There are several basic benefits of using telescopes:
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.
