Evaluation of deflectometry for E-ELT optics

A deflectometrical facility was developed at Italian National Institute for Astrophysics-OAB in the context of the ASTRI project to characterize free-form segments for Cherenkov optics. The test works as an inverse Ronchi test in combination with a ray-tracing code: the under-test surface is illuminated by a known light pattern and the pattern warped by local surface errors is observed. Knowing the geometry of the system it is possible to retrieve the surface normal vectors. This contribution presents the analysis of the upgrades and of the configuration modifications required to allow the use of deflectometry in the realization of optical components suitable for European Extremely Large Telescope and as a specific case to support the manufacturing of the Multi-conjugate Adaptive Optics Relay (MAORY) module. <P /

Adjustable integration molds for X-ray optics with cold shaping: requirements and conceptual design

The cold shaping of thin substrates is a worthwhile process for the realization of x-ray optics. The technique is based on the usage of integration molds to keep the substrate in the theoretical shape while it is fixed to a structure, which will limit at the desired level the residual spring back of the plate after the release of the constrain. Since some years, this process is in use at INAF/OAB to realize Slumped Glass Optics mirror modules by means of interfacing ribs. In principle, the optical design at a given focal length of each mirror shell is different for each radius and therefore several integration molds are necessary for an assembly of plates. Depending on the optical design of the mirror module to be realized and on the desired optical performances of the system, some simplifications can be introduced in order to reduce the number of integration molds to be realized. Nevertheless the most cost-efficient solution to the problem is to realize an adjustable integration mold pair that can be shaped to the different theoretical configurations needed for the plates. This is advantageous not only in terms of number of molds and parts to be realized but also for the reduction of integration time thanks to the simplification of the process procedure. In this paper we describe the conceptual design of the system, describing its optical design, analysing its requirements and we report on the achieved results. <P /

Metrological needs and reliable solutions for optics fabrication related to E-ELT

In order to contribute and support the R&D activities related to the E-ELT (European Extremely Large Telescope) program, the INAF-OAB acquired a dedicated robotic polisher machine, the IRP1200 by Zeeko. It will be used in synergy with the ion beam figuring technique, already available at OAB for the realization of prototypes and advanced optical components, as the optics of the Multi-conjugate Adaptive Optics Relay module (MAORY). In this paper, starting from the discussion of the metrological needs to be fulfilled during the different parts of the machining for the different kind of optics, we define the metrological scheme that will be followed. Moreover, we present the concept for a new measurement machine based of non-contact scanning probes, capable of nanometer uncertainty and compatible with the optics design to be realized. škip 0.5c

Ion beam figuring of thin glass plates: achievements and perspectives

Different hot slumping techniques have been developed in the last decade to shape thin glass plates in Wolter-I configuration for high angular resolution x-ray telescopes. The required high quality surface characteristic, both in terms of figure error and of micro-roughness, is challenging and the best results achieved so far are compatible with an HEW of few arcseconds. In order to push forward the technology enabling x-ray optics with final HEW below 1 arcsec, we investigate the ion beam figuring as a deterministic technology which can correct the low frequency components of the residual error directly on thin glasses. In this paper we present the tests performed so far, giving a first assessment on the deterministic process definition. In particular, we report on the results achieved on flat samples of D263 and Eagle glass, focusing on the removal function characterization, the micro-roughness evolution and the plate shape variation

A bonnet and fluid jet polishing facility for optics fabrication related to the E-ELT

A robotic polishing machine has been implemented at INAF-Brera Astronomical Observatory within the T-REX project. The facility, IRP1200 by Zeeko Ltd., consists of a 7-axis computer-controlled polishing/forming machine capable of producing precision surfaces on several optical materials. The machine enables two methods, the bonnet and the fluid jet polishing. We report on the results of the standard bonnet polishing machine acceptance tests that have been completed at our site. We intend to use the machine to support development and production programs related to the European Extremely Large Telescope (E-ELT), in particular, for making part of the optics of the Multi-conjugate Adaptive Optics RelaY (MAORY) module. <P /

Ion beam figuring technique used as final step in the manufacturing of the optics for the E-ELT

The INAF-Astronomical Observatory of Brera (INAF-OAB) is exploring the technical problems related to the ion beam figuring (IBF) of the Zerodur hexagonal mirrors (1.45 m corner to corner) of M1 for the European Extremely Large Telescope (E-ELT). As starting step a scaled down version mirror of the same material having size of 1 m corner to corner has been used to assess the relevant figuring problems. This specific mirror is spherical and has a radius of curvature of 3 m which allows a simple interferometric measurement setup. A mechanical support was designed to minimize its deformations due to gravity. The Ion Beam Figuring Facility used for this study has been recently completed in the Brera Observatory and has a figuring area of 170 cm x 140 cm. Aim of this study is the estimation and optimization of the time requested for the correction of the surface using also strategies to control the well-known thermal problems related to the Zerodur material. In this paper we report the results obtained figuring the 1 m corner-to-corner test segment

Cold shaping of thin glass foils: a fast and cost-effective solution for making light-weight astronomical x-ray optics

Recent advancements in thin glass materials allowed the development and the mass production of very thin glass foils, like e.g. the Willow glass (thickness of 0.1-0.2 mm) produced by Corning or AF32 produced by Schott (thickness down to 0.055 mm). The thickness, strength and flexibility of these glass foils allow bending them up to very small radius of curvature without breaks. This feature, together with the very low micro-roughness, makes this kind of materials ideal candidates for pursuing a cold replication approach for cost-effective and fast making of grazing incidence astronomical optics. Starting from the very thin flat glass sheets, the process under development foresees to bond them onto the supporting structure while they are wrapped around reference mandrels. The assembly concept, based on the use of Wolter-I counter-form moulds, is also based on the use of reinforcing ribs that connect pairs of consecutive foils in the final assembly. The ribs do not only play the role of mechanical connectors, they keep the shape and increase the structural stiffness. Indeed, the ribs constrain the foil profile to the correct shape during the bonding, damping the low-frequency residuals with respect to the Wolter I configuration. This approach is particularly interesting because of their low weight and cost. They could e.g be used for the production of high throughput optics as those needed for the Chines XTP mission, in which the requirements on the angular resolution are not too tight. In fact, a Half Energy Width in the range of 20-60 arcsec is compatible with the expected residual error due to the spring back of the glass sheets. In this paper we provide an overview of the project, the expected performances and present the first preliminary results

Slumped glass optics development with pressure assistance

Thin glass mirrors are a viable solution to build future X-ray telescopes with high angular resolution and large collecting area. This approach is very attractive for the optics implementation of future X-ray astronomy projects like the X-ray Surveyor Missions in USA, the XTP mission in China and the FORCE mission in Japan (all this projects could have an European participation). In the case of the X-ray Surveyor Mission, where a sub-arcsec angular resolution is requested, the use of actuators or post correction with sputtering deposition is envisaged. The hot slumping assisted by pressure is an innovative technology developed in our laboratories to replicate a mould figure. Our hot slumping process is based on thin substrates of Eagle XG glass to be thermally formed on Zerodur K20 moulds. This technology is coupled with an integration process able to damp low frequency errors. A continuous improvement in the reduction of the mid-frequency errors led to slumped glass foils with a potential angular resolution evaluated from the metrological data of a few arcsec. High frequency errors have been for a long time a critical point of our technology. In particular, the pressure assistance was leading to a partial replication of the mould micro-roughness, causing a non-negligible contribution to the Point Spread Function (PSF), in the incidence angle and X-ray energy range of operation. Therefore, we developed a new process to further reduce the micro-roughness of slumped glass foils, making now the technology attractive also for telescopes sensitive at higher X-ray energies. This paper provides the latest status of our research. <P /

Thin fused silica shells for high-resolution and large collecting area x-ray telescopes (like Lynx/XRS)

The implementation of an X-ray mission with high imaging capabilities, similar to those achieved with Chandra (2 effective area @1 keV), represents a compelling request by the scientific community. To this end the Lynx/XRS mission is being studied in USA, with the participation of international partners. In order to figure out the challenging technological task of the mirror fabrication, different approaches are considered, based on monolithic and segmented shells. Starting from the experience done on the glass prototypal shell realized in the past years, the direct polishing of thin (2 mm thick) fused silica monolithic shells is being investigated as a possible solution. A temporary stiffening structure is designed to support the shell during the figuring and polishing operations and to manage the handling up to its integration in the telescope structure. After the grinding and the polishing phases, in order to achieve the required surface accuracy, a final ion beam figuring correction is foreseen. In this paper, we present the technological process and the results achieved so far on a prototypal shell under development