Toward Large-Area Sub-Arcsecond X-Ray Telescopes II

In order to advance significantly scientific objectives, future x-ray astronomy missions will likely call for x-ray telescopes with large aperture areas (approx. = 3 sq m) and fine angular resolution (approx. = 1"). Achieving such performance is programmatically and technologically challenging due to the mass and envelope constraints of space-borne telescopes and to the need for densely nested grazing-incidence optics. Such an x-ray telescope will require precision fabrication, alignment, mounting, and assembly of large areas (approx. = 600 sq m) of lightweight (approx. = 2 kg/sq m areal density) high-quality mirrors, at an acceptable cost (approx. = 1 M$/sq m of mirror surface area). This paper reviews relevant programmatic and technological issues, as well as possible approaches for addressing these issues-including direct fabrication of monocrystalline silicon mirrors, active (in-space adjustable) figure correction of replicated mirrors, static post-fabrication correction using ion implantation, differential erosion or deposition, and coating-stress manipulation of thin substrates

Toward Large-Area Sub-Arcsecond X-Ray Telescopes

The future of x-ray astronomy depends upon development of x-ray telescopes with larger aperture areas (>1 sq m) and finer angular resolution(100 sq m) of lightweight (1 kg/sq m areal density) high quality mirrors-possibly entailing active (in-space adjustable) alignment and figure correction. This paper discusses relevant programmatic and technological issues and summarizes progress toward large area sub-arcsecond x-ray telescopes. Key words: X-ray telescopes, x-ray optics, active optics, electroactive devices, silicon mirrors, differential deposition, ion implantation

Ion implantation for figure correction of high-resolution x-ray telescope mirrors

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 99-103).Fabricating mirrors for future high-resolution, large-aperture x-ray telescopes continues to challenge the x-ray astronomy instrumentation community. Building a large-aperture telescope requires thin, lightweight mirrors; due to the very low stiffness of thin mirrors, these are difficult to fabricate, coat, and mount, while achieving and maintaining the required surface figure accuracy. Ion implantation offers a potential solution for fabricating high-accuracy mirrors, by providing capability for fine figure correction of mirror substrates. Ion implantation causes local sub-surface stress that is a function of ion fluence, which results in changes in curvature. In principle, implanting to cause the right amount of stress in the right locations on a substrate would allow correction of figure errors in the substrate. In addition, x-ray telescope mirrors must be mechanically stable over decades, and have low surface roughness. In this work, high-energy (150 keV - 1.5 MeV) ions were implanted into silicon and glass substrates, and the implications on figure correction, surface roughness, and surface figure stability studied. Changes in curvature resulting from sub-surface stress were measured, to understand the magnitude of stress that can be applied, and the dependence of stress on ion fluence. Figure corrections of flat silicon substrates were made. To investigate effects on surface roughness, x-ray reflectivity studies were conducted on implanted samples. Stability in surface figure was studied using thermal cycling, and measurements after 1 year of storage. Finally, simulations were conducted for correction of conical substrates similar to what would be used in future x-ray observatories. The results presented in this work suggest that ion implantation is indeed a feasible method of figure correction of mirrors for high-resolution, large-aperture x-ray telescopes.by Brandon D. Chalifoux.S.M

Figure correction of thin plate and shell substrates using stress generated by ion implantation

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2019Cataloged from PDF version of thesis.Includes bibliographical references (pages 213-225).I developed a method to correct height errors in thin substrates. Accurately-figured thin plates and shallow shells are important for large-area space telescopes and for the semiconductor industry. Thin substrates can be bent into the desired shape, for example by tens of microns on a 100 mm diameter substrate, by applying stress to their surface. Ion implantation is one of many possible approaches to applying a controlled stress field to the surface of a substrate. I develop analytical and numerical approaches to calculating stress fields that generate a desired deformation field in thin flat plates and shallow shells. Equibiaxial stress alone is insufficient to generate some deformation fields exactly, making non-equibiaxial stress components critical for figure correction, in general. I experimentally demonstrate the generation of non-equibiaxial stress using ion implantation in glass substrates, by angling the ion beam. To generate a desired deformation field, I developed a process to calculate, and built a system to implement, implantation recipes (i.e. the doses and ion beam angles at each substrate position) on 100 mm glass wafers. Using this system, I demonstrate a 2-4 x improvement in height and slope errors of glass wafers using ion implantation. I also demonstrate the use of ion implantation to compensate for the deformation caused by stress in thin film coatings. Thin films, such as those used for mirror coatings, often have non-uniform equibiaxial stress fields. I developed a process to restore the figure of 100 mm silicon wafers that have been deformed by thin metal films, by applying the same equibiaxial stress field on the other side of the wafer using ion implantation. I demonstrate a 20 x reduction of the coating-induced deformation.by Brandon D. Chalifoux.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Mechanical Engineerin

Ultrafast laser stress figuring for accurate deformation of thin mirrors

Fabricating freeform mirrors relies on accurate optical figuring processes capable of arbitrarily modifying low-spatial frequency height without creating higher-spatial frequency errors. We present a scalable process to accurately figure thin mirrors using stress generated by a focused ultrafast laser. We applied ultrafast laser stress figuring (ULSF) to four thin fused silica mirrors to correct them to 10-20 nm RMS over 28 Zernike terms, in 2-3 iterations, without significantly affecting higher-frequency errors. We measured the mirrors over a month and found that dielectric-coated mirrors were stable but stability of aluminum-coated mirrors was inconclusive. The accuracy and throughput for ULSF is on par with existing deterministic figuring processes, yet ULSF doesn't significantly affect mid-spatial frequency errors, can be applied after mirror coating, and can scale to higher throughput using mature laser processing technologies. ULSF offers new potential to rapidly and accurately shape freeform mirrors