Smart x-ray optics, focussing in on the next generation of x-ray telescopes Carolyn Atkins a *, Peter Doel a, Samantha Thompson a, Hongchang Wang a and David Brooks a a Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK Collaborators logos Introduction The Smart X-ray Optics (SXO) Project is a UK based consortium consisting of several institutions investigating the application of adaptive optics to both large scale (telescopes) and small scale (microscopes) x-ray optics. Work is presented regarding the large scale optics; it is hoped that via the combination of two current astronomical optic techniques that the resolution can be improved from 0.5 arc seconds to 0.1 arc seconds. Figure 1 shows the development of x-ray telescopes through the past decades and how the resolution of an image has improved from 4 arc seconds to 0.5 arc seconds, our work is hoping to improve this even further. Acknowledgements This work has been funded by a Basic Technologies Grant from the UK Engineering and Physical Sciences Research Council (EPSRC). The author appreciates the help and advice offered by the SXO consortium, and would like to thank EPSRC for her PhD studentship. Status Update and Future Work Currently the SXO project is investigating and testing the production of the nickel shells, one problem with electroforming is attaining an even deposition as nickel ions favour the edges of the mould and therefore creating a stiffer overall shell. This problem is being researched and it is hoped that a satisfactory solution will be found. The support structure is currently in production and this will prove invaluable in the initial testing of the optic. X-ray Telescopes The first x-ray telescopes were launched in the 1970’s and the basic theory remains the same today, all x-ray telescopes are space borne, as x-rays from space do not penetrate the Earth’s atmosphere. An x-ray telescope works on the principle of external reflection, essentially an x-ray is ‘bounced’ off a mirror surface, very much like a bullet ricocheting off a wall. The telescope consists of two sets of nested cylinders, where the x-rays reflect twice before coming to a focus (Figure 2). The cylinders are nested to increase the surface area seen by the x-rays, they are also coated in a heavy element metal like: gold, iridium, platinum or nickel, to ensure that the x-ray reflection can occur. Crab Nebula Chandra Adaptive Optics These sophisticated optics correct a ground based telescope image for the distortions caused by the Earth’s atmosphere, these distortions are the same as those that cause stars in the night sky to appear to twinkle. Removing the affect of the atmosphere vastly increases the resolution and therefore the quality of the image, as shown in Figure 3. Adaptive optics work by having a deformable mirror and by constantly monitoring the atmosphere, the mirror is actively deformed to correct for the atmosphere at any particular point in time. Smart X-ray Optics: Prototype Design The SXO project is combining both the classical x-ray telescope design with the theory of adaptive optics, with the goal of achieving a resolution of 0.1 arc seconds. A prototype will be produced consisting of a segment of cylindrical shell, on the back of which will be bonded a series of piezoelectric actuators. A piezoelectric material is one that changes shape in an electric field. The prototype will be tested in the x-ray beam facility at The University of Leicester. The proposed final prototype is based on an elliptical segment that is capable of focussing of an x-ray beam. However prior to that a second prototype has been designed to allow various production methods to be investigated. The second prototype is a cylindrical segment (length 200mm, width 100mm, radius of curvature 154mm, thickness 1mm), Figure 4a displays the design for the cylindrical prototype mould. Prototype Manufacture Following the same production methods as previous x-ray telescopes, the process of shell replication will be used, replication is a process by which several shells can be made from a single mould (Figure 4b). The mould is submerged in an electroforming bath (Figure 4c) where a nickel layer will be deposited. The solution of the bath is made up of positive nickel ions, these ions become attracted to the surface of the mould when a electric field is present, as under these conditions the mould becomes negatively charged. When the nickel ions come into contact with the mould they gain two electrons and become neutral nickel atoms. Once the required deposition thickness has been attained the mould is removed from the bath and the pure nickel shell removed. The actuators will be bonded to the back of the nickel shell using a low shrinkage glue to ensure that distortions caused by the glue shrinking will be kept to a minimum. The actuators will be wired to a drive system, which will control the voltage output of each of the actuators and therefore the final resolution capable of the optic. Testing and Support Structure The x-ray beam facility consists of a long tube, approximately 29m in length, at one end is situated an x-ray point source and at the other end a detector. It is intended that the prototype will be situated approximately 4m from the detector. A support structure was designed to ensure that distortions due to gravity would be kept to a minimum. Figure 5 shows the proposed aluminium support structure and on top of two foam strips the prototype will be placed. Figure 6 highlights the effect due to gravity experienced by the prototype. Figure 2: a) The incoming x-rays reflecting off the mirror’s surface and coming to a focus, b) the form of the x-ray mirrors. (a) (b) Figure 3a: The Cat’s Eye Nebula taken by the Palomar Telescope with no adaptive optics present (courtesy of The Institute of Astronomy Cambridge) Figure 3b: The Cat’s Eye Nebula taken by the Palomar Telescope using adaptive optics (courtesy of The Institute of Astronomy Cambridge) Figure 5: The aluminium support structure Figure 6: The prototype resting on the support structure under the effect of gravity Red indicates a downward displacement of 2 x m Blue indicates a downward displacement of 5 x m Figure 1: The Crab Nebula as imaged by three different x-ray telescopes, a) The Einstein Observatory (courtesy of NASA), b) ROSAT (courtesy of S. L. Snowden USRA, NASA/GSFC) and c) Chandra (courtesy of NASA/CXC/SAO) Resolution 4 arc secondsResolution 3 arc secondsResolution 0.5 arc seconds Chandra X-ray Observatory (a)(b)(c) (a) (b) (c) Figure 4a: The cylindrical prototype mould showing dimensions, the material of the mould is stainless steel and it is polished to a high surface quality. Figure 4b: The finished cylindrical mould, only the curved surface of the mould will be used for nickel deposition. Figure 4c: The electroforming bath (0.6 x 0.6 x 0.6 m³), the mould will be suspended in the solution from the central brass bar facing the anode that supplies the positive nickel ions. central brass bar anode