The magnification beyond which nothing new is revealed: about twice the aperture in millimetres, so 400× for a 200 mm telescope. That limit assumes perfect optics: with real-world mirrors and lenses and imperfect collimation, the image often softens from about 1× per millimetre (200× for a 200 mm). And the steadiness of the air usually caps it at 150× to 250×, whatever the instrument. The “600×” printed on boxes of small telescopes is therefore misleading.
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Related terms
- Magnification
How many times the instrument enlarges the object: telescope focal length divided by eyepiece focal length. A 1,000 mm telescope with a 10 mm eyepiece magnifies 100 times (100×). More is not always better: past a limit the image becomes dim and blurred.
- Seeing
The restlessness of the atmosphere that makes images dance and boil; it is also what makes stars twinkle. It is measured in arcseconds (the size of a star in a photo) or on scales from 1 to 10 (Pickering) or I to V (Antoniadi). It limits high magnification far more often than the instrument does.
- Exit pupil
The small disc of light that leaves the eyepiece and enters the eye; its diameter is the aperture divided by the magnification. A 200 mm telescope at 40× gives a 5 mm exit pupil. Between 5 and 7 mm the image is at its brightest (nebulae); around 0.5 to 1 mm you reach the maximum useful magnification.
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