Hongchang Wang

Bio: Hongchang Wang is an academic researcher from University College London. The author has contributed to research in topics: Adaptive optics & X-ray telescope. The author has an hindex of 5, co-authored 9 publications receiving 68 citations. Previous affiliations of Hongchang Wang include Diamond Light Source (United Kingdom).

Active x-ray optics for the next generation of x-ray telescopes

Abstract: The immediate future for X-ray astronomy is the need for high sensitivity, requiring large apertures and collecting areas, the newly combined NASA, ESA and JAXA mission IXO (International X-ray Observatory) is specifically designed to meet this need. However, looking beyond the next decade, there have been calls for an X-ray space telescope that can not only achieve this high sensitivity, but could also boast an angular resolution of 0.1 arc-seconds, a factor of five improvement on the Chandra X-ray Observatory. NASA's proposed Generation-X mission is designed to meet this demand; it has been suggested that the X-ray optics must be active in nature in order to achieve this desired resolution. The Smart X-ray Optics (SXO) project is a UK based consortium looking at the application of active/adaptive optics to both large and small scale devices, intended for astronomical and medical purposes respectively. With Generation-X in mind, an active elliptical prototype has been designed by the SXO consortium to perform point-to-point X-ray focussing, while simultaneously manipulating its optical surface to improve its initial resolution. Following the completion of the large scale SXO prototype, presented is an overview of the production and operation of the prototype, with emphasis on the X-ray environment and preliminary results.

Development of piezoelectric actuators for active X-ray optics

Abstract: Piezoelectric actuators are widely utilised in adaptive optics to enable mirrors having an actively controlled reflective surface for the purpose of the wavefront correction by reducing the effects of rapidly changing optical distortion. Two new prototype adaptive X-ray optical systems are under development with the aim of approaching the fundamental diffraction limit. One proposed technology is microstructured optical arrays (MOAs) involving two or four piezoelectric strips bonded to a silicon wafer to produce a micro-focused X-ray source for biological applications, and which uses grazing incidence reflection through consecutive aligned arrays of channels obtained using deep silicon etching. Another technology is large scale optics which uses a thin shell mirror bonded with 20–40 piezoelectric actuators for the next generation of X-ray telescopes with an aim to achieve a resolution greater than that currently available by Chandra (0.5"). PZT-based piezoelectric actuators are being developed in this programme according to the design and implementation of the proposed mirror and array structures. Viscous plastic processing is chosen for the preparation of the materials system, which is subsequently formed and shaped into the suitable configurations. Precise controls on the thickness, surface finish and the curvature are the key factors to delivering satisfactory actuators. Unimorph type piezoelectric actuators have been proposed for the applications and results are presented regarding the fabrication and characterisation of such piezo-actuators, as well as the related design concepts and comparison to modelling work.

Development of thin-shell adaptive optics for high angular resolution x-ray telescopes

Abstract: The Smart X-ray Optics project is a UK based consortium of eight institutions investigating the application ofactive/adaptive X-ray optics for both large and small scale. The work being undertaken at the University ofLeicester includes the modelling and testing of a large scale optic, suitable for an X-ray telescope. This willincorporate piezoelect ric devices to enable the surface to be actively deformed, aiming to achieve an angularresolution better than that currently available (e.g. Chandra 0.5Ž). The test optic design is based on a thinNickel ellipsoid segment on the back of which will be bonded a series of piezoelectric actuators. Simulation of theX-ray performance and the eect of the actuated piezoel ectric devices on the detected image is described. Detailsof the models produced for the piezoelectric actuation routine and the simulated annealing algorithm under de-velopment, will be outlined. Planed testing of the ellipsoidal prototype and future objectives for implementationof active X-ray optics in the design of an X-ray telescope will be discussed.Keywords: X-ray astronomy, X-ray telescopes, piezo electric actuators, t hin active mirrors

Advances in active X-ray telescope technologies

Abstract: The next generation of X-ray telescopes will require both high resolution and high sensitivity to target the earliest astronomical objects, to this end the UK based Smart X-ray Optics (SXO) project has been investigating the application of active/adaptive optics to traditional grazing incidence X-ray optics and this has resulted in the fabrication and testing of our first active X-ray prototype in November 2008. Results from these initial tests have proved very encouraging for this advancing technology and have highlighted the prototype's ability to deform its optical surface through piezoelectric actuation. We present a critical analysis of the first prototype system, discussing metrology of the mandrel, the nickel replicated ellipsoidal optics and the prototype. The measured actuator influence functions of the prototype are compared against finite element analysis simulations and the observed characteristics are then described. The advances required in the current technology are then outlined in relation to a second generation of active X-ray prototype, which is scheduled for X-ray testing in 2010.

Future high-resolution x-ray telescope technologies: prototype fabrication methods and finite element analysis

Abstract: The Smart X-ray Optics (SXO) project is a UK based consortium consisting of several institutions investigating the application of active/adaptive optics to both large and small scale grazing incidence x-ray optics. University College London presents work relating to the large scale x-ray optics that is geared towards the next generation of x-ray space telescopes. It is proposed that through the addition of piezoelectric actuators, an active x-ray telescope with a resolution better than that currently achieved (e.g. Chandra 0.5") could be realised. An immediate aim of the SXO project is to produce an operational active ellipsoidal segment prototype, with point-to- point focusing and with the intention of being tested at the University of Leicester's x-ray beam source. Work relating to the fabrication of the prototype will be presented, including shell replication via a nickel sulphamate electroforming process, piezoelectric actuators and prototype assembly and operation. Results from finite element analysis modelling will be discussed; these relate primarily to gravitational distortion effects and the plating tank electrostatics.

XMM-Newton observatory*: I. The spacecraft and operations. Commentary

Abstract: The XMM-Newton Observatory is a cornerstone mission of the European Space Agency's Horizon 2000 programme, and is the largest scientific satellite it has launched to date. This paper summarises the principal characteristics of the Observatory which are pertinent to scientific operations. The scientific results appearing in this issue have been enabled by the unprecedentedly large effective area of the three mirror modules, which are briefly described. The in-orbit performance and preliminary calibrations of the observatory are briefly summarised. The observations from the XMM-Newton calibration and performance verification phase, which are public and from which most papers in this issue have been derived, are listed. The flow of data from the spacecraft, through the ground segment, to the production of preliminary science products supplied to users is also discussed.

3D characterization of thin glass x-ray mirrors via optical profilometry

Abstract: In this paper we present the "Characterization Universal Profilometer" (CUP), a new metrological instrument developed at the Brera Observatory for the 3D surface figure mapping of X-ray segmented mirrors. The CUP working principle is based on the measure of the the distance between the surface under test from a rigid reference dish. This approach is made possible by the coupled use of two sensors, the CHRocodile® optical device and the SIOS triple beam interferometer, mounted onto a proper system of x-y-z stage of translators. In this paper we describe the working principle of the new instrument. We will also present the results of the commissioning performed for a CUP breadboard developed at the Brera Observatory. The CUP offers the possibility to perform an high accuracy metrology of thin glass segments produced via hot slumping, to be used in future segmented X-ray mirrors like those foreseen aboard IXO or other projects that will make use of active X-ray mirrors.

Parallel Multiobjective Evolutionary Algorithms

Abstract: The use of evolutionary algorithms (EA s) for solving multiobjective optimization problems has been very active in the last few years. The main reasons for this popularity are their ease of use with respect to classical mathematical programming techniques, their scalability, and their suitability for finding trade-off solutions in a single run. However, these algorithms may be computationally expensive because (1) many real-world optimization problems typically involve tasks demanding high computational resources and (2) they are aimed at finding a whole front of optimal solutions instead of searching for a single optimum. Parallelizing EAs emerges as a possible way of reducing the CPU time down to affordable values, but it also allows researchers to use an advanced search engine – the parallel model – that provides the algorithms with an improved population diversity and enable them to cooperate with other (eventually nonevolutionary) techniques. The goal of this chapter is to provide the reader with an up-to-date review of the recent literature on parallel EAs for multiobjective optimization.

High-Resolution X-Ray Telescopes

Abstract: Fundamental needs for future x-ray telescopes: a) Sharp images => excellent angular resolution b) High throughput => large aperture areas Generation-X optics technical challenges: a) High resolution => precision mirrors & alignment b) Large apertures => lots of lightweight mirrors Innovation needed for technical readiness: a) 4 top-level error terms contribute to image size b) There are approaches to controlling those errors Innovation needed for manufacturing readiness Programmatic issues are comparably challenging

Adaptive optics for extreme ultraviolet lithography : actuator design and validation for deformable mirror concepts

Abstract: In the production of integrated circuits (e.g. computer chips), optical lithography is used to transfer a pattern onto a semiconductor substrate (wafer). For lithographic systems using light in the ultraviolet band (EUV) with a 13.5nm nm wavelength, only reflective optics with multi-layers can reflect that light by means of interlayer interference, but these mirrors absorb around 30% of the incident light. Depending on pattern and beam shape, there is a nonuniform light distribution over the surface of the mirrors. This causes temperature gradients and therefore local deformations, due to different thermal expansions. To improve the throughput (wafers per hour), there is a demand to increase the source power, that will increase these deformations even further. Active mirrors are a solution to correct these deformations by reshaping the surface. This thesis addresses the challenges to accurate deform a mirror with high repeatability, meeting the requirements for implementation in a lithographic illumination machine. The main design criteria are vacuum compatibility, actuator stroke and the distance between actuators. Four different experimental mirrors, with increasing complexity, are successfully designed, realized and validated. All mirrors are equipped with thermomechanical actuators to either bend, or axially deform them. These actuators are free from mechanical hysteresis and therefore have a high position resolution with high reproducibility. Extensive finite element analysis is done, to maximize actuator stroke and minimize input power. All mirrors are tested and validated with interferometer surface measurements and thermocouple temperature measurements. The first experimental mirror with one thermo-mechanical bending actuator is successfully built and tested (chapter 2). To obtain a high mirror deflection at a given inserted actuator power, aluminum is chosen as the actuator material. The mirror is made from Zerodur® like the mirrors in the first EUV lithographic demonstration machines. A mirror deformation of 4:7 nm/C is achieved, where the inserted actuator power is 0.044 C/mW, meaning 0:21 nm/mW. The measured characteristic time constant is 10 s, meaning that for a given input, 63% of the steady state stroke is reached within that time scale. All values are close to the predicted ones from the models and also meet the requirements for implementation. To further investigate the concept and to measure the mechanical and thermal actuator coupling, an experimental mirror with four actuators is designed, developed and validated (chapter 3). It is an extension of the mirror with one actuator. In a single actuator step-response, a mirror deflection of 3.4 nm/C is achieved. A design optimization is proposed and successfully tested which reduces the actuator coupling from 30% to 10%, while the mirror deflection at the same input is reduced to 55%. Actuator speed is demonstrated while simultaneously heating all actuators with 3mW, which correspond with a mirror deformation of 33 pm/s. When using an adaptive mirror in an EUV lithography system, actuator strokes of 1 nm/min are required. The demonstrated actuator speed of 33 pm/s = 2 nm/min meets that requirement. The third and fourth mirror have actuators placed perpendicular to the surface (chapter 4). By placing the actuators on a thin back plate, the force loop is localized and therefore a lower actuator coupling is achieved. The results obtained from the third mirror with 7 actuators are close to the predicted values from the static and thermal models. Based on these good results, this actuation principle is implemented in a smaller deformable mirror with 19 actuators inside a 25mm beam diameter. A linear relation between actuator power and temperature of 0.190 C/mW and between power and averaged interactuator stroke of 0.13 nm/mW is achieved. So, the successfully realized mirror deflection is 0:68 nm/ C and no hysteresis is observed. For both mirrors a support frame is developed, that minimizes introduced surface deformations by temperature variations. Thermal step responses are fitted and both heating and cooling characteristic time constants are 2:5 s. The thermal actuator coupling from an energized actuator to its direct neighbor is 6:0, to their neighbors it is 1:3%. The total actuator coupling is approximated around 10%, based on the good agreement between simulated and measured inter-actuator stroke. Finally, chapter 5 summarizes the main findings from the different deformable mirrors and compares them. Also, suggestions for future research are given for implementation into a lithographic machine.