An optical telescope collects light from distant objects and forms an image with an optical lens, a mirror, or a combination of both. The aperture controls both collected radiant power and the ideal diffraction limit; the focal length sets how an incoming angular separation becomes a physical separation in the focal plane.
For small angles, stars separated by angle form images separated by approximately at focal length . A larger magnification can make this separation easier to inspect, but cannot undo overlapping point spread functions. Atmospheric turbulence, imperfect focus and tracking motion can broaden them further. A detector at the focus records the image; an eyepiece instead presents it to the eye.
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