The stacked prism lens - new optics for X-ray astronomy

The launch of the first orbiting X-ray telescope in 1978 revolutionised X-ray astronomy.

For the first time X-rays from a celestial source could be focused onto a small and well-shielded detector, and the scientific yield of the mission was not limited by the high levels of background radiation present in orbit. With significantly increased detection sensitivity, a plethora of high-energy sources were resolved on the sky.

X-rays were focussed using grazing-incidence reflection from an arrangement of nested cylindrical mirrors - a technique which persists today.

The large and heavy optics (many 100’s kg) and long focal length (~10 m) challenges spacecraft design and limits the collecting area, which impacts mission sensitivity.This project stands to provide the next quantum leap in X-ray astronomy.

Established micro-fabrication techniques are exploited to enable lightweight X-ray optics with a short (10’s cm) focal length.

The Stacked Prism Lens (SPL) comprises an assembly of millimetre-diameter dielectric disks embedded with prismatic rings. X-rays are brought to a focus as they refract and diffract through the assembly of prisms.

This approach allows a sub-arcsecond resolution X-ray telescope with unprecedentedly large collecting area - paving the way for new discoveries in the high energy universe.

A prototype SPL has been manufactured in-house using grayscale UV lithography and the focussing performance has been studied using X-ray beams generated at a synchrotron light-source facility.

The focussing principle has been demonstrated, but the performance was limited by imperfections in the lithography process, which resulted in imperfect prism geometries.

Funding is requested for the fabrication and testing of second-generation SPLs using nanoimprint lithography, which as well as addressing the fabrication issues encountered previously, is well-suited to the mass-production required by any future space mission, which will require 1000’s of SPLs. At the end of the project, the performance of the optics will be sufficient for application to a Pathfinder mission.

The SPL concept is optimal in the hard X-ray band (>10 keV), so cost-effective tests can be conducted using a stratospheric balloon-flight, which well-matches experience in the group. The conceptual design of this Pathfinder mission is included in the project. The overarching goal is to raise the Technical Readiness Level (TRL) for the SPL from the current value of 3-4 to 6-7.