The cylinder approach (actually paraboloid-hyperboloid or Wolter-I optics) is the main design used today and for future telescopes for lower energy X-rays. Focu

The cylinder approach (actually paraboloid-hyperboloid or Wolter-I optics) is the main design used today and for future telescopes for lower energy X-rays. Focusing optics have a big advantage of having a lower level of background compared to a coded mask. Future Wolter-I optics use clever designs for the telescope, such as stacking silicon wafers into modules planned for the ESA Athena observatory [1] or modular lightweight single crystal silicon mirrors planned for future NASA missions [2]. [1] https://www.esa.int/ESA_Multimedia/Images/2019/02/Athena_mirror_module https://www.esa.int/ESA_Multimedia/Images/2019/02/Athena_mir... [2] https://www.nasa.gov/feature/goddard/2019/nasa-s-new-lightweight-x-ray-mirrors-ready-for-try-outs-in-space https://www.nasa.gov/feature/goddard/2019/nasa-s-new-lightwe...