SPHERE: the exoplanet imager for the Very Large Telescope

Observations of circumstellar environments to look for the direct signal of exoplanets and the scattered light from disks has significant instrumental implications. In the past 15 years, major developments in adaptive optics, coronagraphy, optical manufacturing, wavefront sensing and data processing, together with a consistent global system analysis have enabled a new generation of high-contrast imagers and spectrographs on large ground-based telescopes with much better performance. One of the most productive is the Spectro-Polarimetic High contrast imager for Exoplanets REsearch (SPHERE) designed and built for the ESO Very Large Telescope (VLT) in Chile. SPHERE includes an extreme adaptive optics system, a highly stable common path interface, several types of coronagraphs and three science instruments. Two of them, the Integral Field Spectrograph (IFS) and the Infra-Red Dual-band Imager and Spectrograph (IRDIS), are designed to efficiently cover the near-infrared (NIR) range in a single observation for efficient young planet search. The third one, ZIMPOL, is designed for visible (VIR) polarimetric observation to look for the reflected light of exoplanets and the light scattered by debris disks. This suite of three science instruments enables to study circumstellar environments at unprecedented angular resolution both in the visible and the near-infrared. In this work, we present the complete instrument and its on-sky performance after 4 years of operations at the VLT.Comment: Final version accepted for publication in A&

Modern optical astronomy: technology and impact of interferometry

The present `state of the art' and the path to future progress in high spatial resolution imaging interferometry is reviewed. The review begins with a treatment of the fundamentals of stellar optical interferometry, the origin, properties, optical effects of turbulence in the Earth's atmosphere, the passive methods that are applied on a single telescope to overcome atmospheric image degradation such as speckle interferometry, and various other techniques. These topics include differential speckle interferometry, speckle spectroscopy and polarimetry, phase diversity, wavefront shearing interferometry, phase-closure methods, dark speckle imaging, as well as the limitations imposed by the detectors on the performance of speckle imaging. A brief account is given of the technological innovation of adaptive-optics (AO) to compensate such atmospheric effects on the image in real time. A major advancement involves the transition from single-aperture to the dilute-aperture interferometry using multiple telescopes. Therefore, the review deals with recent developments involving ground-based, and space-based optical arrays. Emphasis is placed on the problems specific to delay-lines, beam recombination, polarization, dispersion, fringe-tracking, bootstrapping, coherencing and cophasing, and recovery of the visibility functions. The role of AO in enhancing visibilities is also discussed. The applications of interferometry, such as imaging, astrometry, and nulling are described. The mathematical intricacies of the various `post-detection' image-processing techniques are examined critically. The review concludes with a discussion of the astrophysical importance and the perspectives of interferometry.Comment: 65 pages LaTeX file including 23 figures. Reviews of Modern Physics, 2002, to appear in April issu

Measurement Uncertainties in Fibre-coupled Spectrographs

The signal quality of fibre-coupled spectrographs can be limited by the inherent properties of the optical fibre. This is especially the case for applications that require high signal-to-noise performance and high spectral resolution. Examples include metallicity and age of star clusters, as well as investigations of Lyman-alpha absorbers. Extra-solar planet research in particular encounters its limitations due to the non-repeatability of the fibre response. Initially, a limited signal quality due to fibres seems counter-intuitive, since one of the most remarkable advantages of fibres is their signal stabilizing property, called image scrambling, which refers to the effect that the fibre output signal is largely insensitive to variations at the input side. However, the fibre photometric and barycentre response is sub ject to external parameters like stress, seeing and guiding variations. State-of-the-art instrumentation has attained a level of sensitivity where these effects will impact upon instrument performance, especially when advancing to a regime of spectral resolving powers where the quantized character of the standard optical fibre can be resolved, which manifests itself in modal noise. Unprecedented effort will be required in order to accomplish high resolving powers in the spectral and spatial domains with 40 m class telescopes. It is therefore essential to predict these fibre-related measurement uncertainties so that the performance of current and future instruments can be optimized. This thesis starts out with a phenomenological description of the different effects that give rise to fibre-related noise and its influence on the observables relevant to astrophysics, such as barycentre and photometric stability. Special emphasis is given to the photometric uncertainties related to modal noise, where first a theoretical model is outlined which in later chapters will be sub ject to experimental investigations. Subsequently, the barycentre repeatability due to incomplete scrambling is the subject of detailed investigation. The remaining sources of noise are estimated using experimental data as well as simulations and put in contrast with the other effects. Alongside the quantitative prediction of instrument instabilities, mitigation strategies will be presented and discussed. I conclude with a brief discussion of the impact of incomplete scrambling and modal noise on current instrumentation, the implications for future instrument pro jects as well as future work that will help to further understand and obviate the underlying mechanisms

Image Restoration for Remote Sensing: Overview and Toolbox

Remote sensing provides valuable information about objects or areas from a distance in either active (e.g., RADAR and LiDAR) or passive (e.g., multispectral and hyperspectral) modes. The quality of data acquired by remotely sensed imaging sensors (both active and passive) is often degraded by a variety of noise types and artifacts. Image restoration, which is a vibrant field of research in the remote sensing community, is the task of recovering the true unknown image from the degraded observed image. Each imaging sensor induces unique noise types and artifacts into the observed image. This fact has led to the expansion of restoration techniques in different paths according to each sensor type. This review paper brings together the advances of image restoration techniques with particular focuses on synthetic aperture radar and hyperspectral images as the most active sub-fields of image restoration in the remote sensing community. We, therefore, provide a comprehensive, discipline-specific starting point for researchers at different levels (i.e., students, researchers, and senior researchers) willing to investigate the vibrant topic of data restoration by supplying sufficient detail and references. Additionally, this review paper accompanies a toolbox to provide a platform to encourage interested students and researchers in the field to further explore the restoration techniques and fast-forward the community. The toolboxes are provided in https://github.com/ImageRestorationToolbox.Comment: This paper is under review in GRS