Numerical simulations of pinhole and single-mode fibre spatial filters for optical interferometers

We use a numerical simulation to investigate the effectiveness of pinhole spatial filters for optical/IR interferometers and to compare them with single-mode optical fibre spatial filters and interferometers without spatial filters. We show that fringe visibility measurements in interferometers containing spatial filters are much less affected by changing seeing conditions than equivalent measurements without spatial filters. This reduces visibility calibration uncertainties, and hence can reduce the need for frequent observations of separate astronomical sources for calibration of visibility measurements. We also show that spatial filters can increase the signal-to-noise ratios (SNRs) of visibility measurements and that pinhole filters give SNRs within 17 per cent of the values obtained with single-mode fibres for aperture diameters up to 3 r ₀. Given the simplicity of the use of pinhole filters we suggest that it represents a competitive, if not optimal, technique for spatial filtering in many current and next generation interferometers.     

Improvements for group delay fringe tracking

Group delay fringe tracking using spectrally dispersed fringes is suitable for stabilizing the optical path difference in ground-based astronomical optical interferometers in low light level situations. We discuss the performance of group delay tracking algorithms when the effects of atmospheric dispersion, high-frequency atmospheric temporal phase variations, non-ideal path modulation, non-ideal spectral sampling, and the detection artifacts introduced by electron-multiplying CCDs are taken into account, and we present ways in which the tracking capability can be optimized in the presence of these effects.     

Interferometric visibility and closure phase of microlensing events with finite source size

Interferometers from the ground and space will be able to resolve the two images in a microlensing event. This will at least partially lift the inherent degeneracy between physical parameters in microlensing events. To increase the signal-to-noise ratio, intrinsically bright events with large magnifications will be preferentially selected as targets. These events may be influenced by finite source size effects both photometrically and astrometrically. Using observed finite source size events as examples, we show that the fringe visibility can be affected by ∼5–10 per cent, and the closure phase by a few degrees – readily detectable by ground and space interferometers. Such detections will offer unique information about the lens–source trajectory relative to the baseline of the interferometers. Combined with photometric finite source size effects, interferometry offers a way to measure the angular sizes of the source and the Einstein radius accurately. Limb-darkening changes the visibility by a small amount compared with a source with uniform surface brightness, marginally detectable with ground-based instruments. We discuss the implications of our results for the plans to make interferometric observations of future microlensing events.     

A generalized measurement equation and van Cittert-Zernike theorem for wide-field radio astronomical interferometry

We derive a generalized van Cittert-Zernike (vC-Z) theorem for radio astronomy that is valid for partially polarized sources over an arbitrarily wide field of view (FoV). The classical vC-Z theorem is the theoretical foundation of radio astronomical interferometry, and its application is the basis of interferometric imaging. Existing generalized vC-Z theorems in radio astronomy assume, however, either paraxiality (narrow FoV) or scalar (unpolarized) sources. Our theorem uses neither of these assumptions, which are seldom fulfiled in practice in radio astronomy, and treats the full electromagnetic field. To handle wide, partially polarized fields, we extend the two-dimensional (2D) electric field (Jones vector) formalism of the standard 'Measurement Equation' (ME) of radio astronomical interferometry to the full three-dimensional (3D) formalism developed in optical coherence theory. The resulting vC-Z theorem enables full-sky imaging in a single telescope pointing, and imaging based not only on standard dual-polarized interferometers (that measure 2D electric fields) but also electric tripoles and electromagnetic vector-sensor interferometers. We show that the standard 2D ME is easily obtained from our formalism in the case of dual-polarized antenna element interferometers. We also exploit an extended 2D ME to determine that dual-polarized interferometers can have polarimetric aberrations at the edges of a wide FoV. Our vC-Z theorem is particularly relevant to proposed, and recently developed, wide FoV interferometers such as Low Frequency Array (LOFAR) and Square Kilometer Array (SKA), for which direction-dependent effects will be important.     

Direct imaging with highly diluted apertures – II. Properties of the point spread function of a hypertelescope

In the future, optical stellar interferometers will provide true images thanks to larger number of telescopes and to advanced cophasing subsystems. These conditions are required to have sufficient resolution elements (resel) in the image and to provide direct images in the hypertelescope mode. It has already been shown that hypertelescopes provide snapshot images with a significant gain in sensitivity without inducing any loss of the useful field of view for direct imaging applications. This paper aims at studying the properties of the point spread functions of future large arrays using the hypertelescope mode. Numerical simulations have been performed and criteria have been defined to study the image properties. It is shown that the choice of the configuration of the array is a trade-off between the resolution, the halo level and the field of view. A regular pattern of the array of telescopes optimizes the image quality (low halo level and maximum encircled energy in the central peak), but decreases the useful field of view. Moreover, a non-redundant array is less sensitive to the space aliasing effect than a redundant array.     

Rapid mapping of the sky at 240 MHz using the antennas of the Giant Metrewave Radio Telescope

In this paper we describe a method for measuring the effective receiver temperature T _erc and its variation for the entire receiver chain of a radio telescope, and use it to make a radio-continuum map of the sky at 240 MHz using the Giant Metrewave Radio Telescope (GMRT). We also show that in the case of GMRT, T _erc varies mainly with elevation and ambient temperature. The calibration techniques evolved here are applicable to similar interferometers with a large number of antennas, several frequency bands and a number of receiver systems at room temperature (where conventional methods are time-consuming). This method ideally requires just one complete day of observations in a frequency band.     

Optimization of the direct imaging properties of an optical-fibred long baseline interferometer

Long baseline interferometry is now a mature technique in the optical domain. Current interferometers are, highly limited in, number of subapertures and concepts are being developed for future generations of very large optical arrays and especially with the goal of direct imaging. In this paper, we study the effects of introducing single-mode fibres in direct imaging optical interferometers. We show how the flexibility of optical fibres is well adapted to the pupil densification scheme. We study the effects of the truncation of the Gaussian beams in the imaging process, either in the Fizeau mode or in the densified pupil mode or in the densified image mode. Finally, in the pupil densification configuration, we identify an optimum of the diaphragm width. This optimum maximizes the on-axis irradiance and corresponds to a trade-off between the loss of transmission and the efficiency of the densification.     

HARP/ACSIS: a submillimetre spectral imaging system on the James Clerk Maxwell Telescope

This paper describes a new Heterodyne Array Receiver Program (HARP) and Auto-Correlation Spectral Imaging System (ACSIS) that have recently been installed and commissioned on the James Clerk Maxwell Telescope. The 16-element focal-plane array receiver, operating in the submillimetre from 325 to 375 GHz, offers high (three-dimensional) mapping speeds, along with significant improvements over single-detector counterparts in calibration and image quality. Receiver temperatures are ∼120 K across the whole band, and system temperatures of ∼300 K are reached routinely under good weather conditions. The system includes a single-sideband (SSB) filter so these are SSB values. Used in conjunction with ACSIS, the system can produce large-scale maps rapidly, in one or more frequency settings, at high spatial and spectral resolution. Fully sampled maps of     size can be observed in under 1 h.
The scientific need for array receivers arises from the requirement for programmes to study samples of objects of statistically significant size, in large-scale unbiased surveys of galactic and extra-galactic regions. Along with morphological information, the new spectral imaging system can be used to study the physical and chemical properties of regions of interest. Its three-dimensional imaging capabilities are critical for research into turbulence and dynamics. In addition, HARP/ACSIS will provide highly complementary science programmes to wide-field continuum studies and produce the essential preparatory work for submillimetre interferometers such as the Submillimeter Array (SMA) and Atacama Large Millimeter/Submillimeter Array (ALMA).     

Offline, multidetector intensity interferometers – II. Implications and applications

Intensity interferometry removes the stringent requirements on mechanical precision and atmospheric corrections that plague all amplitude interferometry techniques at the cost of severely limited sensitivity. A new idea we recently introduced, very high redundancy, alleviates this problem. It enables the relatively simple construction (∼1 cm mechanical precision) of a ground-based astronomical facility able to transform a two-dimensional field of point-like sources to a three-dimensional distribution of microarcsec resolved systems, each imaged in several optical bands. Each system will also have its high-resolution residual timing, high-quality (inside each band) spectra and light curve, emergent flux, effective temperature, polarization effects and perhaps some thermodynamic properties, all directly measured. All the above attributes can be measured in a single observation run of such a dedicated facility. We conclude that after three decades of abandonment, optical intensity interferometry deserves another review, also as a ground-based alternative to the science goals of space interferometers.     

Multiple mode gravitational wave detection with a spherical antenna

Apart from being omnidirectional, a solid elastic sphere is a natural multimode and multifrequency device for the detection of gravitational waves (GW). Motion sensing in a spherical GW detector thus requires a multiple set of transducers attached to it at suitable locations. If these are resonant then they exert a significant back action on the larger sphere and, as a consequence, the joint dynamics of the entire system must be properly understood before reliable conclusions can be drawn from data obtained using this system . In this paper, I present and develop an analytical approach to the study of such dynamics, which generalizes the currently existing ones and clarifies their actual range of validity. In addition, the new formalism shows that resonator layouts exist that are alternatives to the highly symmetric Truncated Icosahedron Gravitational Antenna (TIGA) , and that they have potentially interesting properties. I will describe in detail one resonance layout that has mode channels , only requires five resonators per quadrupole-mode sensed and is based on a Pentagonal HexaContrahedron (PHC) polyhedric shape. Also, the perturbative nature of the proposed approach makes it very well adapted to systematically assessing the consequences of realistic mistunings in the device parameters, by robust analytic methods. In order to check the real value of the mathematical model, its predictions have been compared with experimental data from the Louisiana State University (LSU) prototype detector TIGA and agreement between the predictions and data is consistently found to reach a satisfactory precision of four decimal places.     

Comparing modal noise and FRD of circular and non‐circular cross‐section fibres

Modal noise is a common source of noise introduced to the measurements by optical fibres and is particularly important for fibre‐fed spectroscopic instruments, especially for high‐resolution measurements. This noise source can limit the signal‐to‐noise ratio and jeopardize photon‐noise limited data. The subject of the present work is to compare measurements of modal noise and focal‐ratio degradation (FRD) for several commonly used fibres. We study the influence of a simple mechanical scrambling method (excenter) on both FRD and modal noise. Measurements are performed with circular and octagonal fibres from Polymicro Technology (FBP‐Series) with diameters of 100, 200, and 300μm and for square and rectangular fibres from CeramOptec, among others. FRD measurements for the same sample of fibres are performed as a function of wavelength. Furthermore, we replaced the circular fibre of the STELLA‐échelle‐spectrograph (SES) in Tenerife with an octagonal and found a SNR increase by a factor of 1.6 at 678 nm. It is shown in the laboratory that an excenter with a large amplitude and low frequency will not influence the FRD but will reduce modal noise rather effectively by up to 180%. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The coupling performance of photonic crystal fibres in fibre stellar interferometry

Large mode area (LMA), single-mode photonic crystal fibres (PCFs) have the potential to provide significant instrumental advantages in fibre stellar interferometry, due to their broad-band attenuation spectrum, endlessly single-moded performance and very large core size. We investigate the theoretical performance of coupling the telescope point spread function directly into LMA PCFs. We find that a single LMA fibre can replace as many as three step-index fibres for atmospheric seeing characterized by   D _T/ r _o≥ 2  with approximately the same coupling performance and a slower feed from the telescope. This could lead to considerable simplification of broad-band fibre interferometers.     

Extended source effects in astrometric gravitational microlensing

Extended source size effects have been detected in photometric monitoring of gravitational microlensing events. We study similar effects in the centroid motion of an extended source lensed by a point mass. We show that the centroid motion of a source with uniform surface brightness can be obtained analytically. For a source with a circularly symmetric limb-darkening profile, the centroid motion can be expressed as a one-dimensional integral, which can be evaluated numerically. We find that when the impact parameter is comparable to the source radius, the centroid motion is significantly modified by the finite source size. In particular, when the impact parameter is smaller than the source radius, the trajectories become clover-leaf like. Such astrometric motions can be detected using space interferometers such as the Space Interferometry Mission . Such measurements offer exciting possibilities for determining stellar parameters, such as stellar radius, to excellent accuracy.     

Study of the atmospheric refraction in a single-mode instrument – application to AMBER/VLTI

This paper presents a study of the atmospheric refraction and its effect on the light coupling efficiency in an instrument using single-mode optical fibres. We show the analytical approach which allowed us to assess the need to correct the refraction in J and H bands while observing with an 8-m Unit Telescope. We then developed numerical simulations to go further in calculations. The hypotheses on the instrumental characteristics are those of AMBER (Astronomical Multi BEam combineR), the near-infrared focal beam combiner of the Very Large Telescope Interferometric mode, but most of the conclusions can be generalized to other single-mode instruments. We used the software package caos to take into account the atmospheric turbulence effect after correction by the European Southern Observatory system Multi-Application Curvature Adaptive Optics. The optomechanical study and design of the system correcting the atmospheric refraction on AMBER is then detailed. We showed that the atmospheric refraction becomes predominant over the atmospheric turbulence for some zenith angles z and spectral conditions: for z larger than 30° in J band for example. The study of the optical system showed that it allows to achieve the required instrumental performance in terms of throughput in J and H bands. First observations in J band of a bright star, α Cir star, at more than 30° from zenith clearly showed the gain to control the atmospheric refraction in a single-mode instrument, and validated the operating law.     

Studying focal ratio degradation of optical fibres with a core size of 50 μm for astronomy

Along with the spectral attenuation properties, the focal ratio degradation (FRD) properties of optical fibres are the most important for instrumental applications in astronomy. We present a special study about the FRD of optical fibres with a core size of 50 μm to evaluate the effects of stress when mounting the fibre. Optical fibres like this were used to construct the Eucalyptus integral field unit. This fibre is very susceptible to the FRD effects, especially after the removal of the acrylate buffer. This operation is sometimes necessary to allow close packing of the fibres at the input to the spectrograph. Without the acrylate buffer, the protection of the cladding and core of the fibre may be easily damaged. In the near future, fibres of this size will be used to build the Southern Observatory for Astronomical Research (SOAR) integral field unit spectrograph (SIFS) and other instruments. It is important to understand the correct procedure which minimizes any increase in FRD during the construction of the instrument.     

Compressed sensing for next generation instruments

This document discusses the possibility of using compressed sensing techniques for measuring 2D spectro‐polarimetric information using only one etalon and a broad prefilter. Instead of using an etalon and an extremely narrow prefilter (with all the subsequent problems of alignment), the idea is to use multiplexing techniques to include in the observations all the secondary peaks of the etalon. The reconstruction of the signal is done under the assumption that it can be efficiently reproduced in an orthogonal basis set (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Prominence support in potential field configurations around rotating stars

For the majority of optical observing programmes, the sky brightness provides the fundamental limit to signal detection such that the scientific feasibility is largely dictated by the phase of the Moon. Since most observatories do not have the resources to build expensive high-resolution or infrared instruments, they are increasingly at a loss as to how to exploit bright time. We show that, with due consideration of the field and Moon position, it is possible to undertake 'dark-time' observing programmes under 'bright-time' conditions. Our recommendations are particularly appropriate to all-sky survey programmes.
In certain instances, there are gains in observing efficiency with the use of a polarizer, which can significantly reduce the moonlight (or twilight) sky-background flux relative to an extraterrestrial flux. These gains are possible in background-limited cases because the sky background can be highly polarized, caused by scattering, around 90° away from the Moon (or Sun). To take advantage of this, only minor modifications to existing instruments are needed.     

电荷守恒与CCD孔径测光

Lick天文台的CCD控制器是一类新型控制器,它能够在同一CCD系统内实现MPP(Multi—pinned phase)和通常工作模式间的转换．自2002年起中国至少有4架 CCD照相机使用这类控制器．此控制器的弱点是溢出发生在非线性前,而且远在饱和以前．虽然这对亮的面光源观测不利,并且它妨碍了使用亮星构建点扩散函数(psf)．但是由于电荷守恒原理,对亮的点源(恒星)的孔径测光依然可行．观测证明了此结论．     

一种偏振定标单元方位角误差的约束非线性最小化优化定标方法

偏振定标单元(Polarization Calibration Unit, PCU)对于定标由偏振系统和天文望远镜产生的仪器偏振至关重要, 然而偏振定标单元 中偏振元件光轴的方位角误差是限制定标精度的主要因素之一. 为解决该问题, 提出了一种基于约束非线性最 小化优化的方位角误差定标方法, 该方法具有定标精度高、定标速度快的优点. 首先将偏振定标单元中的线性 偏振片和四分之一波片的光轴方位角误差设置为两个待优化的自由变量, 然后利用产生和测量的Stokes参数 以及偏振定标获得的响应矩阵定义优化目标函数, 最终使用约束非线性最小化优化方法来确定 两个偏振元件的方位角误差. 分别从理论模拟和实际测量两个方面对优化方法进行了验证, 实验结果表明, 该 优化方法能够成功获得上述两个方位角误差, 精度分别优于2.79$''$和2.72$''$. 此外, 从理论上 计算分析了不同方位角误差对各Stokes分量的影响情况. 该优化方法有望应用到我国太阳望远镜中偏振定标 装置的误差定标及研制之中.     

Diffraction losses in ground-based optical interferometers

We present a numerical analysis of free-space propagation of the beams inside a long-baseline optical/infrared interferometer. Unlike the models of beam propagation used in most previous studies, our analysis incorporates the effects of atmospheric seeing on the wavefronts entering the interferometer. We derive results for the changes in throughput, coherence loss and fringe-detection signal-to-noise ratio arising from diffraction along the propagation path. Our results for conditions of moderate seeing show that although the flux throughput decreases with propagation distance for a given beam diameter, the fringe contrast increases at the same time. In this case it becomes possible for diffraction to increase the signal-to-noise ratio of the fringe measurements. Previous studies have only considered an arrangement where all the apertures in the beam-propagation system have the same diameter. If the light at the end of the propagation path is collected with a fixed size aperture, we find that in many cases the signal-to-noise ratio for fringe detection is maximized when the initial beam diameter is approximately 30 per cent smaller than the final collector diameter. We discuss the implications of our results in the context of future interferometer designs.