X-Ray and Gamma-Ray Optics
Micropore Optics
X-ray optics for astrophysics missions requires extremely large collecting areas (>10 m²) in combination with good angular resolution (<5"). The existing technologies of focussing optics use polished glass, electroformed nickel or foils and would lead to excessively heavy and expensive optics, and/or they are not able to produce the required large area. We have developed an entirely novel technology by using pore structures that allow very thin mirrors in the required stiff structure. In the past three different generic optics technologies have been employed conventionally: - Monolithic shells of glass that are figured and highly polished for high angular resolution, but with relatively large mass penalty. They are rigid and relatively easy to support.
- Thin foils that are fabricated from pliable substrates (plastic or metal) and bent into a conical approximation of a Wolter geometry. These offer the lowest mass but are relatively poor in angular response, especially because distortions arise at their mounting points.
- Replication of thin shells (usually nickel) from a highly polished mandrel that offers a compromise in terms of mass versus resolution trade-off. These shells are inherently stiffer than foils and can be mounted at one end through fixation onto an accurately machined spider.
The minimisation of the telescope mass and volume becomes of critical importance for the next generation astronomical X-ray telescopes (e.g. NXO) due to their challenging requirements in collecting area and resolution. Reducing the mass of a grazing incidence X-ray optic can be achieved by - using a material that has a lower density and
- by reducing the thickness of the mirrors.
Very thin mirrors will in general have more distortions which result in lower imaging resolution. This can be prevented by producing the reflecting surfaces in a tightly interconnected structure, so that the mirrors span only small distances. This is achieved in pore optics.
 | | Two sets of pores placed back to back reflect the X-rays onto the detector in the focal point (FP). | In a pore optic one wall of each pore is used as the reflecting surface, and the side walls provide extreme stiffness to the structure. The shape of the entirety of reflecting surfaces does not necessarily have to follow exactly the parabolic and hyperbolic surfaces of the Wolter-I design. If the reflecting surfaces are short compared to the focal length, the effect on the imaging resolution of conical instead of parabolic or hyperbolic surfaces ('conical approximation') can be made sufficiently small. The profile of the pore needs not follow a circle, provided that the width of the pore is smaller than the required size of the focal spot. Because the walls in the pore structure can be very thin the reflecting surfaces can be stacked very densely, effectively leading to small pores and short optics. This enables the production of high-resolution optics even with flat reflecting surfaces of the pores themselves. The complete system, however, still focuses the rays, since the reflection surfaces are concentric and the inclination of the surfaces rise with the radius.
 | | SEM image of etched silicon square pore material. |
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Grazing angle mirrors |
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Layered Synthetic Microstructures |
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Last Update: 24 Feb 2006
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