Slumping of x-ray mirror substrates with Corning Eagle XG glass

The precision fabrication of x-ray mirror substrates is crucial to achieving a lightweight, high resolution and large effective area x-ray telescope mirror system for observational astronomy. Ultrafast Laser Stress Figuring (ULSF) is a promising figuring technique, useful even after assembly and coating, but requires a smooth substrate whose figure is accurate to within tens of microns. Moldless slumping is a potential technique to provide these substrates, by avoiding contact with a mold that introduces mid-spatial frequency error. This study investigated the mid- and low-spatial frequency errors introduced by moldless slumping of Corning Eagle XG glass into cylindrical shapes. We varied the temperature and time of slumping on a predesigned frame, afterwards mapping the curved substrate surface using interferometry to assess these spatial frequencies of the surface through Zernike terms, enabling the repeatability from substrate to substrate, and for spatial frequency error correction through ULSF. In this case, we concluded that the curved substrate surfaces are smooth and that the RMS height error over 50 mm aperture is 2.03 microns with a mid-spatial RMS of 0.0075 microns.