fold mirror

noun

noun

An mirror used to bend an optical path so that it can be made longer than the physical size of the system

To achieve this, a plane mirror, which functions as a fold mirror, is positioned at an angle of 45° with respect to the image formed by the primary-secondary mirrors...1978, D. G. Korsch, Anastigmatic three-mirror telescope, US Patent 4101195 (PDF version), retrieved 27 Dec 2021, page 2:

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