Ann F. Shipley

Laboratory detection of X-ray fringes with a grazing-incidence interferometer

Starting with Galileos observations of the Solar System, improvements of an order of magnitude in either the sensitivity or resolution of astronomical instruments have always brought revolutionary discoveries. The X-ray band of the spectrum, where exotic objects can have extremely high surface brightness, is ideally suited for significant improvements in imaging, but progress has been impeded by a lack of optics of sufficiently high sensitivity and quality. Here we present an X-ray interferometer design that is practical for adaptation to astronomical observatories. Our prototype interferometer, having just under one millimetre of baseline, creates fringes at 1.25 keV with an angular resolution of 100 milliarcseconds. With a larger version in orbit it will be possible to resolve X-ray sources at 10-7 arcseconds, three orders of magnitude better than the finest-resolution images ever achieved on the sky (in the radio part of the spectrum) and over one million times better than the current best X-ray images. With such resolutions, we can study the environments of pulsars, resolve and then model relativistic blast waves, image material falling into a black hole, watch the physical formation of astrophysical jets, and study the dynamos of stellar coronae.

MAXIM Pathfinder x-ray interferometry mission

The MAXIM Pathfinder (MP) mission is under study as a scientific and technical stepping stone for the full MAXIM X-ray interferometry mission. While full MAXIM will resolve the event horizons of black holes with 0.1 microarcsecond imaging, MP will address scientific and technical issues as a 100 microarcsecond imager with some capabilities to resolve microarcsecond structure. We will present the primary science goals of MP. These include resolving stellar coronae, distinguishing between jets and accretion disks in AGN. This paper will also present the baseline design of MP. We will overview the challenging technical requirements and solutions for formation flying, target acquisition, and metrology.

Off-plane grating mount tolerances for Constellation-X

High groove density reflection gratings placed at grazing incidence in the extreme off-plane mount offer increased performance over conventional in-plane mounts in the x-ray. We are developing an off-plane approach to the Reflection Grating Spectrometer of the Constellation-X Mission. In this paper we discuss the geometry of the off-plane mount and present formulae for the key tolerances of the grating array.

Requirements and options for a stable inertial reference frame for a 100-micro-arcsecond imaging telescope

The MAXIM Pathfinder (MP) and Stellar Imager (SI) missions are under study to do 100 microarcsecond resolution imaging for a number of different targets using interferometers divided over formation flying spacecrafts. One of the most challenging technical hurdles for these missions is to have an independent directional reference in the sky to use for target acquisition and tracking. This directional reference will guide the placement of separate free flying elements of the interferometers to have ~<30 microarcseconds of alignment with the target. This paper will discuss some of the specific challenges as well as some possible options to explore for achieving this alignment.

Off-plane gratings for Constellation-X

A reflection grating spectrometer featuring off-plane, radial groove gratings for the Constellation-X mission will be presented. Preliminary work shows that off-plane designs can significantly enhance the scientific capabilities of the mission baseline design. Resolutions up to λ/Δλ~5000 can be achieved. The design accomplishes the mission goal for throughput and may also significantly reduce the assembly tolerances. Detailed raytracing and performance assessment of a strawman configuration are included.

Laboratory studies of petal-shaped occulters

We present laboratory studies of scaled occulting starshades for the New Worlds Observer (NWO). A deep reactive ion etched silicon starshade has been fabricated by NIST, designed to cover the same number of Fresnel zones as in the proposed mission. The broadband shadow is mapped with a photometer in a dark vacuum tunnel fed by a heliostat at HAO. CCD images provide direct contrast measurements of different features around the starshade. Preliminary measurements reach 5x10-6 suppression in the center of the shadow at the focal plane. The two-dimensional structure of the starshade diffraction pattern is compared to that produced by the Fresnel integral.

Off-plane grating performance for Constellation-X

High groove density reflection gratings placed at grazing incidence in the extreme off-plane mount offer improved performance over conventional in-plane mounts in the x-ray. We present test results from the grating evaluation facility at the University of Colorado for gratings optimized for use in the off-plane configuration. The gratings tested are produce via holographic lithography. Gratings tested have radial groove patterns and include both blazed and sinusoidal groove profiles. We present efficiency and sub-aperture resolution results.

White-light demonstration of one hundred parts per billion irradiance suppression in air by new starshade occulters

A new mission concept for direct imaging of exo-solar planets called New Worlds Observer (NWO) has been proposed. It involves flying a meter-class space telescope in formation with a newly-conceived, specially-shaped, deployable star-occulting shade several meters across at a separation of some tens of thousands of kilometers. The telescope would make its observations from behind the starshade in a volume of high suppression of incident irradiance from the star around which planets orbit. For an efficacious mission, the required level of irradiance suppression by the starshade is of order 0.1 to 10 parts per billion in broadband light. We discuss an experiment to accurately measure the irradiance suppression ratio at the null position behind candidate starshade forms to these levels. We also present results of broadband measurements which demonstrated suppression levels of less than 100 parts per billion in air using the Sun as a light source. A simulated spatial irradiance distribution surrounding the null from an analytical model developed for starshades is compared with a photograph of actual irradiance captured in situ behind a candidate starshade.

Highly reflective uranium mirrors for astrophysics applications

The reported optical constants of uranium differ from that of vacuum significantly more than other elements do over the range of about 150 to 350 eV. This suggests that uranium could be used to produce high reflectance imaging mirrors for many soft x-ray applications. Elemental uranium is too chemically active to be used as a front surface mirror without protection. We computed the expected reflectance of carbon-coated uranium films and of uranium-nickel alloys for low-angle reflectors. Carbon is mostly transparent below its K absorption edge at about 283 eV. The reflectance at 10 degrees from grazing is computed to be greater than 50% at 277 eV (C Kα). For comparison, about 5 degrees is the maximum grazing incidence angle for which conventional materials are computed to have comparable reflectance. We sputter deposited and measured the reflectance of carbon-coated uranium layers at 44.7 Å (C Kα). Sample reflectance was a factor of two greater than that of nickel, the material used for low-angle mirrors. The initial oxidation behavior of sputtered uranium-nickel alloys is similar to pure U so their reflectance was not determined. Coatings based on uranium should be considered for all applications where high-reflectance, broadband, low-angle soft x-ray mirrors are required

MAXIM interferometer tolerances and tradeoffs

MAXIM consists of thirty-two individual grazing incidence interferometer channels that act, in combination, like a high-resolution imaging telescope. In this paper, we will describe an optical design for Maxim and calculate principal optical tolerances. These tolerances offer advantages that make anticipated engineering challenges more soluble and affordable within the limitations of current technology. We also discuss key design tradeoffs that contribute to a preliminary tolerance budget.