We report on the design, development and measured performance of the Microchannel plate X-ray optics for the Mercury imaging X-ray spectrometer instrument, due to fly to Mercury on the ESA/JAXA BepiColombo mission in 2018. This paper serves as a progress update, identifying further work to finish the analysis of data from the flight model.
In support of future x-ray telescopes ESA is developing new optics for the x-ray regime. To date, mass and volume have made x-ray imaging technology prohibitive to planetary remote sensing imaging missions. And although highly successful, the mirror technology used on ESA’s XMM-Newton is not sufficient for future, large, x-ray observatories, since physical limits on the mirror packing density mean that aperture size becomes prohibitive. To reduce telescope mass and volume the packing density of mirror shells must be reduced, whilst maintaining alignment and rigidity. Structures can also benefit from a modular optic arrangement. Pore optics are shown to meet these requirements. This paper will discuss two pore optic technologies under development, with examples of results from measurement campaigns on samples.
One activity has centred on the use of coated, silicon wafers, patterned with ribs, that are integrated onto a mandrel whose form has been polished to the required shape. The wafers follow the shape precisely, forming pore sizes in the sub-mm region. Individual stacks of mirrors can be manufactured without risk to, or dependency on, each other and aligned in a structure from which they can also be removed without hazard. A breadboard is currently being built to demonstrate this technology.
A second activity centres on glass pore optics. However an adaptation of micro channel plate technology to form square pores has resulted in a monolithic material that can be slumped into an optic form. Alignment and coating of two such plates produces an x-ray focusing optic. A breadboard 20cm aperture optic is currently being built.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a French-Chinese space mission to be launched in 2021 with the goal of studying gamma-ray bursts, the most powerful stellar explosions in the Universe. The Microchannel X-ray Telescope (MXT) on-board SVOM, is an X-ray focusing telescope with a detector-limited field of view of ∼1 square° , working in the 0.2-10 keV energy band. The MXT is a narrow-field-optimised lobster eye telescope, designed to promptly detect and accurately locate gamma-ray bursts afterglows. The breadboard MXT optic comprises of an array of square pore micro pore optics (MPOs) which are slumped to a spherical radius of 2 m giving a focal length of 1 m and an intrinsic field of view of ∼6° . We present details of the baseline design and results from the ongoing X-ray tests of the breadboard and structural thermal model MPOs performed at the University of Leicester and at Panter. In addition, we present details of modelling and analysis which reveals the factors that limit the angular resolution, characteristics of the point spread function and the efficiency and collecting area of the currently available MPOs.
We identify all the significant aberrations that limit the performance of square pore micro-channel plate optics (MPOs) used as an X-ray lobster eye. These include aberrations intrinsic to the geometry, intrinsic errors associated with the slumping process used to introduce a spherical form to the plates and imperfections associated with the plate manufacturing process. The aberrations are incorporated into a comprehensive software model of the X-ray response of the optics and the predicted imaging response is compared with the measured X-ray performance obtained from a breadboard lobster eye. The results reveal the manufacturing tolerances which limit the current performance of MPOs and enable us to identify particular intrinsic aberrations which will limit the ultimate performance we can expect from MPO-lobster eye telescopes.
We report progress in the design of the BepiColombo Mercury Imaging X-ray Spectrometer (MIXS). This instrument
consists of two modules; a Wolter I soft X-ray telescope based on radially packed microchannel plate
optics (MIXS-T) and a profiled collimator which uses a square pore square packed microchannel plate array to
restrict its field of view (MIXS-C). Both instrument modules have identical focal planes (DEPFET macropixel
array) providing an energy resolution of better than 200 eV FWHM throughout the mission.
The primary science goal of MIXS is to perform X-ray fluorescence spectroscopy of the Hermean surface with
unprecedented spatial and energy resolution. This allows discrimination between different regolith types, and
by combining with data from other instruments, between competing models of crustal evolution and planetary
formation. MIXS will also probe the complex coupling between the planet's surface, exosphere and magnetosphere
by observing Particle Induced X-ray Emission (PIXE).
We report progress in the design, theoretical modeling and experimental characterisation of microchannel plate
(MCP) X-ray optics for the BepiColombo Mercury Imaging X-ray Spectrometer (MIXS). We show that MCP
optics technology allows the design of a highly capable imaging telescope with 1 m focal length, a 1° field of
view and approximately 50 cm2 of on-axis effective area at 1 keV. Of a total instrument mass budget 7.3 kg, less
than 2.3 kg is allocated to the optics assemblies, telescope tubes, support structures and the electron diverters
(used to deflect electrons from the focal plane). The instrument science goals require an imaging resolution of 9
arcminutes, with a design goal of 2 arcminutes. Recent experimental data, taken from individual optic elements
is presented to show that MCP quality is in good agreement with the error budgets assumed in theoretical
calculations of performance.
Glass micro-pore optics technology, developed over the last years for planetary X-ray imagers, has
been used to assemble optical modules in approximation of a Wolter-I configuration. These tandems of
glass sectors consist of hundreds of square, millimetre sized, multi-fibres that each contain more than a
thousand, 3 μm thin, X-ray mirrors with a surface roughness suitable for application at medium X-ray
energies. The performance of the tandems can be traced back to the quality of the individual fibres.
Extensive X-ray testing has been done on all constituents, from several fibres up to tandem level, using
pencil beam and, for the first time, full beam illumination at PANTER. The results of these campaigns
and of reflectometry measurements are discussed in this paper and have been used throughout the
technology development program to monitor the X-ray performance. It will be shown that the quality
of focussing micro-pore X-ray optics is now high enough to achieve an angular resolution of several
arc minutes and that the multi-fibres are as good as 20 arc seconds, demonstrating the potential of this
technology. The tandems can be combined and assembled into larger geometries, hence forming a very
light and compact X-ray lens of ~200 mm diameter and a focal length of 1 m. This is part of an ESA
breadboard program discussed elsewhere in this conference.
For over fifteen years, micro-Channel plate (MCP) optics, later termed "Micro Pore Optics" (MPOs) - have been under
development to replace the heavy Wolter Type 1 replicated or foil mirrors currently used in X-ray astronomy. Noting
other possible applications, including X-ray Lithography and imaging X-ray fluorescence spectroscopy - and after
considerable, sustained investment from the European Space Agency Technology Research Programme (TRP), a
reliable manufacturing process has now been established, able to produce high quality, low mass X-ray and UV optics
in a variety of formats. Optimisation of the glass preparation and drawing technology, in line process controls and
metrology as well as improvements in the fibre stacking processes, core glass etching and plate slumping have all been
developed. Channel coating methods have also been developed to enhance the high energy response. All these
improvements enable Photonis to offer MPOs with square pores from 10x10 μm up to 100x100 μm, with channel
aspect ratios of up to 500:1 in both square and radially packed geometries in various shapes and with focal lengths in
the range 10 cm to several metres. Space science projects such as LOBSTER (an X-ray all-sky monitor), the Wide Field
Auroral Imager for Kuafu B and the Mercury Imaging X-ray Spectrometer (MIXS) for BepiColombo are likely to
benefit from this unique technology. Other applications are, however, under consideration, such as X-ray pulsar- based
navigation systems for autonomous terrestrial and space navigation. The potential industrial-commercial market interest
in developing these compact X-ray lenses for ground-based applications is the subject of our paper.
ESA is developing technologies for x-ray imaging to reduce the mass and volume of future missions. Applications of x-ray
optics are foreseen in future planetary x-ray imagers, x-ray timing observatories and in observatories for high-energy
astrophysics. With reference to planetary x-ray imagers the use of glass micro-pore material is being investigated. This
technology allows the formation of a monolithic, glass structure that can be used to focus x-rays by glancing reflections
off the pore walls. A technique to form x-ray focusing plates that contain thousands of square micro-pores has been
developed with Photonis. The square pores are formed in a process that fuses blocks of extruded square fibres, which
can then be sliced, etched and slumped to form the segment of an optic with a specific radius. A proposed imager
would be created from 2 optics, slumped with different radii, and mounted to form an approximation of a Wolter I optic
configuration. Reflection can be improved by coating the channel surfaces with a heavy element, such as nickel.
Continuing developments have been made to enhance the manufacturing processes and improve the characteristics of
the manufactured x-ray focusing plates, such as improved surface roughness and squareness of pore walls, improved
pore alignment from fibre stacking through to optic segment slumping and development of pore wall coatings. In order
to measure improvements x-ray measurements are performed by ESA and cosine Research BV, using the BESSY-II
synchrotron facility four-crystal monochromator beamline of the Physikalisch-Technische Bundesanstalt, on multifibres,
sectors and slumped sectors. A probing beam is used to investigate a number of pores to determine x-ray
transmission, focussing characteristics as they relate to the overall transmission, x-ray reflectivity of channel walls,
radial alignment of fibres, slumping radius and fibre position in a fused block. SEM measurements and microscope
inspection have also been used to inspect the channel walls and determine improvements made in fibre stacking and
With Photonis and cosine Research BV, ESA has been developing and testing micro pore optics for x-ray imaging. Applications of the technology are foreseen to reduce mass and volume in, for example, a planetary x-ray imager, x-ray timing observatory or high-energy astrophysics. Photonis, a world leader in the design and development of micro pore optics, have developed a technique for manufacturing square channel pores formed from extruded glass fibres. Single square fibres, formed with soluble glass cores, are stacked into a former and redrawn to form multifibres of the required dimension. Radial sectors of an optic are then cut from a block formed by stacking multifibres and fusing them to form a monolithic glass structure. Sectors can be sliced, polished, etched and slumped to form the segment of an optic with specific radius. Two of these sectors will be mounted to form, for example, a Wolter I optic configuration. To improve reflectivity of the channel surfaces coating techniques have also been considered.
The results of x-ray tests performed by ESA and cosine Research, using the BESSY-II synchrotron facility four-crystal monochromator beamline of the Physikalisch-Technische Bundesanstalt (PTB), on multi-fibres, sectors and slumped sectors will be discussed in this paper. Test measurements determine the x-ray transmission and focussing characteristics as they relate to the overall transmission, x-ray reflectivity of the channel walls, radial alignment of the fibres, slumping radius and fibre position in a fused block. The multifibres and sectors have also been inspected under microscope and Scanning electron Microscope (SEM) to inspect the channel walls and determine the improvements made in fibre stacking.
Producing the next generation of X-ray optics, both for large astrophysics missions and smaller missions such as planetary exploration, requires much lower mass and therefore much thinner mirrors. The use of pore structures allows very thin mirrors in a stiff structure. Over the last few years we have been developing ultra-low mass pore optics based on microchannel plate technology in glass, resulting in square, open-core glass fibres in a concentric geometry. The surface roughness inside the pores can be as low as 0.5 nm due to the extreme stretching of the surface during production. We show how improvements in the production process have led to an improved quality of the fibers and the quality of stacking the fibers in the required geometry. To achieve een higher imaging quality as required for XEUS we have developed in parallel a novel pore optics technology based on silicon wafers. The production process of silicon wafers is extremely optimised by the semiconductor industry, leading to optical qualities that are sufficient for high-resolution X-ray focussing. We have developed the technology to stack these wafers into accurate X-ray optics, set up automated assembly facilities for the production of these stacks and present very promising X-ray test results of 5.3 arcsec HEW from single reflection off such a stack, showing the great potential of this technology for XEUS and other high-resolution low mass X-ray optics.
The Science Payload and Advance Concepts Office of the Science Directorate of the European Space Agency is responsible for developing and conducting a coherent and strategic technology program so as to ensure the feasibility of innovative advanced concepts for future science missions. These missions cover a wide range of disciplines ranging from astrophysics and fundamental physics to solar and planetary research, including exo-biology. The underpinning technology research and development is being conducted in collaboration with European industry and research institutes. The field of high energy photon optics for space applications has demonstrated substantial progress in the past decades, but continues to face very interesting challenges for the future missions. Low specific mass (mass per effective collecting area) is the driving parameter for most future mission designs, both for space based astrophysics observatories and planetary missions. New technologies have to be explored for future applications, simultaneously achieving good angular resolution and low mass. The next generation of high energy astrophysics missions will require the development of much improved optical systems for the x-ray range, and the introduction of focussing imaging systems in the gamma-ray regime. While adequate detection systems are already available, or in the process of refinement and optimization, the optical systems have posed the main hurdle in the design of new space missions. In this paper one potential alternative to the production of very lightweight X-ray optics, which is being investigated by ESA and its industrial partners, is discussed. First the applicability of the required optical design is addressed, followed by the currently ongoing work on the production facilities. Finally the impact of such optics on mission design is investigated based on the example of the X-ray Evolving Universe Spectroscopy mission XEUS. The cosmology mission XEUS requires very large effective area, 30 m2 at 1 keV, X-ray optics with high angular resolution of below 5" with a goal of 2". This implies a large aperture for a single telescope system, which will necessarily require assembly or deployment in space, and which will be formed by basic mirror modules known a petals. The petals must remain compatible with compact ground handling and production tools and will require minimum modifications to existing calibration facilities. Such optics are also envisaged for applications such as astrophysics observatories placed in very deep orbits or in the field of planetary remote sensing. In the latter application there are even stronger mass constraints although a more relaxed angular resolution requirement (e.g. arc minutes compared to arc seconds). Such optics systems have as a single common feature a dramatically reduced mirror thickness and therefore mass.
Using the technology that has been developed over many years for the fabrication of glass micro-channel plates, a prototype micro-pore optic has been produced that is a very light and compact implementation of a Wolter-I optic for X-ray imaging. With this prototype true Wolter-I imaging has been observed for the first time in a micro-pore optic. Individual fibers in the plates are found to be quite good, with a surface roughness permitting application at medium X-ray energies. The image quality and effective area is however seriously reduced by random tilt errors of multifibers in the plates. If this limitation can be overcome, this technology would allow very light and compact X-ray telescopes to be built. A design is presented that already provides a considerable effective area for soft X-rays using the properties of the surfaces obtained in this program.
We report on the performance of 6 micrometer pore diameter Microchannel Plates (MCPs) fabricated in 50 X 50 mm2 format, from both standard and radio-isotope free low noise glass, by Photonis SAS for a European Space Agency Technology Research Program. We compare them to MCPs manufactured by Photonis (the former Philips Photonics) for the High Resolution Camera (HRC) on NASA's Chandra X-ray observatory. The new MCPs represent a significant advance in MCP technology, having a much larger area than previously reported 6 micrometer plates, and demonstrating low noise 6 micrometer technology for the first time. The 6 micrometer plates are shown to be, mechanically, exceptionally well made with a defect density reduced by a factor of 2 - 5 compared to samples from the HRC flight blocks. They exhibit excellent gain and the expected 0.28 keV (Carbon K) X-ray quantum efficiency. The low noise plates have a very uniform response to X-rays but the standard glass MCPs do show inhomogeneity on both the global and multifiber scales.
A novel type of micro-pore optics for the X-ray regime has been developed. These optics have a radial design instead of the square packing in the more traditional Lobster-eye optics. With such a design true imaging, without a crucifix in the focus, can be achieved. We demonstrate that the walls inside the square pores are good enough to produce sub- arcminute focussing up to photon energies above 10 keV. The current performance of the optics is limited by large-scale distortions of the plates, probably caused by the method to fuse the fibers together.