The IBIS telescope is the high angular resolution gamma-ray imager on-board the INTEGRAL Observatory, suc- cessfully launched from Baikonur (Kazakhstan) the 17th of October 2002. This medium size ESA project, planned for a 2 year mission with possible extension to 5, is devoted to the observation of the gamma-ray sky in the energy range from 3 keV to 10 MeV (Winkler 2001). The IBIS imaging system is based on two independent solid state detector arrays optimised for low (15 1000 keV) and high (0:175 10:0 MeV) energies surrounded by an active VETO System. This high eciency shield is essential to minimise the background induced by high energy particles in the highly excentric out of van Allen belt orbit. A Tungsten Coded Aperture Mask, 16 mm thick and1 squared meter in dimension is the imaging device. The IBIS telescope will serve the scientific community at large providing a unique combination of unprecedented high energy wide field imaging capability coupled with broad band spectroscopy and high resolution timing over the energy range from X to gamma rays. To date the IBIS telescope is working nominally in orbit since more than 9 month.

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IBIS: The Imager on-board INTEGRAL

The ESA observatory INTEGRAL (International Gamma-Ray Astrophysics Laboratory) is dedicated to the fine spectroscopy (2.5 keV FWHM @ 1 MeV) and fine imaging (angular resolution: 12 arcmin FWHM) of celestial gamma-ray sources in the energy range 15 keV to 10 MeV with concurrent source monitoring in the X-ray ($3{-}35$ keV) and optical ( V -band, 550 nm) energy ranges. INTEGRAL carries two main gamma-ray instruments, the spectrometer SPI (Vedrenne et al. [CITE]) – optimized for the high-resolution gamma-ray line spectroscopy (20 keV–8 MeV), and the imager IBIS (Ubertini et al. [CITE]) – optimized for high-angular resolution imaging (15 keV–10 MeV). Two monitors, JEM-X (Lund et al. [CITE]) in the ($3{-}35$) keV X-ray band, and OMC (Mas-Hesse et al. [CITE]) in optical Johnson V -band complement the payload. The ground segment includes the Mission Operations Centre at ESOC, ESA and NASA ground stations, the Science Operations Centre at ESTEC and the Science Data Centre near Geneva. INTEGRAL was launched on 17 October 2002. The observing programme is well underway and sky exposure (until June 2003) reaches ~1800 ks in the Galactic plane. The prospects are excellent for the scientific community to observe the high energy sky using state-of-the-art gamma-ray imaging and spectroscopy. This paper presents a high-level overview of INTEGRAL.

The INTEGRAL Mission

We introduce a new family of binary arrays for use in coded aperture imaging which are predicted to have properties and sensitivity (SNR) equal to that of the uniformly redundant array (URA). The new arrays, called MURAs (modified URAs), have decoding coefficients all of which are unimodular, resulting in a reconstructed image with noise terms completely independent of image-source structure. Although the new arrays are derived from quadratic residues, they do not belong to the cyclic difference set or set of pseudonoise sequences and consequently are constructible in configurations forbidden to those designs, thus providing the user with a wider selection of aperture patterns to match his particular needs. With the addition of MURAs to the family of binary arrays, all prime numbers can now be used for making optimal coded apertures, increasing the number of available square patterns by more than a factor of 3.

New family of binary arrays for coded aperture imaging

Coded aperture imaging in high energy astronomy represents an important technical advance in instrumentation over the full energy range from X- to γ-rays and is playing a unique role in those spectral ranges where other techniques become ineffective or impracticable due to limitations connected to the physics of interactions of photons with matter. The theory underlying this method of indirect imaging is of strong relevance both in design optimization of new instruments and in the data analysis process. The coded aperture imaging method is herein reviewed with emphasis on topics of mainly practical interest along with a description of already developed and forthcoming implementations.

Coded aperture imaging in X- and gamma-ray astronomy

Recent work in machine learning shows that deep neural networks can be used to solve a wide variety of inverse problems arising in computational imaging. We explore the central prevailing themes of this emerging area and present a taxonomy that can be used to categorize different problems and reconstruction methods. Our taxonomy is organized along two central axes: (1) whether or not a forward model is known and to what extent it is used in training and testing, and (2) whether or not the learning is supervised or unsupervised, i.e., whether or not the training relies on access to matched ground truth image and measurement pairs. We also discuss the tradeoffs associated with these different reconstruction approaches, caveats and common failure modes, plus open problems and avenues for future work.

Deep Learning Techniques for Inverse Problems in Imaging

PICsIT (Pixellated Imaging CaeSium Iodide Telescope) is the high energy detector of the IBIS telescope on-board the INTEGRAL satellite. PICsIT operates in the gamma-ray energy range between 175 keV and 10 MeV, with a typical energy resolution of 10% at 1 MeV, and an angular resolution of 12 arcmin within a100 square degree field of view, with the possibility to locate intense point sources in the MeV region at the few arcmin level. PICsIT is based upon a modular array of 4096 independent CsI(Tl) pixels,0.70 cm 2 in cross-section and 3 cm thick. In this work, the PICsIT on-board data handling and science operative modes are described. This work presents the in-flight performances in terms of background count spectra, sensitivity limit, and imaging capabilities.

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IBIS/PICsIT in-flight performances

A position sensitive detector for a gamma-ray imaging telescope.

The technique of using position-sensitive scintillating bar detectors for gamma-ray telescopes operating in the range 0.3–20 MeV can take advantage of recent progress in the technology of photodiodes and their related electronics. By assembling together several bars it is possible to realize a 3D position-sensitive detector with good background rejection capability, and consequently to improve the telescope sensitivity. The reconstructed energy and spatial resolution of the interacting gamma photon as well as the operational energy threshold are dependent upon the signal to noise ratio of the overall photodiode-electronic chain and change along the bar. Optimization of spatial and energy resolutions with respect to scintillation light attenuation characteristics and length of the bar are discussed. Furthermore, in order to extend the operational range to low energies, (under 300 keV) different methods to lower the signal acceptance threshold are examined and compared with data from experimental tests.

Methods for optimizing the performances of position-sensitive CsI(Tl) scintillation bars with photodiode readout

Coded-mask telescopes are presently considered as one of the best solutions for producing images of the sky in hard-X-ray and γ-ray astronomy, because they can possess optimum signal-to-noise ratio (SNR), which is crucial in an energy range in which the expected fluxes are generally low compared with the background. For a given field of the sky, the values of the SNR, associated with each pixel of the reconstructed image, depend on both the mask design and the decoding technique employed. The expressions for the SNR of a variety of replicated masks are derived for the cases of the inversion and the correlation methods, in different conditions of background and source fluxes. For the inversion method, the SNR is found to be simply inversely proportional to the square root of the trace of the matrix associated with the mask, but generally in the case of the correlation method the SNR is limited by an additional systematic term due to the cross-talk between sources in the field of view. The effect of this term can be avoided by choosing a uniformly redundant array pattern.

A. Spizzichino

Papers

Coded aperture imaging in X- and gamma-ray astronomy

IBIS/PICsIT in-flight performances

A position sensitive detector for a gamma-ray imaging telescope.

Methods for optimizing the performances of position-sensitive CsI(Tl) scintillation bars with photodiode readout

Coded masks in X- and γ-ray astronomy: the problem of the signal-to-noise ratio evaluation