The main purposes of a telescope
are:
(a) to gather as much light as possible – this is done by using a large aperture lens
or mirror.
(b) to resolve fine detail – this is also done by using a large aperture lens
or mirror.
(c) to magnify the image of a distant object – this is
done by using a lens or mirror with a long focal length.
Notice that the image formed by this type of refracting telescope is upside down.