Arjan L. Mieremet

Development of x-ray pore optics: novel high-resolution silicon millipore optics for XEUS and ultralow mass glass micropore optics for imaging and timing

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.

Achromatic phase shifting using adjustable dispersive elements

We describe a theoretical concept of achromatic phase shifting using dispersive elements. This concept is able to provide any desirable phase shift. Its achromatic behavior in terms of spectral bandwidth and/or residual phase shift error can be improved by increasing the number of elements. Furthermore, the ability to adjust the optical path through each element enables to overcome manufacturing inaccuracy. We show some theoretical curves of residual phase shift errors obtained with this method over a wavelength region from 400 nm to 1000 nm.

Fundamental spatial resolution of an x-ray pore optic

We investigate the fundamental spatial resolution of an x-ray pore optic as a function of the pore dimensions, the photon energy, and the focal length. We achieve this by calculating the shape of the focal spot, using diffraction integrals such that the half-energy width is determined. Quantitative results are presented for the X-Ray Evolving Universe Spectroscopy (XEUS) telescope, showing that a resolution of better than 2 arc sec half-energy width is possible by use of an optic with pore sizes of approximately 0.5 mm.

Demonstration of nulling using delay line phase shifters

We present results of experiments obtained using a new nulling technique that enables deep nulling without the use of achromatic phase shifters. The experimental set-up consists of a three-beam interferometer that should provide a nulling depth of several thousands over a wavelength range of 500 to 650 nm. The intended depth of null was not achieved and further experiments on determining the spectrum of each beam revealed why. We describe a method of obtaining accurate beam spectra in a multi-beam interferometer. The insights on the need of spectral shape control were tested with our nulling theory and proved the sensitivity of this nulling approach with respect to spectral mismatches.

Deep nulling by means of multiple-beam recombination

Direct detection of exoplanets is possible by use of a technique called nulling interferometry, which is based on destructive interference of light of the bright object and constructive interference of the faint object. In the infrared wavelength region, this implies that light of a star must be attenuated by a certain factor, the so-called rejection ratio, which typically equals 10(6). This can be achieved by use of phase shifters, which apply a phase shift of pi rad with an average error nogreater than 2 mradover a predefined wavelength region. For a 6-18-microm wavelength interval, this is a tough constraint. We show that the 2-mrad constraint can be relaxed if more than two beams participate in the beam recombination. We focus our attention on dispersive phase shifters and show that rejection ratios beyond 10(6) can be reached easily by use of a system of four or more apertures and simple dispersive phase shifters that consist of only one material.

Nulling interferometry without achromatic phase shifters.

In the infrared wavelength region, a typical star is approximately a million times brighter than the planet that surrounds it, which is a major problem when we attempt to detect exoplanets in a direct manner. Nulling interferometry is a technique that one can use to solve this problem by attenuating the stellar light and enhancing that of the planet. Generally, deep nulling is achieved by use of achromatic phase shifters (APSs). Unfortunately, the technology needed to build these APSs is not yet fully developed. We show that deep nulling can also be achieved by using delay lines only. We investigate the nulling depth as a function of the width of the wavelength interval and the number of telescopes. We also show that we can obtain nulling depths of less than 10(-6), which are required for exoplanet detection. Furthermore, we investigate the properties of the transmission map and make a comparison between our system and an APS system.

Use of silicon pore optics for an SF class deployed telescope

X-ray optics based on pore geometries have opened applications for X-ray telescopes in the planetary and astrophysics areas where very restricted resources are allowed. In this paper the mission design for a limited size X-ray telescope is presented, which is based on a stowed structure to be deployed in a L2 orbit. With the application of silicon based pore optics in the conical approximation of the Wolter geometry [1, 2, 3] an appreciable effective area can be achieved at 1keV. The energy response function can be extended and optimised towards higher energies by the application of more complex reflective coatings including multiplayer designs [4, 5]. The angular resolution is kept compatible with this collecting area, avoiding source confusion. One of the workhorse launchers for the ESA science missions, the Soyuz Fregat, is assumed as vehicle. The main trade-offs of the mission design will be addressed and the performance of such a telescope is discussed.

A Combined Nulling and Imaging Pupil-Plane Beam-Combiner for DARWIN

The primary goal of DARWIN is to detect earth-like extrasolar planets and to search for biomarkers. This is achieved by means of nulling interferometry, using three free-flying telescopes and a Beam-Combiner (BC) hub. DARWIN will be able to perform astrophysical imaging with high spectral and spatial resolution. Should one of Darwins telescope flyers fail, then Darwins capability of detecting earth-sized exo-planets is dramatically reduced. However, with only two telescopes the imaging mode can continue operating with minimal performance degradation, thus ensuring mission success. This work describes a trade-off study between four conceptual three-beam BCs, that are capable of performing both as a nuller and as an imager. A proposed breadboard design will demonstrate end-to-end Fringe-Tracking (FT) and Optical Path-Length (OPL) control. The BC concept is based on a pupil-plane (Michelson) beam combination scheme. Pupil-plane imaging BCs offer a large overlap in terms of optical layout with the nulling BC concept, making it possible to develop a combined nulling- and imaging BC. This means that a reduced number of optical components can be used compared to a scheme with separate BCs. The BC concept inherently compensates for unequal OPLs, which in ground-based interferometers is compensated for by long stroke Optical Delay Lines (ODLs).