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
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.