fold flat

fold flat (plural fold flats)

1. A fold mirror that is a "flat", i.e. a plane mirror
   • 2007 September, Hadjimichaela et al., “An Alignment and Integration Technique for Mirror Segment Pairs on the Constellation X Telescope”, in Proceedings of SPIE, volume 6688, →DOI, retrieved 27 December 2021, pages 66881D-4:

     By reflecting the incident beam off of a rotating fold flat located on the optical axis of mirror segment..., we can conveniently image a range of azimuthal positions.

• fold sphere