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.