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Telescope specifications

To a photographer my telescope is a 1200mm (focal length) f/4.8 mirror lens

To an astronomer it is a 254mm (aperture) f/4.8 Newtonian telescope

Astronomers are more concerned with aperture -
for collecting more light and
for better resolving power

Be careful when buying a telescope: does the mm figure quoted by the salesperson mean aperture or focal length?

Astronomical telescopes are not just about magnifying, especially when it comes to deep sky observing. A large aperture is important both for collecting as much light as possible and for resolving fine details.

For deep sky observing you want a low f-number for maximum light. But for planetary work you want a high f-number: the focal length is greater so you can magnify more without needing extremely short focal length eyepieces. So different kinds of telescope are suitable in these two cases.

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