Giorgia Sironi

Accurate integration of segmented x-ray optics using interfacing ribs

Abstract. Future lightweight and long-focal-length x-ray telescopes must guarantee a good angular resolution (e.g., 5 arc sec HEW) and reach an unprecedented large effective area. This goal can be reached with the slumping of borosilicate glass sheets that allow the fabrication of lightweight and low-cost x-ray optical units (XOU). These XOUs, based on mirror segments, have to be assembled together to form complete multishell Wolter-I optics. The technology for the fabrication and the integration of these XOUs is under development in Europe, funded by European Space Agency, and led by the Brera Observatory (INAF-OAB). While the achievement of the required surface accuracy on the glass segments by means of a hot slumping technique is a challenging aspect, adequate attention must be given to the correct integration and coalignment of the mirror segments into the XOUs. To this aim, an innovative assembly concept has been investigated, based on glass reinforcing ribs. The ribs connect pairs of consecutive foils, stacked into a XOU, with both structural and functional roles, providing robust monolithic stacks of mirror plates. Moreover, this integration concept allows the correction of residual low-frequency errors still present on the mirror foil profile after slumping. We present the integration concept, the related error budget, and the results achieved so far with a semi-robotic integration machine especially designed and realized to assemble slumped glass foils into XOUs.

Making the ATHENA optics using Silicon Pore Optics

Silicon Pore Optics, after 10 years of development, forms now the basis for future large (L) class astrophysics Xray observatories, such as the ATHENA mission to study the hot and energetic universe, matching the L2 science theme recently selected by ESA for launch in 2028. The scientific requirements result in an optical design that demands high angular resolution (5“) and large effective area (2 m2 at a few keV) of an X-ray lens with a focal length of 12 to14 m. Silicon Pore Optics was initially based on long (25 to 50 m) focal length telescope designs, which could achieve several arc second angular resolution by curving the silicon mirror in only one direction (conical approximation). With the advent of shorter focal length missions we started to develop mirrors having a secondary curvature, allowing the production of Wolter-I type optics, which are on axis aberration-free. In this paper we will present the new manufacturing process, discuss the impact of the ATHENA optics design on the technology development and present the results of the latest X-ray test campaigns.

Design and development of the optics system for the NHXM Hard X-ray and Polarimetric Mission

The New Hard X-ray Mission (NHXM) Italian project will be operated by 2016. It is based on 4 hard X-ray optics modules, each formed by 60 evenly spaced multilayer coated Wolter I mirror shells. For the achievement of a long focal length (10 m) an extensible bench is used. The pseudo-cylindrical Wolter I monolithic substrates where the multilayer coating is applied will be produced using the Ni electroforming replica approach. For three of the four mirror modules the focal plane will host a hybrid a detector system, consisting in the combination of a Si-based low energy detector (efficient from 0.5 up to ~ 15 keV) , on top of a high energy CdTe pixellated detector (efficient from 10 keV up to ~ 80 keV); the two cameras will be surrounded by both a passive shield and an anticoincidence shield. The total on axis effective area of the three telescopes at 1 keV and at 30 kev is of 1500 cm2 and 350 cm2 respectively. The angular resolution requirement is better than 20 arcsec HEW at 30 keV, while the Field of View at 50% vignetting is 12 arcmin (diameter). The payload is finally completed with the fourth telescope module, that will have as a focal plane detector a high sensitivity imaging photoelectric polarimetric system, operating from 2 up to 35 keV. In this paper, after an overview of the mission configuration and its scientific goals, we report on the design and development of the multilayer optics of the mission, based on thin replicated Ni mirror shells.

Ion figuring of large prototype mirror segments for the E-ELT

At INAF-Astronomical Observatory of Brera a study is under way to explore the problems related to the ion beam figuring of full scale Zerodur hexagonal mirrors of M1 for the European Extremely Large Telescope (E-ELT), having size of 1.4 m corner to corner. During this study it is initially foreseen the figuring of a scaled down version mirror of the same material having size of 1 m corner to corner to assess the relevant figuring problems and issues. This specific mirror has a radius of curvature of 3 m, which allows for easy interferometric measurement. A mechanical support was designed to minimize its deformations due to the gravity. The Ion Beam Figuring Facility used for this study has been recently completed in the Brera Observatory and has a figuring area of 140 cm x 170 cm. It employs a Kaufman ion source having 50 mm grids mounted on three axis. This system has been designed and developed to be autonomous and self-monitoring during the figuring process. The software and the mathematical tools used to compute the dwell time solution have been developed at INAF-OAB as well. Aim of this study is the estimation and optimization of the time requested to correct the surface adopting strategies to mitigate the well-known thermal problems related to the Zerodur material. In this paper, the results obtained figuring the 1 m corner-to-corner test segment are reported.

Aberration-free silicon pore x-ray optics

Silicon Pore Optics is an enabling technology for future L- and M-class astrophysics X-ray missions, which require high angular resolution (~5 arc seconds) and large effective area (1 to 2 m2 at a few keV). The technology exploits the high-quality of super-polished 300 mm silicon wafers and the associated industrial mass production processes, which are readily available in the semiconductor industry. The plan-parallel wafers have a surface roughness better than 0.1 nm rms and are diced, structured, wedged, coated, bent and stacked to form modular Silicon Pore Optics, which can be grouped into a larger optic. The modules are assembled from silicon alone, with all the mechanical advantages, and form an intrinsically stiff pore structure. The optics design was initially based on long (25 to 50 m) focal length X-ray telescopes, which could achieve several arc second angular resolution by curving the silicon mirror in only one direction (conical approximation). Recently shorter focal length missions (10 to 20 m) have been discussed, for which we started to develop Silicon Pore Optics having a secondary curvature in the mirror, allowing the production of Wolter-I type optics, which are on axis aberration-free. In this paper we will present the new manufacturing process, the results achieved and the lessons learned.

Qualification and Testing of a Large Hot Slumped Secondary Mirror for Schwarzschild–Couder Imaging Air Cherenkov Telescopes

Dual-mirror Schwarzschild–Couder (SC) telescopes are based on highly aspherical optics, and they represent a novel design in the world of very high energy astrophysics. This work addresses the realization and the qualification of the secondary mirror for an SC telescope, named ASTRI, developed in the context of the Cherenkov Telescope Array Observatory. The discussion surveys the overall development from the early design concept to the final acceptance optical tests.

Slumping technique for the manufacturing of a representative x-ray grazing incidence mirror module for future space missions

The Astronomical Observatory of Brera (INAF-OAB, Italy), with the financing support of the European Space Agency (ESA), has concluded a study regarding a glass shaping technology for the production of grazing incidence segmented x-ray optics. This technique uses a hot slumping phase, in which pressure is actively applied on thin glass foils being shaped, to form a cylindrical approximation of Wolter I x-ray segments, and a subsequent cold slumping phase, in which the final Wolter I profile is then freeze into the glass segments during their integration in elemental X-ray Optical Units. The final goal of this study was the manufacturing of a prototype containing a number of slumped pair plates (meaning parabola and hyperbola couples) having representative dimensions to be tested both in UV light and in x-rays at the Panter facility (Germany). In this paper, the INAF-OAB slumping technique, comprising a shaping step and an integration step is described, together with the results obtained on the manufactured prototype modules: the first prototype was aimed to test the ad-hoc designed and built semi-automatic Integration MAchine (IMA) and debug its control software. The most complete module comprises 40 slumped segments of Schott D263 glass type of dimension 200 mm x 200 mm and thickness of 0.4 mm, slumped on Zerodur K20 mould and stacked together through glued BK7 glass structural ribs to form the first entire x-ray optical module ever built totally composed by glass. A last prototype was aimed at demonstrate the use of Schott glass AF32 type instead of D263. In particular, a new hot slumping experimental set-up is described whose advantage is to permit a better contact between mould and glass during the shaping process. The integration procedure of the slumped segments into the elemental module is also reviewed.

The ASTRI camera for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) foresees, in its southern site (Chile), the implementation of up to 70 small-sized telescopes (SSTs), which will extend the energy coverage up to hundreds of TeV. It has been proposed that one of the first set of CTA SSTs will be represented by the ASTRI mini-array, which includes (at least) nine ASTRI telescopes. The endto-end prototype of such telescopes, named the ASTRI SST-2M, is installed in Italy and it is now completing the overall commissioning and entering the science verification phase. ASTRI telescopes are characterized by an optical system based on a dual-mirror Schwarzschild-Couder design and a camera at the focal plane composed of silicon photomultiplier sensors managed by a fast read-out electronics specifically designed. Based on a custom peak-detector mode, the ASTRI camera electronics is designed to perform Cherenkov signal detection, trigger generation, digital conversion of the signals and data transmission to the camera server. In this contribution we will describe the main features of the ASTRI camera, its performance and results obtained during the commissioning phase of the ASTRI SST-2M prototype in view of the ASTRI mini-array implementation.

The ASTRI SST-2M prototype for the Cherenkov Telescope Array: manufacturing of the structure and the mirrors

The Cherenkov Telescope Array (CTA) observatory will represent the next generation of Imaging Atmospheric Cherenkov Telescopes. Using a combination of large-, medium-, and small-scale telescopes (LST, MST, SST, respectively), it will explore the Very High Energy domain from a few tens of GeV up to few hundreds of TeV with unprecedented sensitivity, angular resolution and imaging quality. In this framework, the Italian ASTRI program, led by the Italian National Institute of Astrophysics (INAF), is currently developing a scientific and technological SST prototype named ASTRI SST-2M; a 4-meter class telescope, it will adopt an aplanatic, wide-field, double-reflection optical layout in a Schwarzschild-Couder configuration. In this contribution we give an overview of the technological solutions adopted for the ASTRI SST-2M prototype. In particular we focus on the manufacturing of the telescope structure and mirrors. We will also describe early results from tests.

Deflectometry for optics evaluation: free form segments of polynomial mirror

Deflectometry is a well-known method for astronomical mirror metrology. This paper describes the method we developed for the characterization of free-form concave mirrors. Our technique is based on the synergy between deflectometry and ray-tracing. The deflectometrical test is performed by illuminating the reflecting surface with a known light pattern in a Ronchi – like configuration and retrieving the slope errors by the observed rays deflection. The ray-tracing code allows us to measure the slopes and to evaluate the mirror optical performance. This technique has two main advantages: it is fast and it is applicable on-site, as an intermediate step in the manufacturing process, preventing that out-of-specification mirrors may proceed towards further production steps. Thus, we obtain a considerable time and cost reduction. As an example, we describe the results obtained measuring the primary mirror segments of the Cherenkov prototypal telescope manufactured by the Italian National Institute for Astrophysics in the context of the ASTRI Project. This specific case is challenging because the segmentation of the polynomial primary mirror lead to individual mirrors with deviations from the spherical optical design up to a few millimeters.