Simulations of film stress effects on mirror segments for the Lynx X-ray Observatory concept

Brandon D Chalifoux,Mark L Schattenburg,Ralf K Heilmann,Youwei Yao

doi:10.1117/1.jatis.5.2.021004

Abstract

The Lynx X-ray Observatory concept, under study for the 2020 NASA Decadal Survey, will require a telescope with ∼2 m2 of effective area and a point-spread function (PSF) with ∼0.5-arc sec half-power diameter (HPD) to meet its science goals. This requires extremely accurate thin grazing-incidence mirrors with a reflective x-ray coating. A mirror coating, such as 15-nm-thick iridium, can exhibit stress exceeding 1 GPa, significantly deforming segmented mirrors and blurring the PSF. The film stress and thickness are neither perfectly repeatable nor uniform. We use finite element analysis and ray tracing to quantify the effects of integrated stress inaccuracy, nonrepeatability, nonuniformity, and postmounting stress changes on segmented mirrors. We find that if Lynx uses segmented mirrors, it will likely require extremely small film stress (∼10 MPa) and nonuniformity (<1 % ). We show that realigning mirrors and matching complementary mirror pairs can reduce the HPD from uniform film stress by a factor of 2.3 × and 5 × , respectively. Doubling mirror thickness produces much less than the 4 × HPD reduction that would be expected from a flat mirror. The x-ray astronomy community has developed numerous methods of reducing the PSF blurring from film stress, and Lynx may require several of these in combination to achieve 0.5 arc sec HPD using segmented mirrors.

Highlights

The Lynx X-ray Observatory mission concept is under study for the 2020 NASA Astrophysics Decadal Survey and is expected to enable significant progress in astrophysics and cosmology.[1]
It is well known that segmented mirrors are much more affected by film stress than full-shell mirrors
We have investigated the effects of film stress on the half-power diameter (HPD) of individual mirror shells composed of silicon mirror segments, as well as estimating the HPD from six shells together

Summary

Introduction

The Lynx X-ray Observatory mission concept (or Lynx) is under study for the 2020 NASA Astrophysics Decadal Survey and is expected to enable significant progress in astrophysics and cosmology.[1]. This paper is restricted to the investigation of film stress effects on the shape of segmented silicon mirrors. To produce an x-ray mirror segment, a silicon substrate must be fabricated[5] and coated, typically with a thin metal film, such as iridium[6,7,8,9] or a multilayered coating,[10,11,12] to efficiently reflect x-rays. Coatings are often deposited using magnetron sputtering, usually to maximize x-ray reflectivity, which results in a stressed film that deforms the thin mirror substrates, especially segmented mirrors. This problem is well known to the community; the Lynx Interim Report[13] cites mitigating the effects of coating stress as one of the major technology development goals

Objectives

Methods

Findings

Conclusion