Future investigations of astronomical X-ray sources require light weight telescope systems with large collecting areas and good angular resolution. The Wolter I type telescope design offers a suitable possibility for obtaining performant X-ray mirrors with high collecting areas. The technology based on replicated slumped glass optics using thin glasses thereby provides the opportunity to fulfil the light weight and mass production requirements. In NASA's telescope NuSTAR this technology has been proven as advantageous compared to previous systems. Coating thin glasses with iridium, gold or platinum enhances the reflectivity of X-ray mirrors.
Future space-based X-ray observatories need to be very lightweight for launcher mass constraints. Therefore they will use a reduced mirror thickness, which results in the additional requirement of low coating stress to avoid deformation of the initial precisely shaped mirror substrates. Due to their excellent reflection properties iridium coatings are sometimes applied for grazing incidence mirrors in astronomical X-ray telescopes. At Aschaffenburg University of Applied Sciences the coating of thin iridium films by an RF-magnetron sputtering technique is under development. The work is embedded in collaborations with the Max-Planck-Institute for Extraterrestrial Physics in Germany, the Czech Technical University in Prague, the Osservatorio Astronomico di Brera in Italy, the German Leibniz Institute for Solid State and Materials Research in Dresden, and the French Institute Fresnel. Sputtering with different parameters leads to iridium films with different properties. The current work is focused on the microstructure of the iridium coatings to study the influence of the substrate and of the argon gas pressure on the thin film growing process. Correlations between coating density, surface micro-roughness, the crystalline structure of the iridium layers, and the expected reflectivity of the X-ray mirror as well as coating stress effects are presented and discussed. The final goal of the project is to integrate the produced prototype mirrors into an X-ray telescope module. On a longer timescale measurements of the mirror modules optical performance are planned at the X-ray test facility PANTER.
The paper provides a description of recent progress in the development of lightweight, precision and highthroughput grazing-incidence mirrors for X-ray astronomy made of glass. In particular, the indirect slumping technology under investigation at the Max Planck Institute for Extraterrestrial Physics (MPE) is reviewed and recent activities are presented together with the research approach. The glass slumping technique foresees several steps: a thermal forming process using a suitable mould; a reflective layer application; the alignment and integration of mirror segments into a supporting structure; and the final verification of prototype modules using X-rays. Each step is considered at MPE, with the involvement of partner institutes and universities. The last year of activities was mainly dedicated to the procurement of new moulds and to the application of Iridium coating. The main results will be presented.
Sun´s ultraviolet radiation is classified into UV-A, UV-B, and UV-C bands. Thereby UV-A passes through Earth´s atmosphere, while UV-B is partially absorbed by ozone. The limitations of the commonly accepted statement, that UV-C is always completely absorbed by Earth´s atmosphere, are discussed critically. Below 200 nm the solar spectrum is strongly absorbed by molecular oxygen. The stratospheric ozone layer has strong absorption between 200 nm and 300 nm. However, the “ozone hole” increases UV-B radiation just below 300 nm and may also open a transmitting atmospheric window for harmful UV-C at the overlap region between oxygen absorption and ozone absorption.
X-ray astronomy uses space-based telescopes to overcome the disturbing absorption of the Earth´s atmosphere. The telescope mirrors are operating at grazing incidence angles and are coated with thin metal films of high-Z materials to get sufficient reflectivity for the high-energy radiation to be observed. In addition the optical payload needs to be light-weighted for launcher mass constrains. Within the project JEUMICO, an acronym for “Joint European Mirror Competence”, the Aschaffenburg University of Applied Sciences and the Czech Technical University in Prague started a collaboration to develop mirrors for X-ray telescopes. The X-ray telescopes currently developed within this Bavarian- Czech project are of Lobster eye type optical design. Corresponding mirror segments use substrates of flat silicon wafers which are coated with thin iridium films, as this material is promising high reflectivity in the X-ray range of interest. The deposition of the iridium films is based on a magnetron sputtering process. Sputtering with different parameters, especially by variation of the argon gas pressure, leads to iridium films with different properties. In addition to investigations of the uncoated mirror substrates the achieved surface roughness has been studied. Occasional delamination of the iridium films due to high stress levels is prevented by chromium sublayers. Thereby the sputtering parameters are optimized in the context of the expected reflectivity of the coated X-ray mirrors. In near future measurements of the assembled mirror modules optical performances are planned at an X-ray test facility.
In the field of astronomical X-ray telescopes, different types of optics based on grazing incidence mirrors can be used. This contribution describes the special design of a lobster-eye optics in Schmidt's arrangement, which uses dual reflection to increase the collecting area. The individual mirrors of this wide-field telescope are made of at silicon wafers coated with reflecting iridium layers. This iridium coatings have some advantages compared to more common gold layers as is shown in corresponding simulations. The iridium coating process for the X-ray mirrors was developed within a cooperation of the Aschaffenburg University of Applied Sciences and the Czech Technical University in Prague. Different mirror parameters essential for a proper function of the X-ray optics, like the surface microroughness and the problematic of a good adhesion quality of the coatings were studied. After integration of the individual mirrors into the final lobster-eye optics and the corresponding space qualification testing it is planned to fly the telescope in a recently proposed NASA rocket experiment.
Previously used mirror technologies are not suitable for the challenging needs of future X-ray telescopes. This is why the required high precision mirror manufacturing triggers new technical developments around the world. Some aspects of X-ray mirrors production are studied within the interdisciplinary project INTRAAST, a German acronym for "industry transfer of astronomical mirror technologies". The project is embedded in a cooperation of Aschaffenburg University of Applied Sciences and the Max-Planck-Institute for extraterrestrial Physics. One important task is the development of low-stress Iridium coatings for X-ray mirrors based on slumped thin glass substrates. The surface figure of the glass substrates is measured before and after the coating process by optical methods. Correlating the surface shape deformation to the parameters of coating deposition, here especially to the Argon sputtering pressure, allows for an optimization of the process. The sputtering parameters also have an influence on the coating layer density and on the micro-roughness of the coatings, influencing their X-ray reflection properties. Unfortunately the optimum coating process parameters seem to be contrarious: low Argon pressure resulted in better micro-roughness and higher density, whereas higher pressure leads to lower coating stress. Therefore additional measures like intermediate coating layers and temperature treatment will be considered for further optimization. The technical approach for the low-stress Iridium coating development, the experimental equipment, and the obtained first experimental results are presented within this paper.
Previously used mirror technologies are not able to fulfil the requirements of future X-ray telescopes due to challenging requests from the scientific community. Consequently new technical approaches for X-ray mirror production are under development. In Europe the technical baseline for the planned X-ray observatory ATHENA is the radical new approach of silicon pore optics. NASA´s recently launched NuSTAR mission uses segmented mirrors shells made from thin bended glasses, successfully demonstrating the feasibility of the glass forming technology for X-ray mirrors. For risk mitigation also in Europe the hot slumping of thin glasses is being developed as an alternative technology for lightweight X-ray telescopes. The high precision mirror manufacturing requires challenging technical developments; several design trades and trend-setting decisions need to be made and are discussed within this paper. Some new technical and economic aspects of the intended glass mirror serial production are also studied within the recently started interdisciplinary project INTRAAST, an acronym for "industry transfer of astronomical mirror technologies". The goal of the project, embedded in a cooperation of the Max-Planck-Institute for extraterrestrial Physics and the University of Applied Sciences Aschaffenburg, is to master the challenge of producing thin mirror shells for future X-ray telescopes. As a first project task the development of low stress coatings for thin glass mirror substrates have been started, the corresponding technical approach and first results are presented.
Future X-ray telescopes aim for large effective area within the given mass limits of the launcher. A promising method is the hot shaping of thin glass sheets via a thermal slumping process. This paper presents the status and progress of the indirect glass slumping technology developed at the Max-Planck-Institut for extraterrestrial Physics (MPE). Recent developments in our research include the use of the mould material Cesic under vacuum, as well as the fabrication of a high-precision slumping mould, which meets the requirements of large, high angular resolution missions like ATHENA. We describe the way forward to optimise the slumping process on these materials, the force-free integration concept and its progress, as well as the first test on reflective coating application.
Large X-ray segmented telescopes will be a key element for future missions aiming to solve still hidden mysteries of the hot and energetic Universe, such as the role of black holes in shaping their surroundings or how and why ordinary matter assembles into galaxies and clusters as it does. The major challenge of these systems is to guarantee a large effective area in combination with large field of view and good angular resolution, while maintaining the mass of the entire system within the geometrical and mass budget posed by space launchers. The slumping technology presents all the technical potentiality to be implemented for the realization of such demanding systems: it is based on the use of thin glass foils, shaped at high temperature in an oven over a suitable mould. Thousands of slumped segments are then aligned and assembled together into the optical payload. An exercise on the mass production approach has been conducted at Max Planck Institute for Extraterrestrial Physics (MPE) to show that the slumping technology can be a valuable approach for the realization of future X-ray telescopes also from a point of view of industrialization. For the analysis, a possible design for the ATHENA mission telescope was taken as reference.
Astronomical mirrors are key elements in modern optical telescopes, their dimensions are usually large and their
specifications are demanding. Only a limited number of skilled companies respectively institutions around the world are
able to master the challenge to polish an individual astronomical mirror, especially in dimensions above one meter. This
paper presents an overview on the corresponding market including a listing of polishers around the globe. Therefore
valuable information is provided to the astronomical community: Polishers may use the information as a global
competitor database, astronomers and project managers may get more transparency on potential suppliers, and suppliers
of polishing equipment may learn about unknown potential customers in other parts of the world. An evaluation of the
historical market demand on large monolithic astronomical mirrors is presented. It concluded that this is still a niche
market with a typical mean rate of 1-2 mirrors per year. Polishing of such mirrors is an enabling technology with impact
on the development of technical know-how, public relation, visibility and reputation of the supplier. Within a
corresponding technical discussion different polishing technologies are described. In addition it is demonstrated that
strategic aspects and political considerations are influencing the selection of the optical finisher.
Professional astronomical telescopes are complex optical systems at the limit of technical feasibility. Often monolithic primary mirrors and sometimes even secondary mirrors with huge dimensions are used. Prominent examples are the two reflectors of the Large Binocular Telescope and the giant mirrors of VLT, GEMINI, and SUBARU. The performance of such precision optical components significantly depends on the physical parameters and the quality of their substrate materials. Within this paper selection criteria for mirror substrates will be discussed, thereby considering the important technical parameters as well as commercial points and aspects of project management. Qualities and limitations of classical mirror substrate materials like Zerodur, ULE, Sitall, borosilicate glass and Cervit will be evaluated and compared to new substrate materials like silicon carbide and beryllium. The different suppliers and their production processes are presented. In addition large mirrors of existing observatories and of telescopes under construction will be listed, thereby concentrating on mirrors above three meter in diameter. An outlook on material trends and on future astronomical telescopes closes this overview on the market of huge monolithic mirror substrates for optical astronomy.
This review paper summarizes the extensive investigations that have been performed at SCHOTT to achieve a deeper
understanding of the CTE homogeneity of ZERODUR® within single blanks and the casted formats (boules). Especially
for the upcoming Extremely Large Telescope (ELT) projects like E-ELT or TMT with at least several hundreds of
mirror segments the reproducibility of the mean CTE, CTE homogeneity and axial gradient is very important while
keeping the CTE quality assurance process economic at the same time. Statistics of CTE homogeneity measurements on
a ZERODUR® boule suitable for an economical production of ELT mirror substrates using the improved dilatometer
will be presented. It will be shown, that it is possible to achieve tight CTE specifications by utilisation of processes
existing at SCHOTT, while at the same time guaranteeing a long term reproducibility. The CTE measurement is
optimized for a temperature interval from 0°C to 50°C. We developed a model to extrapolate the CTE behaviour to
specific temperature conditions at the telescope site.
Recently SCHOTT has shown in a series of investigations the suitability of the zero expansion glass ceramic material
ZERODUR® for applications like mirrors and support structures of complicated design used at high mechanical loads.
Examples are vibrations during rocket launches, bonded elements to support single mirrors or mirrors of a large array, or
controlled deformations for optical image correction, i.e. adaptive mirrors.
Additional measurements have been performed on the behavior of ZERODUR® with respect to the etching process,
which is capable of increasing strength significantly. It has been determined, which minimum layer thickness has to be
removed in order to achieve the strength increase reliably.
New data for the strength of the material variant ZERODUR K20® prepared with a diamond grain tool D151 are
available and compared with the data of ZERODUR® specimens prepared in the same way. Data for the stress corrosion
coefficient n of ZERODUR® for dry and normal humid environment have been measured already in the 1980s. It has
been remeasured with the alternative double cleavage drilled compression (DCDC) method.
The zero-expansion glass ceramic ZERODUR® from SCHOTT is widely used for ground-based astronomical mirrors
and in industrial applications. This paper points out that it is also well suited for satellite applications, especially with
respect to the space radiation environment. Recent developments show that highly lightweighted components can be
manufactured and that such structures are strong enough to survive launch vibrations. A series of thirty reference
applications, where ZERODUR® has been or is currently used (including METEOSAT, SPOT, ROSAT, CHANDRA,
and HST), demonstrate the high and long lasting performance of ZERODUR® components in orbit. The ongoing
successful missions and upcoming new satellites continue to enlarge the space heritage of this unique material.
Initiated in 1968 by the first order of Max-Planck-Institute in Heidelberg successful history of ZERODUR<sup>(R)</sup> continues
now since 40 years. ZERODUR<sup>(R)</sup> zero expansion glass ceramic from SCHOTT has been the material of choice in
astronomy for decades, thanks to its special properties such as its extremely high thermal and mechanical stability. Today
most of the major modern optical telescopes of the 4 m class and of the 8 m to 10 m class are equipped with ZERODUR<sup>(R)</sup> .
For the future several Extremely Large Telescope (ELT) projects are in development, which are designed with even
larger primary mirrors ranging from 30 m to 42 m. Also here ZERODUR<sup>(R)</sup> is under consideration. A historical review,
prominent examples of astronomical projects with glass ceramic mirror substrates, and an outlook to the future is given
in this paper.
With an increasing number of applications mirrors and support structures made of the zero expansion glass ceramic
material ZERODUR® has to endure high mechanical loads, e.g. rocket launches or controlled deformations for optical
image correction. Like for other glassy materials, the strength of glass ceramics is dominated by its surface condition.
Test specimens have been ground with fine grain tools (e.g. D64 diamond grains) and / or subsequently etched. The
strength data basis for the design of highly stressed structures has been extended and new information has been derived
for the extrapolation to low failure probabilities.
Initiated in 1968 by the first order of the Max-Planck-Institute in Heidelberg the successful history of ZERODUR<sup>®</sup>
continues now since 40 years. ZERODUR<sup>®</sup> zero expansion glass ceramic from SCHOTT has been the material of choice
in astronomy for decades, thanks to its special properties such as its extremely high thermal and mechanical stability.
Today most of the major modern optical telescopes of the 4 m class and of the 8 m to 10 m class are equipped with
ZERODUR<sup>®</sup>. For the future several Extremely Large Telescope (ELT) projects are in development, which are designed
with even larger primary mirrors ranging from 30 m to 42 m. Also here ZERODUR<sup>®</sup> is under consideration. A historical
review, the actual status of developments and an outlook to the future is given in this paper.
The new generation of survey telescopes and future giant observatories such as E-ELT or TMT do not only require very
fast or very large mirrors, but also high sophisticated instruments with the need of large optical materials in outstanding
The huge variety of modern optical materials from SCHOTT covers almost all areas of specification needs of optical
designers. Even if many interesting optical materials are restricted in size and/or quality, there is a variety of optical
materials that can be produced in large sizes, with excellent optical homogeneity, and a low level of stress birefringence.
Some actual examples are high homogeneous N-BK7 blanks with a diameter of up to 1000 mm, CaF2 blanks as large as
300 mm which are useable for IR applications, Fused Silica (LITHOSIL<sup>®</sup>) with dimensions up to 700 mm which are
used for visible applications, and other optical glasses like FK5, LLF1 and F2 in large formats.
In this presentation the latest inspection results of large optical materials will be presented, showing the advances in
production and measurement technology.
The zero expansion glass ceramic material, ZERODUR®, is well known for night-time telescope mirror substrates. Also
for solar telescopes ZERODUR® is often selected as mirror blank material. Examples are the Swedish 1 m Solar
Telescope (SST), the balloon-born telescope SUNRISE, and the New Solar Telescope (NST) of the Big Bear Solar
Observatory. The properties of ZERODUR® are discussed with respect to the special technical requirements of solar
observatories, resulting in the conclusion that mirrors made of this glass ceramic material are an excellent choice for
There is a broad range of applications for lightweighted components made from ZERODUR<sup>(R)</sup> glass ceramic. The main
markets are secondary and tertiary mirrors for astronomical telescopes, mirrors and structural components for satellites,
and mechanical structures for industrial applications, mainly in microlithography. Prominent examples from astronomy
are VLT-M3, GEMINI-M2, SOFIA-M1, MAGELLAN-M2, MMT-M2, and METEOSAT-SEVIRI. At SCHOTT
components with blind or undercut semiclosed holes are manufactured, typically with circular, hexagonal, rectangular or
triangular shapes. The classical grinding process results in weight reduction factors of about 70 %. By additional acid
etching technologies even higher lightweighting factors and rib thicknesses below 1 mm have been achieved.
In some applications mirrors and support structures from the zero expansion glass ceramic material ZERODUR<sup>(R)</sup> have
to endure mechanical loads, e.g. rocket launches or controlled deformations for optical image correction. Like for other
glassy materials the strength of glass ceramics is dominated by its surface condition. Similar to other glass ceramics
ZERODUR<sup>(R)</sup> has higher strengths than glasses for comparable surface conditions. For the design of ZERODUR<sup>(R)</sup> parts
well known rules of thumb for its strength are not sufficient in any case. So new information and data with enlarged
sample sets and hence better statistics have been collected to improve the understanding of its behavior under mechanical
loads. Finally an outlook is given on the application of ZERODUR<sup>(R)</sup> in ambitious current and future space projects.
For the planned extremely large telescope projects not only the primary mirror diameters, but also the dimensions of the other optical components are increasing. For the involved manufacturers of astronomical filters technical issues like polishing, coating, measurement, handling and cementing are demanding. Not only the availability of monolithic glass substrates of the required dimensions is critical, but also the required glass quality regarding homogeneity, bubbles, inclusions, and striae. Additionally an individual production of such unique astronomical components is an economical risk, as it does not fit to the usual mass production of small filter components. It is the goal of this paper to call attention to this potential critical path for the future astronomical projects. The status of the filter glass production at SCHOTT and the development needs for these challenging components are discussed.
SCHOTT produces the zero expansion glass ceramics material ZERODUR since 35 years. More than 250 ZERODUR mirror blanks were already delivered for the large segmented mirror telescopes KECK I, KECK II, HET, GTC, and LAMOST. Now several extremely large telescope (ELT) projects are in discussion, which are designed with even larger primary mirrors (TMT, OWL, EURO50, JELT, CFGT, GMT). These telescopes can be achieved also only by segmentation of the primary mirror. Based on the results of the recent production of segment blanks for the GTC project the general requirements of mirror blanks for future extremely large telescope projects have been evaluated. The specification regarding the material quality and blank geometry is discussed in detail. As the planned mass production of mirror blanks for ELT's will last for several years, economic factors are getting even more important for the success of the projects. SCHOTT is a global enterprise with a solid economical basis and therefore an ideal partner for the mirror blank delivery of extremely large telescopes.
SCHOTT has a history of more than 35 years with the production of the zero expansion glass ceramic material ZERODUR. More than 250 ZERODUR mirror blanks were already delivered to the large segmented telescopes KECK I, KECK II, HET, GTC, and LAMOST. The increasing worldwide demand on large ZERODUR components for LCD display lithography machines is similar to the expected demand for an Extremely Large Telescope. Last year SCHOTT has ramped up its ZERODUR production capacity. These recent investments in additional melting and ceramisation capabilities are accompanied by improvements of quality assurance and processing technology. SCHOTT is now prepared also for a future production of mirror blanks for Extremely Large Telescopes. The present status of the production capacity and the mass production of ZERODUR mirror blanks for industrial applications are discussed.
The low thermal expansion glass ceramic ZERODUR is the material of choice for many big astronomical telescope projects like VLT, Keck I + II, HET, LAMOST and GRANTECAN (GTC). For future giant telescope projects like OWL or TMT with at least several hundreds of mirror blanks the CTE homogeneity within a single blank and from blank to blank is an crucial issue.
The ZERODUR production process is based on established and proven methods used in the production of high homogeneity optical glasses. Therefore ZERODUR itself is a material of highest homogeneity even in large dimensions and huge quantities. This paper presents an evaluation of the homogeneity of the thermal expansion coefficient within more than 250 mirror blanks. The observed homogeneity range is only slightly larger than the repeatability of the standard dilatometer measurement of ±0.005*10<sup>-6</sup> K<sup>-1</sup>.
To improve the accuracy of measurement and to get a deeper understanding of the thermal expansion behaviour of ZERODUR a new dilatometer was built exhibiting a repeatability of ±0.001*10<sup>-6</sup> K<sup>-1</sup>. Detailed evaluations of the thermal expansion coefficient homogeneity of a 100 mm x 100 mm ZERODUR test block showed no variation within the repeatability of measurement of the improved dilatometer.
VISTA (Visible and Infrared Survey Telescope for Astronomy) is designed to be the world's largest wide field telescope. After finishing of the construction the telescope will be part of ESO and located in Chile close to the VLT observatory at Cerro Paranal. In November 2001 SCHOTT was selected by the VISTA project office at the Royal Observatory of Edinburgh to deliver the 4.1 m diameter primary mirror blank. The manufacturing of the mirror blank made from the zero expansion material Zerodur was challenging especially due to the f/1 design. Several tons of the glass ceramic material were removed during the grinding operation. A meniscus blank with a diameter of 4100 mm and a thickness of 171.5 mm was generated, having a large central hole of 1200 mm and an aspherical shape of the concave surface. Also the handling and turning operations needed special effort and were performed by a skilled team. This paper presents details and pictures of the corresponding production and inspection sequence at SCHOTT. The geometrical parameters were measured during manufacturing by help of a laser tracker system and the achieved parameters were compared with the initial technical specification. The final quality inspection verified the excellent quality of the mirror blank. The close co-operation between the astronomers and industry resulted in a project management without problems. In April 2003 the VISTA blank was delivered successfully within a ceremony dedicated to the anniversary of "100 years of astronomical mirror blanks from SCHOTT."
SCHOTT has a history of 100 years in delivering mirror blanks for astronomy. Since more than 30 years the zero expansion glass ceramic material ZERODUR is well recognized in the astronomical community. More than 250 ZERODUR mirror blanks for large segmented telescopes have been successfully produced at SCHOTT and were already delivered to KECK I, KECK II, HET, GTC, and LAMOST. For the increasing world wide demand on large ZERODUR components for industrial applications SCHOTT is presently ramping up its production capacity. The investment in additional melting and ceramisation capabilities are accompanied by improvements of quality assurance and processing technology. SCHOTT is now prepared for a future production of ZERODUR mirror blanks for next generation of Extremely Large Telescopes with diameters of 30 m to 50 m. For other large optical elements needed SCHOTT can supply the requested materials like optical glasses, filter glasses, fused silica and calcium fluoride.
The XEUS mission (X-ray Evolving-Universe Spectroscopy Mission) is a future ESA project currently under study. With a mirror collecting area of up to 30 m<sup>2</sup> @ 1 keV and 3 m<sup>2</sup> @ 8 keV it will outperform the x-ray space observatories like XMM-Newton. In fact it will have a source flux sensitivity and angular resolution respectively 250 times and 7.5 times better if compared to that mission. This huge collecting area is obtained with a 10 m diameter telescope of 50 m focal length. It is foreseen that the whole telescope will be formed by two free flying satellites, one for the mirror assembly and the other for the detectors. The two satellites will be kept aligned by an active tracking/orbit control system. The angular resolution of the optics is set to 5 arcsec with a goal of 2 arcsec. Of course the requirement of high resolution and large diameter of the optics create new technological problems which have to be overcome. First of all the impossibility to create closed Wolter I shells (due to the large diameter) means that the optics will be assembled using rectangular segments of ~1 m x ~0.5 m size. A set of these segments will form a petal. The petals will be assembled to form the whole mirror assembly. Another difficulty arises from the fact that the current design foresees a mass/geometric-area ratio of 0.08 kg/cm<sup>2</sup>, which is very small and much lower compared with XMM-Newton. Hence the use of materials that can offer both low weight and high stiffness is mandatory. The impossibility to have a thermal control for the huge area of the optics means also that the mirrors have to operate at temperatures between -30 and -40°C. This requirement excludes the epoxy-replication method as option for their manufacturing (CTE mismatch between resin and substrate). Considering all these constrains a possible solution for the realization of the XEUS mirrors has been found that foresees the use of glass or ceramics materials. In this paper we will describe an investigation currently on-going aimed at the development of a procedure to produce large mirror segments from thin Borofloat glass and the preliminary results obtained, that corroborate the viability of the proposed approach. A previous article has introduced the basic ideas and concepts behind this investigation.
For the next generation of X-ray observatories (CONSTELLATION-X and XEUS) a mass production of glass mirror segments is considered. The mirror substrates (SCHOTT D263 and SCHOTT BOROFLOAT 33) will be pre-shaped in a high temperature slumping process by use of precision forming mandrels. SCHOTT GLAS developed the glass ceramic material ZERODUR K20 to meet the requirements of these mandrels. The new material is a modification of the well-known ZERODUR. A heat driven transformation thereby changes the crystalline phase from high-quartz to keatite structure. The resulting ZERODUR K20 exhibits an increased stability at high temperatures of up to 850°C and a low thermal expansion coefficient (CTE) of approximately 20•10<sup>-7</sup> K<sup>-1</sup> (20°-700°C). Numerical simulations of the slumping process based on experimental parameters of Zerodur K20 and the mirror substrate materials are presented.
Following the actual X-ray satellites XMM-NEWTON and CHANDRA future missions are in discussion. ESA is planning the XEUS-satellite and NASA the CONSTELLATION-X mission. The increasing effective areas of the telescopes require nested thin-walled mirrors of large diameters. For the mass production of segmented shells the techniques of nickel electroforming and of epoxy replication are in evaluation. In both cases ZERODUR glass ceramic was chosen for the replication mandrels due to its high thermal stability and its proven ability to be polished to excellent surface qualities. SCHOTT GLAS has produced pre-shaped prototypes of a-spherical replication mandrels. The final polishing is done at CARL ZEISS, who is also the prime contractor for the finished mandrels. A demonstration mandrel for XEUS has been finished in 2000; the first prototype mandrel for CONSTELLATION-X will be delivered this year. It has been demonstrated that high precision mandrels can be produced with the required accuracy. Thereby ZERODUR is developing from a mirror substrate material (ROSAT, CHANDRA) to the preferred material of mandrels for the replication of X-ray mirrors. This demonstrates the broad variety of applications for this zero expansion glass ceramics.
Schott has delivered blanks for large lenses and prisms since many decades. Glass and glass ceramics objects with dimensions above 300 mm diameter or edge lengths will remain challenges for a glass manufacturer. This holds especially when the quality specifications exceed the standard level significantly. Optical glass blocks of more than half a ton have been produced with outstanding internal quality. Although the manufacturing process is well controlled there are restrictions on the availability of such objects (glass types, long process times e.g.). Implications of the glass production process are presented as a guideline for designers in order to avoid unnecessary time losses. The similarity of the production process of the glass ceramic ZERODUR to that of optical glasses results in high homogeneity with regard to the coefficient of thermal expansion as well as to the optical properties. This qualifies ZERODUR for even higher demanding applications especially when reproducibility in series production is required.
ESA's XEUS x-ray telescope design asks for segmented Wolter 1 mirror plates with radii up to 5 m and a focal length of 50 m. The mirror plates shall have an excellent optical performance (< 5 arcsec HEW). They shall be made by metal (e.g. Nickel) electroforming. This design approach requires highest quality segmented Wolter 1 mandrel plates, with an on-axis HEW < 2 arcsec and a micro-roughness better than 0.3 nm (rms). We will report about the novel design concept, fabrication approach and verification of the x-ray optical performance of the first XEUS demonstration mandrel.
ABRIXAS is a German satellite project - to be launched in spring 1999 - which will perform the first imaging x-ray all-sky survey in the 0.5-10 keV band thus being a complement to the ROSAT all-sky which covered the 0.1-2.4 keV range. Its telescope consists of seven mirrors modules, each with a diameter of 16 cm and a focal length of 160 cm. the mirror modules are tested and calibrated at the MPE X- ray test facility PANTER. Several mirrors from the qualification program and one flight module have been tested and calibrated up to now. The imaging performance of the optics was successively improved until the flight module reached an on-axis resolution of 22 arcseconds. The total scattering level at 8 keV is about 16 percent for two reflections which indicates a microroughness of less than 0.5 nm. The measured on-axis effective area of one flight mirror module is 81 cm<SUP>2</SUP> at 1.5 keV and 25 cm<SUP>2</SUP> at 8 keV. These values indicate that the reflectivities of the mirror surface are on the average about 92 percent of the theoretical expectation.
For the wavelength region above the Si-<i>L</i> edge normal incidence, soft x-ray mirrors are produced with peak reflectivities close to 60%. The multilayer systems consist of molybdenum and silicon and are fabricated by electron beam evaporation in ultrahigh vacuum. A smoothing of the boundaries, and thereby a drastic enhancement of the reflectivity, is obtained by thermal treatment of the multilayer systems during growth. The thermal stability of the multilayer stacks could be improved considerably up to 850° C by mixing Mo and Si in the absorber layers and producing thus Mo<sub><i>x</i></sub>Si<sub><i>y</i></sub>/Si multilayers with <i>x</i> and <i>y</i> denoting the amounts of Mo and Si in the absorber layer, respectively. First attempts are reported to produce mirrors with a bilayer thickness of 2.6 nm. An improvement in the quality of these interfaces can be obtained by bombardment with Ar<sup>+</sup> ions. We report on normal incidence reflectivity measurements of the mirrors with synchrotron radiation and finally on the normal incidence diffraction efficiencies of a Mo/Si multilayer coated grating, for which values of 5.5% are achieved for the + 1'st and - 1'st diffraction orders.
For the wavelength region above the Si-L edge normal incidence soft X-ray mirrors are produced with peak reflectivities around 55 percent. The Mo/Si multilayer systems are fabricated by electron beam evaporation in ultrahigh vacuum. Analysis of the quality of the stack is made by using an in situ monitoring system measuring the reflection of the C-K line and ex situ grazing X-ray reflection of the Cu-K-alpha line. A smoothing of the boundaries and thereby a drastic enhancement of the reflectivity can be obtained by thermal treatment of the multilayer system during growth. The microstructure of the multilayer systems is investigated by means of Rutherford Backscattering spectroscopy and Sputter/AES technique. Baking the final stack after deposition up to 900 C is applied to study the thermal stability of the soft X-ray mirror. Near normal incidence mirrors even for short wavelengths, e.g., the water window (2.4 - 4.4 nm), are produced with a Mo/Si bilayer thickness of 2.6 nm. An improvement in the quality of the interfaces for such ultrathin multilayer systems can be obtained by bombardment of the deposited layers with Ar(+) ions as well as by thermal treatment of the multilayer system and mixing of Mo and Si in the absorber layer during the deposition run. We report on reflectivity measurements of the mirrors and their behavior as polarizers and analyzers and on the diffraction efficiencies of laterally structured multilayer systems as gratings.