1. The aperture of a telescope is several times larger than the aperture of a human eye so objects that are not normally seen with the unaided eye can be seen. The Light- gathering power of a telescope is proportional to the area of its aperture and hence depends on the square of the radius of the mirror. Therefore a 20 cm diameter telescope collects four times more photons than a 10 cm diameter telescope.

2. A telescope can be equipped to record light over a long period of time, by using photographic film or electronic detectors such as photometers or CCD detectors while the eye has no capability to store light. A long-exposure photograph taken through a telescope reveals objects too faint to be seen with the eye, even by looking through the same telescope.

3. A third major advantage of large telescopes is that they have superior resolution, the ability to discern fine detail. Small resolution is good. The resolution is directly proportional to the wavelength being observed and inversely proportional to the diameter of the telescope.An ampirical law for resolving power could be given as:

where d is in arcseconds, and D should have the same units, Angstroms or meters.

Resolution can be greatly increased by using telescopes in pairs or groups. Interferometry is a technique that makes it possible to measure the precise direction toward a source of radiation by analyzing the interference of waves arriving at separate telescopes.

One technical problem must be overcome in telescope construction: The Earth rotates and if nothing is done to compensate for this, a star quickly moves out of the field of view. To avoid this problem, telescopes are mounted so that they can be moved by a motor in the direction opposite the Earth’s rotation, keeping a target centered in the field of view.