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Dioptrics are simple telescopes using two convex lenses. The first lens has a longer focal length and is bigger in aperture. The second lens has a shorter focal length and serves as an eyepiece. The image focused by the first lens converges at the focal point and then is enlarged by the eyepiece. Focusing is achieved by adjusting the distance between eyepiece and point at which the image is formed.
See List of telescope types.
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