Assembly of thin gratings for soft x-ray telescopes

Diffraction gratings used in space telescopes are desired to provide high diffraction efficiency and large collecting area while maintaining minimal mass to meet cost limitations. As a result, the method of assembling such gratings into modules such that all requirements are met becomes critical. We report on the development of a new assembly scheme that densely stacks thin reflection grating substrates using precision spacers. A custom-designed, optically-polished vacuum chuck is used to constrain the substrates for grating surface metrology during assembly. This rigid subassembly composed of the vacuum chuck and grating is manipulated in space until the grating surface final angular and lateral positions are obtained, at which point the grating is transported from the chuck onto the spacers and glued in that final position. This method not only precisely aligns the gratings with respect to each other, but also improves the overall surface flatness of the substrates, since they are constrained by a flat vacuum chuck within the assembly process. This helps reduce the tolerances on the substrate shaping methods followed prior to assembly.