The Fresnel Imager: learning from ground-based generation I prototype

The ??Generation I Fresnel Imager Prototype?? is a ground-based prototype of a Fresnel Imager (Koechlin et al., Astron Astrophys 443:709?C720, 2005), reduced in size (optical elements have a dimension of a few cm) but which features all the elements of an operational Fresnel Imager, and integrated in a clean room. Its design has started in October 2004, the first images were obtained early 2006, the key optical element: a cophased Fresnel Zone Lens, was integrated during summer 2006, and since then it has been used to evaluate new types of target shapes, of Fresnel Array cuttings... . It has been decommissioned end of 2008, some constitutive elements becoming parts of the so-called ??Generation II Fresnel Imager Prototype??. We present the constitutive elements and the achieved results, and the lessons learned from this prototype.     

A space Fresnel Imager for ultra-violet astrophysics: example on accretion disks

The Fresnel Diffractive Imager concept is proposed for space borne astronomical imaging at Ultra-Violet wavelengths, using diffractive focalization. The high angular resolution and high dynamic range provided by this new concept makes it an ideal tool to resolve circumstellar structures such as disks or jets around bright sources, among them, pre-main sequence stars and young planetary disks. The study presented in this paper addresses the following configuration of Fresnel diffractive imager: a diffractive array 4 m large, with 696 Fresnel zones operating in the ultra-violet domain. The diffractive arrays are opaque foils punched with a large number of void subapertures with carefully designed shapes and positions. In the proposed space missions, these punched foils would be deployed in space. Depending on the size of the array and on the working spectral band, the focal length of such imagers will range from a few kilometers to a few tens of kilometers. Thus, such space mission requires a formation flying configuration for two satellites around the L2 Sun-Earth Lagragian point. In this article, we investigate numerically the potential of Fresnel arrays for imaging circumstellar dust environments. These simulations are based upon simple protostellar disk models, and on the computed optical characteristics of the instrument. The results show that protoplanetary disks at distances up to a few thousand parsecs can be successfully studied with a 4 m aperture Fresnel imager in the UV.     

Extragalactic science with the FRESNEL imager

The space Fresnel Interferometric Imager, originally proposed to the ESA Cosmic Vision plan, is an innovative concept providing enhanced capabilities in terms of spatial resolution and dynamical range. In this paper we describe some of the most promising applications of the Fresnel Interferometric Imager concept in extragalactic studies. There are two different topics where the FII could make major progress. The first one is the mapping of star-formation in galaxies, from the local universe to ??75?C85% look back time, based on the first version of FII which is optimized for the UV and optical domains. The second topic is a test case for a subsequent improved version of FII, with a larger collecting area and optimized for the near-IR, focusing on the physical properties of the first galaxies.     

Fresnel imager testbeds: setting up, evolution, and first images

The Fresnel Diffractive Array Imager (FDAI) is a new optical concept proposed for large telescopes in space. To evaluate its performance on real sky objects, we have built a new testbed of FDAI, especially designed for on-sky operation. It is an evolution of the laboratory setup previously used to validate the concept on artificial sources. In order to observe celestial objects, this new two-module testbed was installed in July 2009 at Observatoire de la Côte d??Azur (Nice, France). The two modules of the testbed (the Fresnel array module and the receiver module), were secured at both ends of the 19 m long tube of an historical refractor, used as an optical bench on an equatorial mount. In this article, we focus on the evolution steps from a laboratory experiment to the first observation prototype, and on the targets chosen for performance assessment. We show the first on-sky results of a FDAI, although they do not reflect the nominal performances of the final testbed. These nominal performances have been attained only with the latest and most sophisticated prototype, and are presented in a separate article in this special issue.     

Improvements on Fresnel arrays for high contrast imaging

The Fresnel Diffractive Array Imager (FDAI) is based on a new optical concept for space telescopes, developed at Institut de Recherche en Astrophysique et Planétologie (IRAP), Toulouse, France. For the visible and near-infrared it has already proven its performances in resolution and dynamic range. We propose it now for astrophysical applications in the ultraviolet with apertures from 6 to 30 meters, aimed at imaging in UV faint astrophysical sources close to bright ones, as well as other applications requiring high dynamic range. Of course the project needs first a probatory mission at small aperture to validate the concept in space. In collaboration with institutes in Spain and Russia, we will propose to board a small prototype of Fresnel imager on the International Space Station (ISS), with a program combining technical tests and astrophysical targets. The spectral domain should contain the Lyman-α line (λ =?121 nm). As part of its preparation, we improve the Fresnel array design for a better Point Spread Function in UV, presently on a small laboratory prototype working at 260 nm. Moreover, we plan to validate a new optical design and chromatic correction adapted to UV. In this article we present the results of numerical propagations showing the improvement in dynamic range obtained by combining and adapting three methods : central obturation, optimization of the bars mesh holding the Fresnel rings, and orthogonal apodization. We briefly present the proposed astrophysical program of a probatory mission with such UV optics.     

Preparing the way to space borne Fresnel imagers

The Fresnel Diffractive Array Imager (FDAI) relies on diffraction focusing to potentially ouput very high wavefront quality particularly in the Ultraviolet. After Chesnokov (Russ Space Bull 1(2), 1993) or Barton (Appl Opt 40(4):447?C451, 2001), we intend to develop tangible optical designs for space missions at the horizon 2025. This paper refers to the phase 0 study completed at CNES. We canvass here different optical scenarios adapted to space formation flying, discussing the technologies involved, their level of maturity and criticity. Large spectral domains were investigated from Lyman-?? to Infra-Red, with competitive aperture size and ambitious objectives. We conclude by a 4-m class UV space mission scenario that could be the first launched imager of this kind.     

The Fresnel interferometric imager

The Fresnel interferometric imager is a new kind of high angular resolution space instrument for the UV domain, and the related astrophysical targets. This optical concept is meant to allow larger and lighter apertures in space than solid state optics. It yields high dynamic range images and same resolution as that of a solid aperture of the same size. The long focal lengths of the Fresnel imager (a few kilometers) require operation by two-vessel formation flying in space. The first vessel holds a large and thin opaque foil punched with thousands of holes: the interferometric array, the second vessel holds the focal instrumentation. This Fresnel imager has been designed for mapping high contrast stellar environments: dust disks, close companions and (we hope) exoplanets. Compact objects such as large stellar photospheres may be imaged with array sizes of a few meters in the UV. Larger and more complex fields can also be imaged, although with a lesser dynamic range, such as small fields on galactic clouds or extragalactic fields, or in an other domain: small solar system bodies. We present the first images obtained on artificial sources with an 8 cm laboratory testbed array having 26680 apertures, the measured dynamic range of these images and their diffraction limited angular resolution. A 3 m class probatory space mission will be studied and follow a validation path, It has been submitted as a proposal to the ESA Cosmic Vision program.     

Instrumental Response Function for Filtergraph Instruments

Dopplergrams and magnetograms arising from filtergraph instruments such as the Michelson Doppler Imager (MDI), the Helioseismic and Magnetic Imager (HMI), or the Hinode Narrow Band Filter Imager are generally associated with observation heights that are derived from the contribution function of the targeted absorption line, irrespective of the instrument characteristics. Observation heights are important for interpreting the phases of propagating waves, and for the diagnostics of the solar atmosphere. I show in this paper that the formalism presented by Ruiz Cobo and del Toro Iniesta (Astron. Astrophys. 283, 129, 1994) provides a straightforward approach to associate an observation height for each observable given the instrumental algorithm, the transmission profiles and the local stratification at the point of observation. To demonstrate the method, I construct a simple radially symmetric sunspot model and calculate the mean observation height for various MDI observables as a function of horizontal location. It is shown that different ways of measuring the same quantity can result in different observation heights, that the offset velocity caused by the spacecraft motion has to be taken into account, and that observation heights in sunspots vary beyond the pure geometric effect of the Wilson depression.     

The Distribution of Craters Within Craters

Photometric correction is a necessary step in planetary image pre-processing since the images of planetary surfaces are acquired by orbiting spacecraft at various observational geometries. In this study, visible (748 nm) and near-infrared (948 nm) bands of Hyper Spectral Imager (HySI) onboard Chandrayaan-1 have been used to derive a preliminary photometric correction for lunar data. The purpose of the proposed photometric correction for HySI is to convert observations taken at solar incidence (i), sensor emission (e), and the solar phase angles (α) to a fixed geometry by applying i?=?α?=?30° and e?=?0° to each image. The Lommel–Seeliger function was used to model the lunar limb darkening effect, while topography data from the merged Digital Elevation Model of Lunar Reconnaissance Orbiter—Lunar Orbiter Laser Altimeter (LRO-LOLA) and SELENE Terrain Camera (TC) was used to correct local topographic effects. Data from Moon Mineralogy Mapper (M³), SELENE Multiband Imager (MI) and Clementine Ultraviolet and Visible Camera (UV/VIS) were also used to compare radiance, reflectance and phase functions derived from HySI. Our analysis reveals that HySI is darker than M³ primarily due to low surface radiance conditions observed by HySI. The derived phase functions for the two HySI bands indicate a good correlation between the derived reflectance and phase angle as well as with the phase functions derived for the empirically corrected M³ data. This approach led to the derivation of a photometric correction for maria regions. Finally, it is expected that the proposed correction would be applicable to all HySI images covering the lunar mare region.

     

Temporal variation of large scale flows in the solar interior

We attempt to detect short-term temporal variations in the rotation rate and other large scale velocity fields in the outer part of the solar convection zone using the ring diagram technique applied to Michelson Doppler Imager (MDI) data. The measured velocity field shows variations by about 10 m/s on the scale of few days.     

Gamma Ray Fresnel Lenses – Why Not?

Fresnel lenses offer the possibility of concentrating the flux of X-rays or gamma-rays flux falling on a geometric area of many square metres onto a focal point which need only be a millimetre or so in diameter (and which may even be very much smaller). They can do so with an efficiency that can approach 100%, and yet they are easily fabricated and have no special alignment requirements. Fresnel lenses can offer diffraction-limited angular resolution, even in a domain where that limit corresponds to less than a micro second of arc.Given all these highly desirable attributes, it is natural to ask why Fresnel gamma ray lenses are not already being used, or at least why there is not yet any mission that plans to use the technology. Possible reasons (apart from the obvious one that nobody thought of doing so) include the narrow bandwidth of simple Fresnel lenses, their very long focal length, and the problems of target finding. It is argued that none of these is a ‘show stopper’ and that this technique should be seriously considered for nuclear astrophysics.     

First high dynamic range and high resolution images of the sky obtained with a diffractive Fresnel array telescope

This paper presents high contrast images of sky sources, obtained from the ground with a novel optical concept: Fresnel arrays. We demonstrate the efficiency of a small 20?cm prototype Fresnel array for making images with high brightness ratios, achieving contrasts up to 4 × 10⁵ on sky sources such as Mars and its satellites, and the Sirius?A?CB couple. These validation results are promising for future applications in space, for example the 4 m array we have proposed to ESA in the frame of the ??Call for a Medium-size mission opportunity for a launch in 2022??. Fresnel imagers are the subject of a topical issue of Experimental Astronomy published in 2011, but only preliminary results were presented at the time. Making images of astronomical bodies requires an optical component to focus light. This component is usually a mirror or a lens, the quality of which is critical for sharp and high contrast images. However, reflection on a mirror and refraction through a lens are not the only ways to focus light: an alternative is provided by diffraction through binary masks (opaque foils with multiple precisely etched sub-apertures). Our Fresnel arrays are such diffractive focusers, they offer weight, price and size advantages over traditional optics in space-based astronomical instruments. This novel approach requires only void apertures of special shapes in an opaque material to form sharp images, thus avoiding the wavefront distortion, diffusion and spectral absorption associated with traditional optical media. In our setup, lenses and/or mirrors are involved only downstream (at small sizes) for focal instrumentation and chromatic correction. Fresnel arrays produce high contrast images, the resolution of which reaches the theoretical limit of diffraction. Unlike mirrors, they do not require high precision polishing or positioning, and can be used in a large domain of wavelengths from far IR to far UV, enabling the study of many science cases in astrophysics from exoplanet surfaces and atmospheres to galaxy evolution.     

The fresnel interferometric imager

The Fresnel Interferometric Imager has been proposed to the European Space Agency (ESA) Cosmic Vision plan as a class L mission. This mission addresses several themes of the CV Plan: Exoplanet study, Matter in extreme conditions, and The Universe taking shape. This paper is an abridged version of the original ESA proposal. We have removed most of the technical and financial issues, to concentrate on the instrumental design and astrophysical missions. The instrument proposed is an ultra-lightweight telescope, featuring a novel optical concept based on diffraction focussing. It yields high dynamic range images, while releasing constraints on positioning and manufacturing of the main optical elements. This concept should open the way to very large apertures in space. In this two spacecraft formation-flying instrument, one spacecraft holds the focussing element: the Fresnel interferometric array; the other spacecraft holds the field optics, focal instrumentation, and detectors. The Fresnel array proposed here is a 3.6 ×3.6 m square opaque foil punched with 10⁵ to 10⁶ void “subapertures”. Focusing is achieved with no other optical element: the shape and positioning of the subapertures (holes in the foil) is responsible for beam combining by diffraction, and 5% to 10% of the total incident light ends up into a sharp focus. The consequence of this high number of subapertures is high dynamic range images. In addition, as it uses only a combination of vacuum and opaque material, this focussing method is potentially efficient over a very broad wavelength domain. The focal length of such diffractive focussing devices is wavelength dependent. However, this can be corrected. We have tested optically the efficiency of the chromatism correction on artificial sources (500 < λ < 750 nm): the images are diffraction limited, and the dynamic range measured on an artificial double source reaches 6.2 10^− 6. We have also validated numerical simulation algorithms for larger Fresnel interferometric arrays. These simulations yield a dynamic range (rejection factor) close to 10^− 8 for arrays such as the 3.6 m one we propose. A dynamic range of 10^− 8 allows detection of objects at contrasts as high as than 10^− 9 in most of the field. The astrophysical applications cover many objects in the IR, visible an UV domains. Examples are presented, taking advantage of the high angular resolution and dynamic range capabilities of this concept.     

On the optical studies of the atmospheric water vapour from the surface of Mars

Remote observations of the atmospheric water vapour from the Mars orbit were usually carried out to study its global distribution and variability. Measurements of the water vapour abundance onboard the landers have recently become an important complement to the orbital sounding. Narrow-band filter photometry and spectroscopy of the solar radiation from the surface of the planet proved to be a powerful tool in the study of atmospheric water. The Imager for Mars Pathfinder (IMP) was the first instrument to measure its amount from the surface. The Surface Stereo Imager (SSI) onboard the Mars Polar Lander (MPL) was to follow but the spacecraft was lost at landing. Nevertheless significant expertise in the optical measurements of atmospheric H₂O was gained during these missions. This paper summarizes this experience emphasizing the radiative transfer aspects of the problem. The results of this study could be of importance for future missions to Mars.     

Fresnel zone plate telescopes for X-ray imaging I: experiments with a quasi-parallel beam

Combination of Fresnel Zone Plates (FZP) can make an excellent telescope for imaging in X-rays. We present here the results of our experiments with several pairs of tungsten made Fresnel Zone plates in presence of an X-ray source kept at a distance of about 45 ft. The quasi-parallel beam allowed us to study sources placed on the axis as well as off the axis of the telescope. We present theoretical study of the fringe patterns produced by the zone plates in presence of a quasi-parallel source. We compare the patterns obtained from experiments with those obtained by our Monte-Carlo simulations. The images are also reconstructed by deconvolution from both the patterns. We compare the performance of such a telescope with other X-ray imaging devices used in space-astronomy.     

基于SIFT特征检测和密度峰值聚类的太阳活动区自动检测算法研究

太阳活动区是太阳大气中产生各种活动现象的区域,精确地检测和识别太阳活动区对理解太阳磁场的形成机制具有极为重要的科学意义.根据太阳活动区结构较为复杂的特点,基于尺度不变特征变换(ScaleInvariant Feature Transform, SIFT)和密度峰值聚类(Clustering by Fast Search and Find of Density Peaks,DPC)算法的优越性,提出了一种太阳活动区的自动检测和识别方法.首先,对太阳动力学天文台(Solar Dynamics Observatory, SDO)日震和磁场成像仪(Helioseismic and Magnetic Imager, HMI)的纵向磁图进行对比度增强;然后采用SIFT方法提取出全日面磁图中的特征点;最后利用DPC算法将特征点进行聚类,从而自动检测和识别出太阳活动区.研究结果表明, SIFT和DPC算法相结合的方法可以在不需要人工交互的情况下准确地自动检测出太阳活动区.     

Two Years of the STEREO Heliospheric Imagers

Imaging of the heliosphere is a burgeoning area of research. As a result, it is awash with new results, using novel applications, and is demonstrating great potential for future research in a wide range of topical areas. The STEREO (Solar TErrestrial RElations Observatory) Heliospheric Imager (HI) instruments are at the heart of this new development, building on the pioneering observations of the SMEI (Solar Mass Ejection Imager) instrument aboard the Coriolis spacecraft. Other earlier heliospheric imaging systems have included ground-based interplanetary scintillation (IPS) facilities and the photometers on the Helios spacecraft. With the HI instruments, we now have routine wide-angle imaging of the inner heliosphere, from vantage points outside the Sun-Earth line. HI has been used to investigate the development of coronal mass ejections (CMEs) as they pass through the heliosphere to 1 AU and beyond. Synoptic mapping has also allowed us to see graphic illustrations of the nature of mass outflow as a function of distance from the Sun – in particular, stressing the complexity of the near-Sun solar wind. The instruments have also been used to image co-rotating interaction regions (CIRs), to study the interaction of comets with the solar wind and CMEs, and to witness the impact of CMEs and CIRs on planets. The very nature of this area of research – which brings together aspects of solar physics, space-environment physics, and solar-terrestrial physics – means that the research papers are spread among a wide range of journals from different disciplines. Thus, in this special issue, it is timely and appropriate to provide a review of the results of the first two years of the HI investigations.     

New progress on the Fresnel imager for UV space astronomy

We propose a next generation space instrument: the Fresnel imager, a large aperture and lightweight focusing device for UV astrophysics. This paper presents the laboratory setup used to validate the Fresnel imager at UV at wavelengths around 260 nm, and the results obtained. The validation of this optical concept in the visible domain has been previously published, with the first results on sky objects. In this paper we present new optical tests in the UV, of diffractive focusing and chromatic correction at wavelengths around 260 nm. The results show images free from chromatic aberration, thanks to a chromatic corrector scheme similar to the one used in the visible. To complete these tests and reach real astrophysical UV sources, we propose a short space mission featuring a Fresnel imager prototype placed on the international space station: during the mission this small aperture instrument would be aimed at UV sources such as bright stars and solar system objects, to assess at relatively low cost the limits in contrast and resolution of diffractive focusing in space conditions, on real UV astrophysical objects. At wavelengths from 100 to 300 nm, covering Lyman-α, we expect some scientific return from this mission, but the main goal is to increase the TRL, improving the chances of success for a later proposal featuring a full fledged Fresnel imager 10 meters in aperture or more, that would explore new domains of UV astrophysics at very high angular resolution and very high contrast.     

A space Fresnel imager concept assessment study led by CNES for astrophysical applications

In 2009, the Centre National d??Etudes Spatiales (CNES) carried out an assessment study on a ??Fresnel telescope?? concept based on a two-spacecraftformation flying configuration. This concept uses a binary Fresnel zone plate, and the principle of diffraction focusing, which allows high resolution optical imaging for astrophysics. In addition to CNES, the Laboratoire d??Astrophysique de Toulouse Tarbes (LATT) was deeply involved at two levels: through Research & Technology (R&T) studies to simulate and validate on a test bench the Fresnel concept performance, and through active participation in the CNES team for the optical aspects and to define the astrophysical fields of Fresnel-based space missions. The study was conducted within the technical limitations that resulted from a compromise between the R&T state of the art and the potential scientific domains of interest. The main technical limitations are linked to the size of the primary Fresnel array and to its usable spectral bandwidth. In this framework, the study covers ambitious architectures, correlating the technology readiness of the main critical components with the time-scale and programmatic horizons. The possible scientific topics arise from this range of missions. In this paper, I present a mission launched by a Soyuz, dedicated to astrophysics in the Ultra Violet (UV) band: 120 to 300 nm using a 4-m Fresnel array. It could be competitive in the next fifteen years, whereas a 10-m aperture mission in different bands; UV, visible or Infra Red (IR) (up to 6 ??m) could be achievable in the future. Larger missions, using a primary array larger than 20 m, request technologies not yet available but that will probably be based on new inflatable structures with membranes, as already tested in the USA for other ends.     

RHESSI as a Hard X-Ray Polarimeter

Although designed primarily as a hard X-ray imager and spectrometer, the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) is also capable of measuring the polarization of hard X-rays (20–100 keV) from solar flares. This capability arises from the inclusion of a small unobstructed Be scattering element that is strategically located within the cryostat that houses the array of nine germanium detectors. The Ge detectors are segmented, with both a front and rear active volume. Low-energy photons (below about 100 keV) can reach a rear segment of a Ge detector only indirectly, by scattering. Low-energy photons from the Sun have a direct path to the Be and have a high probability of Compton scattering into a rear segment of a Ge detector. The azimuthal distribution of these scattered photons carries with it a signature of the linear polarization of the incident flux. Sensitivity estimates, based on Monte Carlo simulations and in-flight background measurements, indicate that a 20–100 keV polarization sensitivity of less than a few percent can be achieved for X-class flares.