X-ray Telescopes

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 them.

This photograph by Leon van Speybroeck shows technicians working on the Einstein Observatory's nested X-ray mirrors before launch.