The design concept of the mirrors for the ESA high throughput x-ray spectroscopy mission XMM is characterized by thin (< 1 mm) graphite epoxy shells as mirror structure and a replication technique to produce the mirror surface. The mirror microroughness is specified to σ < 0.5 nm (r.m.$), profile errors shall not exceed + 2.5 arc sec. An ESA research study has been performed by Zeiss and subcontractors (Dornier GmbH, Max Planck Institut fUr Extraterrestrische Physik/ Garching (MPE), University of Leicester, X-ray Astronomical Group, Dept. of Physics) to clarify whether graphite epoxy mirror structures are able to meet the tight XMM mirror figuring and surface requirements and which measures have to be taken to overcome the typical problems associated with composite materials used as mirror substrates:• Print-through of fibres and inner structure •Short term/long term stability• Water absorption and outgassing. The starting conditions and progress during development are described. The present status of optical and x-ray performance of replicated flat graphite epoxy mirrors is reported and aspects for further improvement of graphite epoxy mirrors will be discussed.
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