"fold mirror" meaning in English

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          "ref": "1978, D. G. Korsch, Anastigmatic three-mirror telescope, US Patent 4101195 (PDF version), retrieved 2021-12-27, page 2:",
          "text": "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...",
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      "glosses": [
        "An mirror used to bend an optical path so that it can be made longer than the physical size of the system"
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  "word": "fold mirror"
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  "word": "fold mirror"
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