Grazing angle mirrors focus only a small fraction of the X-ray photons
incident on them. Thus, to increase the effective collecting area
(thereby providing more X-rays to the final image), X-ray mirrors are
nested inside one another. The X-rays then enter into the concentric
annuli and are focused by the mirrors to the detector sitting behind
This photograph by Leon van Speybroeck shows technicians working on the Einstein Observatory's
nested X-ray mirrors before launch.