Download An optical telescope is a telescope that gathers and focuses light

An optical telescope is a telescope that gathers and focuses light, mainly from the visible part of
the electromagnetic spectrum, to create a magnified image for direct view, or to make
a photograph, or to collect data through electronic image sensors.
There are three primary types of optical telescope: refractors, which use lenses (dioptrics),
reflectors, which use mirrors (catoptrics), catadioptric telescopes, which combine lenses and
mirrors
A telescope's light gathering power and ability to resolve small detail is directly related to the
diameter (or aperture) of its objective (the primary lens or mirror that collects and focuses the
light). The larger the objective, the more light the telescope collects and the finer detail it
resolves.
People use telescopes and binoculars for activities such as observational
astronomy, ornithology, pilotage and reconnaissance, and watching sports or performance arts.
The basic scheme is that the primary light-gathering element focuses that light from the distant
object to a focal plane where it forms a real image. This image may be recorded or viewed
through an eyepiece, which acts like a magnifying glass. The eye then sees an
inverted magnified virtual image of the object.
Inverted images
Most telescope designs produce an inverted image at the focal plane; these are referred to
as inverting telescopes. In fact, the image is both turned upside down and reversed left to right,
so that altogether it is rotated by 180 degrees from the object orientation. In astronomical
telescopes the rotated view is normally not corrected, since it does not affect how the telescope is
used. However, a mirror diagonal is often used to place the eyepiece in a more convenient
viewing location, and in that case the image is erect, but still reversed left to right. In terrestrial
telescopes, such as spotting scopes, monoculars and binoculars, prisms or a relay lens between
objective and eyepiece are used to correct the image orientation. There are telescope designs that
do not present an inverted image such as the Galilean refractor and the Gregorian reflector.
These are referred to as erecting telescopes.
Design variants
Many types of telescope fold or divert the optical path with secondary or tertiary mirrors. These
may be integral part of the optical design or may simply be used to place the eyepiece or detector
at a more convenient position. Telescope designs may also use specially designed additional
lenses or mirrors to improve image quality over a larger field of view.
Design specifications relate to the characteristics of the telescope and how it performs optically.
Several properties of the specifications may change with the equipment or accessories used with
the telescope; such as Barlow lenses, star diagonals and eyepieces. These interchangeable
accessories don't alter the specifications of the telescope, however they alter the way the
telescopes properties function, typically magnification, angular resolution and field of view.
Light-gathering power
The light-gathering power of an optical telescope, also referred to as aperture gain is the ability
of a telescope to collect a lot more light than the human eye. Its light-gathering power is
probably its most important feature. The telescope acts as a light bucket, collecting all of the
photons that come down on it from a faraway object, where a larger bucket catches
more photons resulting in more received light in a given time period, effectively brightening the
image. This is why the pupils of your eyes enlarge at night so that more light reaches the retinas.
The gathering power compared against a human eye is the squared result of the division of
the aperture over the observer's pupil diameter with an average adult having a pupil diameter of
7mm. Younger persons host larger diameters, typically said to be 9mm, as the diameter of the
pupil decreases with age.
Magnification
The magnification through a telescope magnifies a viewing object while limiting the field of
view. Magnification is often misleading as the optical power of the telescope; its characteristic is
the most misunderstood term used to describe the observable world. At higher magnifications the
image quality significantly reduces, usage of a Barlow lens—which increases the effective focal
length of an optical system—multiplies image quality reduction.
Similar minor effects may be present when using star diagonals, as light travels through a
multitude of lenses that increase or decrease effective focal length. The quality of the image
generally depends on the quality of the optics (lenses) and viewing conditions—not on
magnification.
Magnification itself is limited by optical characteristics. With any telescope or microscope,
beyond a practical maximum magnification, the image looks bigger but shows no more detail. It
occurs when the finest detail the instrument can resolve is magnified to match the finest detail
the eye can see. Magnification beyond this maximum is sometimes called empty magnification.
To get the most detail out of a telescope, it is critical to choose the right magnification for the
object being observed. Some objects appear best at low power, some at high power, and many at
a moderate magnification. There are two values for magnification, a minimum and maximum. A
wider field of view eyepiece may be used to keep the same eyepiece focal length whilst
providing the same magnification through the telescope. For a good quality telescope operating
in good atmospheric conditions, the maximum usable magnification is limited by diffraction.