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Smart x-ray optics, focussing in on the next
generation of x-ray telescopes
Carolyn Atkinsa*, Peter Doela, Samantha Thompsona, Hongchang Wanga and David Brooksa
a Department
of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
*[email protected]
(a)
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.
(a)
(b)
Chandra X-ray Observatory
(c)
(c)
(b)
central brass bar
anode
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.
Crab Nebula
Chandra
Resolution 4 arc seconds
Resolution 3 arc seconds
Resolution 0.5 arc seconds
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)
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.
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.
Figure 5: The aluminium
support structure
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.
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.
Figure 3a: The Cat’s
Eye Nebula taken by
the Palomar Telescope
with no adaptive optics
present (courtesy of The
Institute of Astronomy
Cambridge)
(a)
(b)
Figure 3b: The Cat’s
Eye Nebula taken by
the Palomar Telescope
using adaptive optics
(courtesy of The
Institute of Astronomy
Cambridge)
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.
Collaborators logos
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 6: The prototype resting on the support
structure under the effect of gravity
Red indicates a downward
displacement of 2 x 10-6 m
Blue indicates a downward
displacement of 5 x 10-6 m
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.
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.