‘Space-qualified’ components for optical systems, as follows: (a) Components lightweighted to less than 20 % ‘equivalent density’ compared with a solid blank of the same aperture and thickness; (b) Raw substrates, processed substrates having surface coatings (single-layer or multi-layer, metallic or dielectric, conducting, semiconducting or insulating) or having protective films; (c) Segments or assemblies of mirrors designed to be assembled in space into an optical system with a collecting aperture equivalent to or larger than a single optic 1 m in diameter; (d) Components manufactured from ‘composite’ materials having a coefficient of linear thermal expansion equal to or less than 5 × 10 – 6 in any coordinate direction;