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The future of xͲray astronomy depends upon development of xͲray telescopes with larger aperture areas (> 1 m 2 ) and finer angular resolution (< 1s). Combined with the special requirements of nested grazingͲincidence optics, the mass and envelope constraints of spaceͲborne telescopes render such advances technologically challenging. Achieving this goal will require precision fabrication, alignment, mounting, and assembly of large areas (>100 m 2 ) of lightweight (у 1 kg m Ͳ2 areal density) highͲquality mirrors-possibly entailing active (inͲspace adjustable) alignment and figure correction. This paper discusses relevant programmatic and technological issues and summarizes progress toward largeͲarea subͲarcsecond xͲray telescopes.

Material Density (g/cm 3 ) CTE (10 -6 / K -1 ) Elastic Modulus GPa Yield Strength MPa Fused Silica 2.2 0.5 72 48* Beryllium 1.8 12 318 240 Al (6061) 2.7 24 69 276 AlSi 2.8 17 90 235