Giancarlo Parodi

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.

Cold-shaping of thin glass foils as a method for mirror processing: from basic concepts to mass production of mirrors

Abstract. We present a method for the production of segmented optics. It is a process developed at INAF-Osservatorio Astronomico di Brera (INAF-OAB) employing commercial of-the-shelf materials. It is based on the shaping of thin glass foils by means of forced bending that occurs at room temperature [cold-shaping (CS)]. The glass is then assembled into a sandwich structure for retaining the imposed shape. The principal mechanical features of the mirrors are their low weight, rigidity and environmental robustness. The cost and production time also are competitive. We sum up the results achieved during research and development performed in the past years. We have investigated the theoretical limits of the structural components by means of parametric finite elements analyses; we also discuss the effects caused by the most common structural loads. Finally, the process implementation, the more significant validation tests and the mass production at the industry are described.

X-ray optics developments at ESA

Future high energy astrophysics missions will require high performance novel X-ray optics to explore the Universe beyond the limits of the currently operating Chandra and Newton observatories. Innovative optics technologies are therefore being developed and matured by the European Space Agency (ESA) in collaboration with research institutions and industry, enabling leading-edge future science missions. Silicon Pore Optics (SPO) [1 to 21] and Slumped Glass Optics (SGO) [22 to 29] are lightweight high performance X-ray optics technologies being developed in Europe, driven by applications in observatory class high energy astrophysics missions, aiming at angular resolutions of 5” and providing effective areas of one or more square meters at a few keV. This paper reports on the development activities led by ESA, and the status of the SPO and SGO technologies, including progress on high performance multilayer reflective coatings [30 to 35]. In addition, the progress with the X-ray test facilities and associated beam-lines is discussed [36].

Development of grazing-incidence multilayer mirrors by direct Ni electroforming replication: a status report

The Ni electroforming replication process has been used successfully by Beppo-SAX, JET-X/SWIFT, and XMM-Newton, to produce their gold-coated X-ray mirrors. The important feature of the technique is that, also with thin substrates, it is possible to achieve a good angular resolution, which is important for obtaining high signal-to-noise ratios in deep observations and imaging extended sources, while the assembly and integration of the monolithic shells is a relatively easy task. Two approaches can be used for the up grade of this technique also to the case of mirrors with multilayer coating, to be used in future hard X-ray missions: i) the direct replication of the mirror shell, after the deposition of the multilayer film on the master (mandrel) surface followed by the electroforming of the Ni walls, ii) the application of the multilayer film to the internal surface of Ni mirror shells, previously realized by replication. In this paper the last results achieved in Italy in the context of an activity aiming at the development of the former of the two methods will be presented and discussed.

Development of soft and hard x-ray optics for astronomy: progress report II and considerations on material properties for large-diameter segmented optics of future missions

In this paper we will review the activities devoted to the development of soft (0.1-10 keV) and hard (10-100 keV) X-ray optics for future astronomical missions that were carried out at the Brera Astronomical Observatory (OAB, Italy) during the last year. Concerning the soft X-ray optics, we are studying the approach based on the use of ceramic carriers for making monolithic Wolter I mirror shells of large diameter by epoxy replication. The ceramic materials investigated in our study are SiC and Alumina (Al2O3), respectively produced by Chemical Vapor Deposition and plasma spray. We fabricated a number of mirror shell prototypes (

Replication by Ni electroforming approach to produce the Con-X/HXT hard x-ray mirrors

PHI equals 60 cm) using carriers based on both materials. X-ray imaging tests performed at the PANTER X-ray facility (Germany) with a full illumination of the optics demonstrated that the mirror shells based on SiC show much better performances than in the case of Alumina. These results can be explained in terms of the thermal-mechanical parameters of the two materials, being in the case of SiC much more performing than for Alumina. Concerning the development of hard X-ray multilayer optics, we are exploring the approach based on Ni electroforming replication. In the last period of activity we in particular concentrated our work on the surface superpolishing methods for the mandrel to be used in the replication process, to be much improved with respect the case Au coated single layer mirrors for soft X-rays. Concerning the specific aspect of the mandrel superpolishing, the results that we obtained can be considered very good and it is possible to claim that we achieved the goal prefixed at the beginning of the development program. The last part of the paper is dedicated to theoretical considerations on large-size and low-weight optics based on segmented mirrors like e.g., those under study for the petals of the XEUS project. In particular, the expected imaging performances by segmented optics produced using different kinds of materials will be compared.

Design of the IXO optics based on thin glass plates connected by reinforcing ribs

The NASAs Constellation X-Ray Mission consists of a Soft X-Ray Telescope (SXT) based on large collecting area optics plus a focusing Hard X-Ray Telescope (HXT) operating between 8 and 70 keV and possibly at even higher energy. The Con-X HXT will have a focal length of 10 m and graze angles are small (0.25 - 0.1 deg). The substrates will be coated with multilayers to enhance the reflectivity but single heavy element coatings are an alternative for the small diameter substrates of the set. Twelve copies of the HXT are distributed evenly among the four Con-X spacecrafts. With multiple telescopes it is appropriate to consider electroforming, the replication process used successfully by Beppo-SAX, JET-X/SWIFT, and XMM-Newton, to produce their substrates. The important feature of the technique is that for mirrors with aperture diameters less than 40 cm also with thin substrates it is possible to achieve good angular resolution, which is important for obtaining high signal-to-noise ratios in deep observations and imaging extended sources. We review the main results of our development study devoted to proving the feasibility of the process for the Con-X/HXT, with particular stress on demonstrating, not only by theoretical considerations but also presenting an important experimental proof, that we can satisfy the severe mass constraints of the mission still maintaining good imaging capabilities.

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

Effective area requirements for the large X-ray mirror of the International X-ray Observatory (IXO) are about 3 m2 at 1keV, 0.65 m2 at 6 keV and 150 cm2 at 30 keV. Because of its large dimension, the telescope cannot be realized as a monolithic structure but rather it requires the integration and assembly in the telescope optical bench of a number of basic module units, called X-ray Optical Unit (XOU). We are currently studying a method for the production of these basic units that is based on the slumping technology for the production of thin glass segmented mirrors. It foresees the implementation of a stacking integration concept based on the use of reinforcing ribs connecting the glass segments in order to create very stiff structures. This paper reports on the last design of the single optical module and describe the results of FEM analyses that show how it is possible to use an innovative approach to the integration of the slumped glass foils.

Thin fused silica optics for a few arcsec angular resolution and large collecting area x-ray telescope

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.

Slumped glass option for making the XEUS mirrors: preliminary design and ongoing developments

The implementation of a X-ray mission with high imaging capabilities, similar to those achieved with Chandra (< 1 arcsec Half Energy Width, HEW), but with a much larger throughput is a very attractive perspective, even if challenging. For such a mission the scientific opportunities, in particular for the study of the early Universe, would remain at the state of the art for the next decades. At the beginning of the new millennium the XEUS mission has been proposed, with an effective area of several m2 and an angular resolution better than 2 arcsec HEW. Unfortunately, after the initial study, this mission was not implemented, mainly due to the costs and the low level of technology readiness. Currently the most advanced proposal for such a kind of mission is the SMART-X project, led by CfA and involving several other US Institutes. This project is based on adjustable segments of thin foil mirrors with piezo-electric actuators, aiming to achieve an effective area < 2 m2 at 1 keV and an angular resolution better than 1 arcsec HEW. Another attractive technology to realize an X-ray telescope with similar characteristics is being developed at NASA/Goddard. In this case the mirrors are based on Si substrates that are super-polished and figured starting from a bulky Si ingot, from which they are properly cut. Here we propose an alternative method based on precise direct grinding, figuring and polishing of thin (a few mm) glass shells with innovative deterministic polishing methods. This is followed by a final correction via ion figuring to obtain the desired accuracy in order to achieve the 1 arc sec HEW requirement. For this purpose, a temporary stiffening structure is used to support the shell from the polishing operations up to its integration in the telescope supporting structure. We will present the technological process under development, the results achieved so far and some mission scenarios based on this kind of optics, aiming to achieve an effective area more than 10 times larger than Chandra and an angular resolution of 1 arcsec HEW on axis and of a few arcsec off-axis across a large field of view (1 deg in diameter).