The implementation of an X-ray mission with high imaging capabilities, similar to those achieved with Chandra (< 1 arc second Half Energy Width, HEW), but with a much larger throughput (2.5 m2 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.

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

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.

Industrialization scenario for X-ray telescopes production based on glass slumping

Development of the glass slumping technology for the production of teh X-raymirrors aboard the international X-ray observatory optical module.

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).

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

In China, X-ray timing and polarization (XTP) observatory will have a collection area of 9,000 cm 2 at 2~6 keV. The observatory consists of five identical hard X-ray telescopes and ten identical soft X-ray telescopes. The angular resolution is about 1 arcminute of HPD (half-power diameter). Each telescope consists of a large number of mirror segments precisely assembled together. Our development of the mirror glass substrate is presented in this manuscript. These substrates are produced by slumping commercially available thin glass sheets. Here, we report on our work of manufacturing these substrates. The optimization of the slumping procedure is described and optimal procedure parameters are reported. The figure error of slumped glass substrates was measured by a laser scanner and an interferometer with CGH. The measurement demonstrated that the figure error is lower enough for the construction of XTP telescopes.

Development of manufacture of mirror glass substrate for x-ray timing and polarization observatory

The Wide Field X-ray Telescope (WFXT) is a medium class mission for X-ray surveys of the sky with an unprecedented
area and sensitivity. In order to meet the effective area requirement, the design of the optical system is based on very thin
mirror shells, with thicknesses in the 1-2 mm range. In order to get the desired angular resolution (10 arcsec requirement,
5 arcsec goal) across the entire 1x1 degree FOV (Field Of View), the design of the optical system is based on nested
modified grazing incidence Wolter-I mirrors realized with polynomial profiles, focal plane curvature and plate scale
corrections. This design guarantees an increased angular resolution at large off-axis angle with respect to the normally
used Wolter I configuration, making WFXT ideal for survey purposes. The WFXT X-ray Telescope Assembly is
composed by three identical mirror modules of 78 nested shells each, with diameter up to 1.1 m. The epoxy replication
process with SiC shells has already been proved to be a valuable technology to meet the angular resolution requirement
of 10 arcsec. To further mature the telescope manufacturing technology and to achieve the goal of 5 arcsec, a
deterministic direct polishing method is under investigation. The direct polishing method has already been used for past
missions (as Einstein, Rosat, Chandra): the technological challenge now is to apply it for almost ten times thinner shells.
Under investigation is quartz glass (fused silica), a well-known material with good thermo-mechanical and polishability
characteristics that could meet our goal in terms of mass and stiffness, with significant cost and time saving with respect
to SiC. Our approach is based on two main steps: first quartz glass tubes available on the market are grinded to conical
profiles, and second the obtained shells are polished to the required polynomial profiles by CNC (Computer Numerical
Control) polishing machine. In this paper, the first results of the direct grinding and polishing of prototypes shells made
by quartz glass with low thickness, representative of the WFXT optical design, are presented.

/pdf/design-and-development-of-thin-quartz-glass-wfxt-polynomial-sghvanxayh.pdf

Design and development of thin quartz glass WFXT polynomial mirror shells by direct polishing

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Design and development of thin quartz glass WFXT polynomial mirror shells by direct polishing