessential field parameter, the radius of the shadow also increases. With the increase of the radius of the shadow of the rotating black hole, the quintessential field parameter causes decrease of the distortion of the shadow shape: in the presence of the quintessential field parameter, the shadow of the fast rotating black hole becomes too close to the circle. We assume the distant observer of the black hole shadow to be located near the so-called static radius where the gravitational attraction of the black hole is just balanced by the cosmic repulsion. The shape and size of the shadow of quintessential rotating black hole surrounded by plasma depends on (i) plasma parameters, (ii) black hole spin and (iii) quintessential field parameter. With the increase of the plasma refraction index, the apparent radius of the shadow increases. However, for the large values of the quintessential field parameter, the change of the black hole shadow shape due to the presence of plasma is not significant, i.e. the effect of the quintessential field parameter dominates over the plasma effect. [ABSTRACT FROM AUTHOR]

Spectra of comet C/2014 Q2 (Lovejoy) were taken with a low-resolution spectrograph mounted on the 0.5-m telescope at the Mount Abu Infrared Observatory (MIRO), India during 2015 January to May, covering the perihelion and post-perihelion periods. The spectra showed strong molecular emission bands (C2, C3 and CN) in January, close to perihelion. We obtained the scale-lengths for these molecules by fitting the Haser model to the observed column densities. The variations of gas production rates and production rate ratios with heliocentric distance were studied. The extent of dust continuum and its variation with heliocentric distance was also investigated using the Aƒρ parameter. The comet is seen to become more active in the post-perihelion phase, thus showing an asymmetric behaviour about the perihelion. [ABSTRACT FROM AUTHOR]

Stellar refraction-based celestial navigation is an attractive method with high accuracy and low cost. Most existing research works only focus on orbit determination problems for orbital vehicles in outer space while its working domain is limited. In order to expand its applicable area and improve information utilization, a new fault-tolerant stellar refraction-based inertial/celestial integrated navigation system is designed in this work, which is supposed to provide accurate position, velocity, and attitude information for aerospace vehicles that either make a maneuvering flight in near space or move in a predetermined Earth orbit in outer space. First, a new nonlinear navigation system dynamic model is established by error-prorogation equations of the strapdown inertial navigation system (SINS) in the Earth-centered inertial (ECI) frame, in which additive quaternion with less model error is used for attitude computation. Secondly, celestial navigation subsystem (CNS) is used to indirectly sense the horizon by utilizing the starlight atmospheric refraction model and determine the vehicle attitude by means of a multistar vector observation method. Thirdly, a fault-tolerant federated unscented Kalman filtering (FFUKF) algorithm is employed to perform information fusion, aiming at improving the navigation accuracy and reliability. Numerical simulations are conducted for an aerospace vehicle either in a near space maneuvering flight or in an orbital flight along a low Earth orbit. The results show that the proposed strategy achieves higher accuracy than traditional methods, and all the navigation parameters can be estimated. Moreover, different measurement faults are introduced in the simulations, and the FFUKF algorithm manages to detect and isolate them and keep the navigation system working in normal operation. [ABSTRACT FROM AUTHOR]

Refraction and diffraction of incoming radio waves by the ionosphere induce time variability in the angular positions, peak amplitudes and shapes of radio sources, potentially complicating the automated cross-matching and identification of transient and variable radio sources. In this work, we empirically assess the effects of the ionosphere on data taken by the Murchison Widefield Array (MWA) radio telescope. We directly examine 51 h of data observed over 10 nights under quiet geomagnetic conditions (global storm index Kp < 2), analysing the behaviour of short-time-scale angular position and peak flux density variations of around ten thousand unresolved sources. We find that while much of the variation in angular position can be attributed to ionospheric refraction, the characteristic displacements (10-20 arcsec) at 154 MHz are small enough that search radii of 1-2 arcmin should be sufficient for crossmatching under typical conditions. By examining bulk trends in amplitude variability, we place upper limits on the modulation index associated with ionospheric scintillation of 1-3 per cent for the various nights. For sources fainter than ~1 Jy, this variation is below the image noise at typical MWA sensitivities. Our results demonstrate that the ionosphere is not a significant impediment to the goals of time-domain science with the MWA at 154 MHz. [ABSTRACT FROM AUTHOR]

ABSTRACT To analyse and invert refraction seismic travel time data, different approaches and techniques have been proposed. One common approach is to invert first-break travel times employing local optimization approaches. However, these approaches result in a single velocity model, and it is difficult to assess the quality and to quantify uncertainties and non-uniqueness of the found solution. To address these problems, we propose an inversion strategy relying on a global optimization approach known as particle swarm optimization. With this approach we generate an ensemble of acceptable velocity models, i.e., models explaining our data equally well. We test and evaluate our approach using synthetic seismic travel times and field data collected across a creeping hillslope in the Austrian Alps. Our synthetic study mimics a layered near-surface environment, including a sharp velocity increase with depth and complex refractor topography. Analysing the generated ensemble of acceptable solutions using different statistical measures demonstrates that our inversion strategy is able to reconstruct the input velocity model, including reasonable, quantitative estimates of uncertainty. Our field data set is inverted, employing the same strategy, and we further compare our results with the velocity model obtained by a standard local optimization approach and the information from a nearby borehole. This comparison shows that both inversion strategies result in geologically reasonable models (in agreement with the borehole information). However, analysing the model variability of the ensemble generated using our global approach indicates that the result of the local optimization approach is part of this model ensemble. Our results show the benefit of employing a global inversion strategy to generate near-surface velocity models from refraction seismic data sets, especially in cases where no detailed a priori information regarding subsurface structures and velocity variations is available. [ABSTRACT FROM AUTHOR]

In this tutorial we summarize the physics and mathematics behind refractive electromagnetic wave bending and delay. Refractive bending and delay through the Earth's atmosphere at both radio/millimetric and optical/IR wavelengths are discussed, but with most emphasis on the former, and with Atacama Large Millimeter Array (ALMA) applications in mind. As modern astronomical measurements often require subarcsecond position accuracy, care is required when selecting refractive bending and delay algorithms. For the spherically-uniform model atmospheres generally used for all refractive bending and delay algorithms, positional accuracies ≲1" are achievable when observing at zenith angles ≲75°. A number of computationally economical approximate methods for atmospheric refractive bending and delay calculation are presented, appropriate for astronomical observations under these conditions. For observations under more realistic atmospheric conditions, for zenith angles ≲75°, or when higher positional accuracy is required, more rigorous refractive bending and delay algorithms must be employed. For accurate calculation of the refractive bending, we recommend the Auer and Standish method, using numerical integration to ray-trace through a two-layer model atmosphere, with an atmospheric model determination of the atmospheric refractivity. For the delay calculation we recommend numerical integration through a model atmosphere. [ABSTRACT FROM AUTHOR]

Context. Atmospheric dispersion and field differential refraction impose severe constraints on widefield, multiobject spectroscopic (MOS) observations, where the two joint effects cannot be continuously corrected. Flux reduction and spectral distortions must then be minimised by a careful planning of the observations, which is especially true for instruments that use slits instead of fibres. This is the case of VIMOS at the VLT, where MOS observations have been restricted, since the start of operations, to a narrow two-hour range from the meridian to minimise slit losses, the so-called two-hour angle rule. Aims. We revisit in detail the impact of atmospheric effects on the quality of VIMOS-MOS spectra with the aim of enhancing the instrument's overall efficiency, and improving the scheduling of observations. Methods. We model slit losses across the entire VIMOS field of view as a function of target declination. We explore two different slit orientations at the meridian: along the parallactic angle (north-south), and perpendicular to it (east-west). Results. We show that, for fields culminating at zenith distances larger than 20 degrees, slit losses are minimised with slits oriented along the parallactic angle at the meridian. The two-hour angle rule holds for these observations using north-south orientations. Conversely, for fields with zenith angles smaller than 20 degress at culmination, losses are minimised with slits oriented perpendicular to the parallactic angle at the meridian; MOS observations can be effectively extended to plus/minus three hours from the meridian in these cases. In general, night-long observations of a single field will benefit from using the east-west orientation. All-sky or service mode observations, however, require a more elaborate planning that depends on the target declination, and the hour angle of the observations. Conclusions. We establish general rules for the alignment of slits in MOS observations that will increase target observability, enhance the efficiency of operations, and speed up the completion of programmes, a particularly relevant aspect for the forthcoming spectroscopic public surveys with VIMOS. Additionally, we briefly address the non-negligible impact of field differential refraction on future widefield MOS surveys. [ABSTRACT FROM AUTHOR]

Relativistic light bending and gravitational lensing have traditionally been viewed purely as effects of spacetime curvature. Yet for many years they have also been treated as a quasi-refraction of light in a special optical medium, wherein the refractive index is considered proportional to the gravitational potential. We now propose that this 'optical-mechanical analogy' in general relativity can also account for gravity. Using classical optics we show that a photon moving through the refractive medium about a mass transfers momentum first to the medium and then to the mass itself. Due to transfer of momentum primarily from ultra-remote CMB photons, masses are then subject to a cosmic pressure on all sides. Where two masses occur, mutual screening by their respective envelopes of refractive medium is shown to result in an attractive force of the Le Sage or 'pushing gravity' type. We suggest that the gravito-optical medium is comprised of gravitons, which may be modeled as a quasi-Einstein-Bose conjugate interconnecting all the masses of the visible universe. [ABSTRACT FROM AUTHOR]

We present a passive method for the retrieval of atmospheric pressure profiles based on the measurement of the apparent flattening of the solar disk as observed through the atmosphere by a spaceborne imager. This method was applied to simulated sunsets. It relies on accurate representation of the solar disk, including its limb darkening, and how its image is affected by atmospheric refraction. The Zernike polynomials are used to quantify the flattening in the Sun images. The inversion algorithm relies on a transfer matrix providing the link between the atmospheric pressure profile and a sequence of Zernike moments computed on the sunset frames. The transfer matrix is determined by a training dataset of pressure profiles generated from a standard climatology. The performance and limitations of the method are assessed by two test cases. Pressure profiles similar to the training dataset show that retrieval error can be up to 10 times smaller than the natural variability in the lower mesosphere, and up to 500 times smaller in the upper troposphere. Tests with other independent profiles emphasize the need for better representativeness of the training dataset. [ABSTRACT FROM AUTHOR]

At any launch base radar tracking is critical for assessing the state of the launch vehicle for range safety function as well as for trajectory monitoring. Tracking data corrected for refraction effects is essential for estimating the flight performance. This paper presents the refraction effects of troposphere and ionosphere on the radio waves of C-Band radars in terms of errors in the measured tracking parameters. Application of mapping functions for correcting refraction errors in C-Band radar track data is studied and a comparison of the performance of mapping functions is presented. The mapping functions not only show good convergence at low elevations but also correct around 90 per cent of the refraction error in elevation. [ABSTRACT FROM AUTHOR]

In this work, a three-dimensional refraction model is proposed, which allows one to calculate the delay of signal due to the Earth's troposphere-the main source of error limiting the accuracy of modern radio astronomical observations. The method of raytracing through the three-dimensional troposphere (numerical weather model) is used. The results of VLBI observations of spacecraft are presented, showing that, in terms of accuracy, the developed model performs at the level of the best modern analogues and in some cases even exceeds them. [ABSTRACT FROM AUTHOR]

Abstract: The refracted, reflected and scattered signals of global navigation satellite systems (GNSS) have been successfully used to remotely sense the Earth’s surface and atmosphere. It has demonstrated its potential to sense the atmosphere and ionosphere, ocean, land surfaces (including soil moisture) and the cryosphere. These new measurements, although in need of refinement and further validation in many cases, can be used to complement existing techniques and sensors, e.g., radiosonde, ionosonde, radar altimetry and synthetic aperture radar (SAR). This paper presents the current status and new developments of remote sensing using GNSS signals as well as its future directions and applications. Some notable emerging applications include monitoring sea ice, dangerous sea states, ocean eddy and storm surges. With the further improvement of the next generation multi-frequency GNSS systems and receivers and new space-based instruments utilizing GNSS reflections and refractions, new scientific applications of GNSS are expected in various environment remote sensing fields in the near future. [Copyright &y& Elsevier]

Abstract: For the first time we propose a method for detecting the mid-latitude field-aligned disturbances (FADs) prolated on the magnetic field. The method is based on GPS detection of isolated ionosphere disturbances and analysis of the angle between the “GPS satellite-receiver” line-of-sight and the local magnetic field vector, for which maximum (minimum) of aperiodic variation of the total electron content (TEC) is registered. The method possibilities are shown by example of recording abnormal mid-latitude isolated disturbances over California during the 5 October 2001 magnetic storm recovery phase. If the plasma FADs with a positive deviation relative to the background TEC were caused by the extension of a plasma blobs at the magnetic equator, then this would imply that the plasma blobs reached an altitude of 5890km in the apex of magnetic field line. When crossing FADs, the TEC gradients may reach quite great values (up to 10 TECU/1000km). Such gradients determine large short-term range errors in single-frequency (differential) GPS systems and refraction errors in radioastronomical interferometers when the line-of-sight passes next to the magnetic zenith. [Copyright &y& Elsevier]

urate heliographic coordinates of objects on the Sun have to be known in several fields of solar physics. One of the factors that affect the accuracy of the measurements of the heliographic coordinates is the accuracy of the orientation of a solar image. In this paper the well-known drift method for determining the orientation of the solar image is applied to data taken with a solar telescope equipped with a CCD camera. The factors that influence the accuracy of the method are systematically discussed, and the necessary corrections are determined. These factors are as follows: the trajectory of the center of the solar disk on the CCD with the telescope drive turned off, the astronomical refraction, the change of the declination of the Sun, and the optical distortion of the telescope. The method can be used on any solar telescope that is equipped with a CCD camera and is capable of taking solar full-disk images. As an example to illustrate the method and its application, the orientation of solar images taken with the Gyula heliograph is determined. As a byproduct, a new method to determine the optical distortion of a solar telescope is proposed. [ABSTRACT FROM AUTHOR]

When dealing with philosophical questions such as the choice of a world system or the substance of heaven, some sixteenth-century astronomers, including Tycho Brahe and Christophe Rothmann, devised more accurate experimental setups so that they could refine their celestial observations. With this desire to listen to nature arose new questions, in particular that of atmospheric refractions, the understanding and resolution of which became decisive to guarantee the best accuracy. However, to solve such practical problems, it was necessary to consider their philosophical and theological background, as well as the knowledge of optics inherited from the medieval period. We examine this heritage and its transmissions through the works of Peter Ramus's assistants, especially Jean Pena and Friedrich Risner. From this first survey, we try to analyse the lively debate on atmospheric refractions in the late sixteenth and early seventeenth centuries by exploring the argumentative structure, the philosophical background, and the epistemological grounds of the discourse of authors such as Tycho Brahe, Christoph Rothmann, and Johannes Kepler. [ABSTRACT FROM AUTHOR]

- The Dawn spacecraft carries a gamma-ray and neutron detector (GRaND), which will measure and map the abundances of selected elements on the surface of asteroid 4 Vesta. We compare the variability of moderately volatile/refractory incompatible element ratios (K/Th and K/Ti) in howardite, eucrite, and diogenite (HED) meteorites with those in other achondrite suites that represent asteroidal crusts, because these ratios may be accurately measured by GRaND and likely reflect initial chemical compositions of the HED parent body. The K/Th and K/Ti variations can differentiate HED meteorites from angrites and some unique eucrite-like lithologies. The results suggest that K, Th, and Ti abundances determined from GRaND data could not only confirm that Vesta is the parent body of HED meteorites but might also allow recognition of as-yet unsampled compositional terranes on Vesta. Besides the K-Th-Ti systematics study, we propose a new three-component mixing model for interpretation of GRaND spectra, required because the spatial resolution of GRaND is coarser than the spectral (compositional) heterogeneity of Vesta's surface. The mixing model uses abundances of K, Ti, Fe, and Mg that will be analyzed more accurately than other prospective GRaND-analyzed elements. We examine propagated errors due to GRaND analytical uncertainties and intrinsic errors that stem from an assumption introduced into the mixing model. The error investigation suggests that the mixing model can adequately estimate not only the diogenite/eucrite mixing ratio but also the abundances of most major and minor elements within the GRaND propagated errors. [ABSTRACT FROM AUTHOR]

Abstract: Refractory objects such as Calcium, Aluminum-rich Inclusions, Amoeboid Olivine Aggregates, and crystalline silicates, are found in primitive bodies throughout our Solar System. It is believed that these objects formed in the hot, inner solar nebula and were redistributed during the mass and angular momentum transport that took place during its early evolution. The ages of these objects thus offer possible clues about the timing and duration of this transport. Here we study how the dynamics of these refractory objects in the evolving solar nebula affected the age distribution of the grains that were available to be incorporated into planetesimals throughout the Solar System. It is found that while the high temperatures and conditions needed to form these refractory objects may have persisted for millions of years, it is those objects that formed in the first 105 years that dominate (make up over 90%) those that survive throughout most of the nebula. This is due to two effects: (1) the largest numbers of refractory grains are formed at this time period, as the disk is rapidly drained of mass during subsequent evolution and (2) the initially rapid spreading of the disk due to angular momentum transport helps preserve this early generation of grains as opposed to later generations. This implies that most refractory objects found in meteorites and comets formed in the first 105 years after the nebula formed. As these objects contained live 26Al, this constrains the time when short-lived radionuclides were introduced to the Solar System to no later than 105 years after the nebula formed. Further, this implies that the as defined by meteoritic materials represents at most, the instant when the solar nebula finished accreting significant amounts of materials from its parent molecular cloud. [Copyright &y& Elsevier]

Abstract: We study the impact of the atmospheric differential chromatic refraction on the measurements and precision of relative astrometry. Specifically, we address the problem of measuring the separations of close pairs of binary stars with adaptive optics in the J and K bands. We investigate the influence of weather conditions, zenithal distance, star’s spectral type and observing wavelength on the astrometric precision and determine the accuracy of these parameters that is necessary to detect exoplanets with existing and planned large ground based telescopes with adaptive optics facilities. The analytical formulae for simple monochromatic refraction and a full approach, as well as moderately simplified procedure, are used to compute refraction corrections under a variety of observing conditions. It is shown that the atmospheric refraction must be taken into account in astrometric studies but the full procedure is not necessary in many cases. Requirements for achieving a certain astrometric precision are specified. [Copyright &y& Elsevier]

Abstract: Because of the influence of atmospheric refraction the astronomical observations of the objects with the angles of elevation below 15° are generally avoided, but for the sake of the complete theoretical research the atmospheric refraction under the condition of lower angles of elevation is still worthy to be analyzed and explored. Especially for some engineering applications the objects with low angles of elevation must be observed sometimes. A new idea for determining atmospheric refraction by utilizing the differential method is proposed. A series of observations of the starry sky at different heights are carried out and by starting from the zenith with a telescope with larger field of view, the derivatives of various orders of atmospheric refraction function at different zenith distances are calculated and finally the actually observed values of atmospheric refraction can be found via numerical integration. The method does not depend upon the strict local parameters and complex precise observational instrumentation, and the observational principle is relatively simple. By the end of 2007 a simply constructed telescope with a larger field of view at Xinglong Observing Station was employed to carry out trial observations. The values of atmospheric refraction at the true zenith distances of 44.8° to 87.5° were obtained from the practical observations based on the differential method, and the feasibility of the method of differential measurement of atmospheric refraction was preliminarily justified. Being limited by the observational conditions, the accuracy of the observed result was limited, the maximal accidental error was about 6” and there existed certain systematic errors. The value of the difference between the result obtained at the zenith distance of 84° and that given in the Pulkovo atmospheric refraction table was about 15”. How to eliminate the cumulative error introduced due to the integration model error is the key problem which needs to be solved in future. [Copyright &y& Elsevier]

The propagation of electromagnetic waves emitted from ground-based meteorological radars is determined by the stratification of the atmosphere. In extreme superrefractive situations characterized by strong temperature inversions or strong vertical gradients of moisture, the radar beam can be deflected toward the ground (ducting or trapping). This phenomenon often results in spurious returned echoes and misinterpretation of radar images such as erroneous precipitation detection. In this work, a 5-yr global climatology of the frequency of superrefractive and ducting conditions and of trapping-layer base height has been produced using refractivity computations from ECMWF temperature, moisture, and pressure analyses at a 40-km horizontal resolution. The aim of this climatology is to better document how frequent such events are, which is a prerequisite for fully benefiting from radar data information for the multiple purposes of model validation, precipitation analysis, and data assimilation. First, the main climatological features are summarized for the whole globe: high- and midlatitude oceans seldom experience superrefraction or ducting whereas tropical oceans are strongly affected, especially in regions where the trade wind inversion is intense and lying near the surface. Over land, seasonal averages of superrefraction (ducting) frequencies reach 80% (40%) over tropical moist areas year-round but remain below 40% (15%) in most other regions. A particular focus is then laid on Europe and the United States, where extensive precipitation radar networks already exist. Seasonal statistics exhibit a pronounced diurnal cycle of ducting occurrences, with averaged frequencies peaking at 60% in summer late afternoon over the eastern half of the United States, the Balkans, and the Po Valley but no ducts by midday. Similarly high ducting frequencies are found over the southwestern coast of the United States at night. A potentially strong reduction of ducting occurrences with increased radar height (especially in midlatitude summer late afternoon) is evidenced by initiating refractivity vertical gradient computations from either the lowest or the second lowest model level. However, installing radar on tall towers also brings other problems, such as a possible amplification of sidelobe clutter echoes. [ABSTRACT FROM AUTHOR]

In the present work, the light-scattering properties of comet Levy 1990XX are studied through simulations using Ballistic Particle-Cluster Aggregation (BPCA) or Ballistic Cluster–Cluster Aggregation (BCCA) aggregates of up to 128 spherical monomers of different compositions (e.g. silicates, carbonaceous materials etc.) and the best-fitting theoretical polarization curve is generated using theSuperposition T-matrix code. The best-fitting refractive indices coming out from the present analysis show silicate behaviour when monomer radius is and provide excellent results on the maximum and negative degrees of linear polarization at a single wavelength for BCCA aggregates. [ABSTRACT FROM AUTHOR]

Abstract: Focusing on lowering the cut-off elevation in the neutral atmosphere refraction delay correction and on raising the accuracy of the correction, we derive the formulae for calculating the correction for the bending of the light path caused by atmospheric refraction. This is the sort of correction that is given after the principal term in theoretical models of neutral atmospheric refraction delay correction, but is often neglected because it is a small quantity. However, in practice, for a not too low elevation like 15°, this term reaches 1cm order of magnitude and can not be neglected. Li Yan-xing et al. specially gave a derivation of this correction and a computational method by successive approximation and some calculated values. Yan Hao-jian also proposed a formula of direct calculation but his calculated result was more than 3 times smaller than that of Li Yan-xing, which shows that further study of this correction is called for. Here we give a simple, convenient and reliable formula for calculating the correction. [Copyright &y& Elsevier]

Refraction is a detrimental problem in terrestrial optical measurements and can be regarded as major source of systematic errors in the precise determination of distances and directions. In general, refraction is a function of the density inhomogeneities of the propagation medium. As the "classical" method of temperature-gradient determination does not meet the requirement of a representative integral determination of the refractive index gradient field, at the Institute of Geodesy and Photogrammetry of the ETH Zurich, two methods to determine and correct the refraction influence have been developed further during the last few years. One approach focuses on the determination of the refractive index gradient in measuring the turbulence of the air by scintillometry or CDD-based image processing, which is presently the key technology in tracking tacheometers and digital levels. Turbulence is a measure of the energy in the heat exchanging process and can be converted by the Monin-Obukov-Similarity into temperature gradients. The advantage of optical scintillation measurements is to derive line averaged turbulence parameters of the atmospheric surface layer. Another challenging approach was the successful development of a compact laser-dispersometer at the ETH, which could be a component of actual geodetic instruments in the future. A dispersometer theodolite, based on the dual-wavelength method for dispersive air, is capable of refraction-free direction measurements. The results of both technologies, turbulence determination and dispersometry, will be presented and discussed in this paper. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this work was to analyse genetic influences on ocular refractive value and axial length using the hypothesis of a polygenic control. The genealogical records of 55 families were used in the analyses. The cohort included 723 individuals and clinical data were collected for 445 individuals with a mean age of 37.86years. Ocular refraction was determined by standard autorefractometry. Axial length was evaluated by scan ultrasonography. Gender, age and ethnic origin were included as covariates in the statistical analyses. Using variance component analysis via a Markov Chain Monte Carlo (MCMC) method, we estimated the heritability of refractive value and axial length in the pedigree. We then performed a segregation analysis, using Loki, a (MCMC) linkage analysis program for multilocus inheritance models, examining different inheritance models with polygenic components. Polygenic control was modelled under an additive infinitesimal model (which assumes infinite loci with small effects, with additive actions) and under a finite locus model (i.e. several causal loci). The estimates of heritability were 0.20 (95% confidence interval (CI) 0.04–0.36) for refractive value and 0.20 (95% CI 0.03–0.43) for axial length. Segregation analyses suggested that ocular refraction and axial length are under a polygenic control. A finite number of genes were identified with or without a polygenic, infinitesimal component. Ocular refraction is mildly-moderately heritable in the studied population. [Copyright &y& Elsevier]

The photoexcited carrier lifetime in ZnSe and CdSe (direct band gap crystals) and CdI2 and PbI2 (indirect band gap crystals) has been investigated by the use of a single beam Z-scan technique at 532 nm. The carrier lifetime (τ) is determined to be (1.7 ± 0.5) ns in the case of direct band gap and (1.2 ± 0.03) ps in the case of indirect band gap crystals. Lower values of τ in the case of indirect band gap crystals can be attributed to the reduction in value of the carrier density (N). [ABSTRACT FROM AUTHOR]

Abstract: It is found that the amphoteric refraction, i.e., the refraction can be either positive or negative depending on the incident angles, could occur at a planar interface associated with a uniaxially anisotropic positive index media (PIM) or an anisotropic negative index media (NIM). Particularly, the anomalous negative refraction can occur at a planar interface from an isotropic PIM to an anisotropic PIM, whereas the anomalous positive refraction occurs at the interface from an isotropic PIM to an anisotropic NIM. The optimal conditions to yield the two unusual refractions are obtained. The difference of the two types of amphoteric refraction is discussed. [Copyright &y& Elsevier]

We study characteristics of refraction of plane waves in a non-absorbing isotropic–uniaxial interface in conditions of total and partial reflection when the incident wave has arbitrary direction with regard to the optical axis of the crystal and polarization. We analyse the ordinary and extraordinary fields and a phase shift that appears in the propagating wave which subsists in ordinary or extraordinary total reflection. We obtain that, unlike what happens in isotropic interfaces, under conditions of ordinary total reflection, the ordinary time-averaged Poynting vector changes its direction with the angle of incidence which does not depend on the existence of extraordinary total reflection. Extraordinary total reflection gives place to an analogous behaviour of the extraordinary time-averaged Poynting vector. In addition, we define and calculate in an explicit way complex coefficients of transmission for geometries with and without symmetry in partial and total reflection. [ABSTRACT FROM AUTHOR]

Refraction of wave propagation in a corotating pulsar magnetospheric plasma is considered as a possible interpretation for observed asymmetric pulse profiles with multiple components. The pulsar radio emission produced inside the magnetosphere propagates outwards through the rotating magnetosphere, subject to refraction by the intervening plasma that is spatially inhomogeneous. Both effects of a relativistic distribution of the plasma and rotation on wave propagation are considered. It is shown that refraction coupled with rotation can produce asymmetric conal structures of the profile. The differential refraction due to the rotation can cause the conal structures to skew towards the rotation direction and leads to asymmetry in relative intensities between the leading and trailing components. Both of these features are potentially observable. [ABSTRACT FROM AUTHOR]

ABSTRACT An inside–out model for the formation of haloes in a hierarchical clustering scenario is studied. The method combines the picture of the spherical infall model and a modification of the extended Press–Schechter theory. The mass accretion rate of a halo is defined to be the rate of its mass increase due to minor mergers. The accreted mass is deposited at the outer shells without changing the density profile of the halo inside its current virial radius. We applied the method to a flat Λ-cold dark matter universe. The resulting density profiles are compared with analytical models proposed in the literature, and a very good agreement is found. A trend is found of the inner density profile to become steeper for larger halo mass, which also results from recent N -body simulations. Additionally, present-day concentrations as well as their time evolution are derived and it is shown that they reproduce the results of large cosmological N -body simulations. [ABSTRACT FROM AUTHOR]

Examines the usability of astro-photometer for magic lantern projection. Development of techniques to manipulate, control and employ scientific judgments; Aspects of subjectivity; Kinds of refraction phenomena.

The refraction of ordinary superluminous waves in a pulsar plasma is considered. The effect is found essentially to influence the apparent spatial structure of a pulsar emission region. This is of particular importance for the interpretation of the observations of pulsar interstellar scintillations, which provide an opportunity of resolving the magnetospheres of nearby pulsars. When calculated with account for magnetospheric refraction, the dependence of transverse ray separation on pulse longitude appears to be similar to that inferred by means of interstellar interferometry. It is shown that the differences in the character of the dependences observed at various frequencies may be attributed to the frequency-dependent efficiency of refraction. Taking into account magnetospheric refraction makes it necessary to reconsider the estimate of the emission altitude derived from the apparent size of the emission region. The renewed estimate appears to be almost an order of magnitude less, so that it is consistent with the emission altitudes obtained by other methods. [ABSTRACT FROM AUTHOR]

Describes the mapping functions of the refractive delay and astronomical refraction at radio and optical frequencies. Derivation of an improved continued fraction expansion of the complementary error function; Evidence that the complementary error function can be related to expressions for the atmospheric refraction integrals of the signal delay.

We present an update of the "refractive index of air" experiment, which is commonly used in undergraduate advanced laboratories. The refractive index of air depends on the average molecular polarizability, which can be determined from the period of the phase shift in a Michelson interferometer as a function of air pressure. The measured average molecular polarizability of air is γ=(2.118±0.091) × 10-29 m3 (95% CI). The corresponding refractive index of air at atmospheric pressure is n=1.000265(11), which agrees with the accepted value of n = 1.000271375(6). [ABSTRACT FROM AUTHOR]