What is the purpose of a telescope?

• The main purpose of a telescope is to gather light, i.e. to collect and focus photons. We can think of a telescope then as a "light bucket" - the bigger the bucket, the more photons a telescope can collect.
• In addition, a telescope can allow a more detailed view of the structure of a celestial source or to infer that there are two sources, not just one. In an astronomical image, the smallest angular separation between sources (or structures within the image of a single extended source) is called the ANGULAR RESOLUTION. The size of the smallest "point" source in an image is called the image's SEEING.
• The minimum angular separation of two sources that can be distinguished by a telescope depends on the wavelength of the light being observed and the diameter of the telescope. This angle is called the DIFFRACTION LIMIT