Using nicmos arrays to study galaxies

The 256×256 HgCdTe arrays developed for the NICMOS (Near-Infrared Camera and Multi-object Spectrometer) project have proven to be very capable devices for extragalactic imaging. This paper describes a sampling of extragalactic results from NICMOS arrays. A brief summary of the history of the development of these arrays, and their outstanding performance characteristics is also given.     

Formation Flying for a Fresnel Lens Observatory Mission

The employment of a large area Phase Fresnel Lens (PFL) in a gamma-ray telescope offers the potential to image astrophysical phenomena with micro-arcsecond (μ^′′) angular resolution [1]. In order to assess the feasibility of this concept, two detailed studies have been conducted of formation flying missions in which a Fresnel lens capable of focussing gamma-rays and the associated detector are carried on two spacecraft separated by up to 10⁶ km. These studies were performed at the NASA Goddard Space Flight Center Integrated Mission Design Center (IMDC) which developed spacecraft, orbital dynamics, and mission profiles. The results of the studies indicated that the missions are challenging but could be accomplished with technologies available currently or in the near term. The findings of the original studies have been updated taking account of recent advances in ion thruster propulsion technology.     

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.     

A Fully Depleted pn-Junction CCD for Infrared-, UV- and X- ray detection

This paper describes the performance of the Fully Depleted pn-junction CCD (pn-CCD) system, developed for ESA's XMM-satellite mission for soft x-ray imaging and spectroscopy in the single photon counting mode in the 100 eV to 10 keV photon range. The 58 mm x 60 mm large pn-CCD array, designed and fabricated at the Semiconductor Lab (Halbleiterlabor) of the Max-Planck-Institut, uses pn-junctions for registers and as backside structure. This concept naturally enables full depletion of the detector volume independent of the silicon wafer's resistivity and thickness, and as such make it an efficient detector for the x-ray region and the infrared. For high detection efficiency in the soft x-ray region and UV, an ultrathin pn-CCD backside deadlayer has been realized. Each pn-CCD-channel is equipped with its own on-chip JFET amplifier which, in combination with the CAMEX-amplifier and multiplexing chip, facilitates parallel readout and fast data rate: the cooled pn-CCD system can be read out at a data rate up to 3 MHz with an electronic noise floor of ENC < 5 e-.     

Very Large Format Back Illuminated CCDs

The University of Arizona Imaging Technology Laboratory has processed several types of very large format (>4K × 4K pixel) charge coupled devices for low light level scientific applications. These back illuminated devices were produced from frontside die fabricated by or for Fairchild Imaging Systems, Semiconductor Technology Associates, the Jet Propulsion Laboratory, and Kodak. A Philips 7K × 9K frontside device has also been processed using similar techniques. The backside sensors yield >90% quantum efficiency (QE). All devices show excellent charge transfer efficiency (CTE > 0.999997) at operating temperatures (typically –100 °C). Devices specifically designed for low signal applications have been demonstrated with less than 4 electrons read noise. This revised version was published online in July 2006 with corrections to the Cover Date.     

Prospects for far infrared arrays

We report initial performance measurements of a 1/8 scale version of a 32×32 pixel array under development for SIRTF. This array demonstrates that we can reach the sensitivity limits set by the natural backgrounds in space while providing good imaging and photometric performance. Based on the achieved performance levels, we project the imaging capabilities of SIRTF in the far infrared to exceed by a factor of more than 10,000 those achieved by any preceding telescope.     

Secrets of E2V Technologies CCDs (ex Marconi CCDs)

To complement previously published information on E2V CCDs, we present here some less well-known information about Marconi CCDs. We show details of the performance of deep depletion silicon variants, and discuss other subtleties of performance. We also present an update on L3vision (sub-electron noise) devices, indicating current availability, options, and ongoing development work. Finally, we will give a flavour of the range of custom designs that have been made. Many of these are not commercially available, and may not be familiar to all astronomers or CCD technologists. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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.     

Direct method of image and spectrum recovery in high-energy astronomy

In this paper we propose to use the method of iterative calculation to solve directly the imaging equation for image recovery. The same method can be applied to the recovery of one-dimensional spectra. During the iteration suitable constraints are applied to effect nonlinear control over the process. The use of fitting technique in the treatment of the imaging equation suppresses noise and raises sensitivity. Analyses of both Monte Carlo samples and actual high-energy astronomical data showed that the direct method is more sensitive and having greater resolving power than the traditional method, is capable of revealing simultaneously both diffuse and discrete structures in the target object and can be used on data of lower statistical quality. In principle, the direct method can be used in all types of image and spectrum recovery in other fields.     

Guidelines for Future UV Observatories

Ultra-violet image sensors and UV optics have been developed for a variety of space borne UV astronomy missions. Technology progress has to be made to improve the performance of future UV space missions. Throughput is the most important technology driver for the future. Required developments for different UV detector types – detectors are one of the most problematic and critical parts of a space born mission – and for optical components of the instruments are given in these guidelines. For near future missions we need high throughput optics with UV sensors of large formats, which show simultaneously high quantum efficiency and low noise performance.     

Using the photons from the Crab Nebula seen by GLAST to calibrate MAGIC and the imaging air Cherenkov telescopes

In this article we discuss the possibility of using the observations by GLAST of standard gamma sources, as the Crab Nebula, to calibrate imaging air Cherenkov detectors, MAGIC in particular, and optimise their energy resolution. We show that at around 100 GeV the absolute energy calibration uncertainty of Cherenkov telescopes can be reduced to 10% by means of such cross-calibration procedure.     

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.     

Replicated Nickel Optics for the Hard-X-Ray Region

Replicated nickel optics has been used extensively in x-ray astronomy, most notably for the XMM/Newton mission. The combination of relative ease of fabrication and the inherent stability of full-shell optics, make them an attractive approach for medium-resolution, high-throughput applications. MSFC has been developing these optics for use in the hard-x-ray region. Efforts at improving their angular resolution, particularly for the very-thin-wall shells required to meet the weight budgets of future missions, are described together with the prospects for future significant improvements.     

The Concordia station on the Antarctic plateau: The best site on earth for the 21st century astronomers

On the Antarctic plateau, a joint project of French and Italian polar programmes is nearing completion: the Concordia station will be open for winter-over operation in 2005. The high altitude and high latitude of this site, the exceptionally cold, clear and stable atmosphere, its incredible astronomical seeing, the almost indefinitely flat snow surface and the not-so-difficult access make this site the most promising on Earth for future ground-based astronomical projects in various fields, including long term photometry, infrared high sensitivity imaging and high angular resolution and high contrast imaging.     

The Fresnel space imager as a disk evolution watcher

The Fresnel Diffractive Imaging Arrays form high resolution images by diffraction with low radiometric efficiencies. They are extremely good devices to make high resolution imaging and integral field spectroscopy of bright sources. Thirty meter arrays will provide a spatial resolution of 0.8 mas at Lyman-?? that will open a completely new field of research: the study of matter distribution around disks and their gravitational drives. In this contribution, the potentials of the 3.6 m precursors (or probes) for astrophysical disks and jets research, are described. Main emphasis is made on young planetary disks.     

Using large radio telescopes at decametre wavelengths

With the aim of evaluating the actual possibilities of doing, from the ground, sensitive radio astronomy at decametre wavelengths (particularly below ), an extensive program of radio observations was carried out, in 1999–2002, by using digital spectral and waveform analysers (DSP) of new generation, connected to several of the largest, decametre radio telescopes in the world (i.e., the UTR-2 and URANs arrays in Ukraine, and the Nançay Decametre Array in France).

We report and briefly discuss some new findings, dealing with decametre radiation from Jupiter and the Solar Corona: namely the discovery of new kinds of hyper fine structures in spectrograms of the active Sun, and a new characterisation of Jupiter's “millisecond” radiation, whose waveform samples, with time resolution down to 40 ns, and correlated measurements, by using far distant antennas (3000 km), have been obtained. In addition, scattering effects, caused by the terrestrial ionosphere and the interplanetary medium, could be disentangled through high time resolution and wide-band analyses of solar, planetary and strong galactic radio sources. Consequences for decametre wavelength imaging at high spatial resolution (VLBI) are outlined. Furthermore, in spite of the very unfavourable electromagnetic environment in this frequency range, a substantial increase in the quality of the observations was shown to be provided by using new generation spectrometers, based on sophisticated digital techniques. Indeed, the available, high dynamic range of such devices greatly decreases the effects of artificial and natural radio interference. We give several examples of successful signal detection in the case of much weaker radio sources than Solar System ones, down to the intensity level.

In summary, we conclude that searching for sensitivity improvement at the decametre wavelength is scientifically quite justified, and is now technically feasible, in particular by building giant, phased antenna arrays of much larger collecting area (as in the LOFAR project). In this task, one must be careful of some specifics of this wavelength range—somewhat unusual in “classical” radio astronomy—i.e., very high level and density of radio interference (telecommunications) and the variable terrestrial ionosphere.     

High diffraction efficiency, broadband, diffraction crystals for use in crystal diffraction lenses

A major goal of the MAX program is to detect and measure gamma rays produced following the nuclear reactions that take place in a supernova explosion. To detect a reasonable number of supernovae, sensitivities of the order of a few times 10-7 γ cm-2sec-1 are needed – much better than possible with current instruments. The approach in the MAX program is to use a crystal diffraction lens to collect photons over a large area and concentrate them on a small well-shielded detector, greatly improving the signal to noise ratio. The crystals need to have both high diffraction efficiency and a relatively broad energy bandwidth. With mosaic crystals there is a trade-off between bandwidth and diffraction efficiency – one can have either high efficiency or large bandwidth, but not both without losing too much intensity through atomic absorption. A recent breakthrough in our understanding of crystal diffraction for high-energy gamma rays has made it possible to develop crystals that have both high diffraction efficiency and a relatively broad energy bandwidth. These crystals have near perfect crystal structure, but the crystalline planes are slightly curved. Such curved planes can be obtained in 3 different ways, by using mixed crystals with a composition gradient, by applying a thermal gradient, and by mechanically bending a near perfect crystal. A series of experiments have been performed on all three types of crystals using high-energy x-ray beams from the Advanced Photon Source at the Argonne National Laboratory. Experiments performed at 3 energies, 93 keV, 123 keV and 153 keV, with both the thermal gradient Si crystals and with the mechanically bent Si crystals, demonstrated that one can achieve diffraction efficiencies approaching 100% with moderate energy bandwidths (ΔE/E = 1.4%) and low atomic absorption (transmission = 0.65), in excellent agreement with theory. The use of this type of diffraction crystal is expected to increase the sensitivity of gamma ray telescopes by a factor of 5 over that possible with normal mosaic crystals.     

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.     

On post-SKA radio astronomy

It is suggested that the development of the SKA will drastically change the face of radio astronomy in the 21st Century. A FAST-style SKA would admit observations of low contrast features, and would be the best design for studying the `dark ages' of the Universe (x≫ 1) where sub-arcmin total power instruments can usefully be employed. To date there have been no proposals for post-SKA, billion square-metra instruments; we speculate that mobile communication systems can be used. In the very distant future, SKA multi-beam systems could be used to collect signals reflected by Solar system bodies such as the asteroid belt. This revised version was published online in July 2006 with corrections to the Cover Date.