Requirements on ELTs from X-ray astronomy

The requirements on ELTs from the perspective of X-ray astronomy are explored. These requirements will be driven largely by deep X-ray surveys, like those conducted with XMM-Newton and the Chandra X-ray Observatory. Up to the present time, ground-based telescopes have largely kept abreast of the needs arising from deep X-ray surveys, i.e. the current generation of 10m-class telescopes is able to (barely) match the required spectroscopic needs for optical identifications in most cases (up to 70%). There are two X-ray astronomy facilities currently proposed and under study: the European-led X-ray Early Universe Spectroscopic mission (XEUS) and the NASA proposal Constellation-X. The emphasis of both these missions, like XMM-Newton, is X-ray spectroscopy, but they will both perform deep surveys which will need optical follow-up spectroscopy towards the middle or end of the next decade.