The lens that works near the focal plane of the objective lens and can effectively reduce the size of the detector is called a field mirror.
To be precise, the field mirror should work in the image plane of the objective lens. On the one hand: the object plane of the field mirror coincides with the main plane, which has a magnification of 1 from the characteristics of the main plane, so it does not contribute to the magnification of the system; on the other hand, the field mirror should project the diaphragm surface of the objective on the pupil of the eyepiece, which is also the exit pupil for a single lens, so as to ensure that the light emitted from the objective can pass through the eyepiece to the maximum extent. If used in the scanning system, the detector replaces the pupil of the eyepiece.
By wavelength: 1064nm field mirror (also called YAG field mirror), 10.6 micron field mirror
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