At high magnifications of the visual telescope, the contrast of details of extended objects decreases, tremors caused by atmospheric turbulence become noticeable, and diffraction phenomena appear. Experiments have shown that it is inappropriate to use magnifications larger than T = 1.4D, where D is the diameter of the telescope objective, expressed in millimeters.
The minimum magnification is the so-called equoplastic magnification, in which the diameter of the exit pupil of the eyepiece is equal to the diameter of the pupil of the eye. At night, d = 8 mm, hence rmin = D / d.
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