How Telescope works
WENJUN TAO
BOYUAN LIN
YUAN YANG
JIAYIN WANG
JIAHUI YUAN
Telescope
A telescope is an instrument that aids in
the observation of remote objects by
collecting electromagnetic radiation (such as
visible light). The first known practical
telescopes were invented in the Netherlands
at the beginning of the 1600s (the 17th
century), using glass lenses. They found use
in terrestrial applications and astronomy.
Different Types of Telescope
Optical Telescope
Radio Telescope
X-ray Telescope
Gamma Ray Telescope
Optical Telescope
An optical telescope is a
telescope which is used to
gather and focus light mainly
from the visible part of the
electromagnetic spectrum
for directly viewing a
magnified image for making
a photograph, or collecting
data through electronic
image sensors.
Three primary types of
optical telescope
Refractors: which use lenses (dioptrics)
Reflectors: which use mirrors (catoptrics)
Catadioptric telescopes: which use both
lenses and mirrors in combination.
BACK
Refracting Telescope
Refractors were the earliest type of
optical telescopes which uses a lens
as its objective to form an image
(also referred to a dioptric
telescope). The refracting telescope
design was originally used in spy
glasses and astronomical telescopes
but is also used for long focus
camera lenses. These telescopes are
called refracting
telescopes or refractors since the
image was formed by the bending of
light, or refraction.
Back
Refracting Telescope Design
A simple refractor, or refracting telescope is a hollow tube which uses a
primary lens at its opening to diffract, or change the path of incoming
light waves. This primary lens is called the “objective lens” and is used
to collect more light than the human eye. When light passes through the
objective lens, it is bent – or refracted. Light waves that enter on a
parallel path converge, or meet together at a focal point. Light waves
which enter at an angle converge on the focal plane. It is the
combination of both which form an image that is further refracted and
magnified by a secondary lens called the eyepiece. Refracting telescopes
can come in many different configurations to correct for image
orientation and types of aberration.
Reflecting Telescope
A reflecting telescope (also
called a reflector) is an optical
telescope which uses a single or
combination of curved mirrors
that reflect light and form an
image. It is a design that allows
for very large diameter
objectives. Almost all of the
major telescopes used in
astronomy research are
reflectors.
24 inch convertible Newtonian/Cassegrain reflecting telescope
on display at the Franklin Institute.
Reflecting telescopes come in many design variations and may
employ extra optical elements to improve image quality or
place the image in a mechanically advantageous position.
Back
Reflecting Telescope Design
A reflecting telescope uses an arrangement of one or more curved
mirrors to gather light and return it along an optical path to a point of
focus. The most critical element of this type of telescope is the major
light gathering source – the primary mirror. Light strikes the parabolic,
reflective surface of the primary and returns to a point of focus called
the focal plane. Because each spherical or parabolic shaped primary
mirror is slightly different, the distance the light needs to travel to
achieve focus is called the focal length. At its focus point, the image (in
a simple reflector telescope) is collected on another mirror surface
called the secondary. The secondary mirror is then aimed towards the
viewer who uses a series of lenses called an eyepiece to magnify the
image and send it to the eye.
Several Designs for
Reflecting Telescope
The Gregorian telescope, described by James
Gregory in his 1663 book Optica Promota, employs
a concave secondary mirror that reflects the image
back through a hole in the primary mirror. This
produces an upright image, useful for terrestrial
observations.
The Newtonian telescope was the first successful
reflecting telescope, completed by Isaac Newton in 1668.
It usually has a paraboloid primary mirror but at focal ratios
of f/8 or longer a spherical primary mirror can be sufficient
for high visual resolution. A flat secondary mirror reflects
the light to a focal plane at the side of the top of the
telescope tube.
The Cassegrain telescope was first published in an
1672 design attributed to Laurent Cassegrain. It has a
parabolic primary mirror, and a hyperbolic secondary
mirror that reflects the light back down through a hole in
the primary. Folding and diverging effect of the
secondary creates a telescope with a long focal length
while having a short tube length.
Catadioptric Telescope
Catadioptric telescopes are
optical telescopes that
combine specifically shaped
mirrors and lenses to form an
image. This is usually done so
that the telescope can have an
overall greater degree of error
correction than their all lens or
mirror counterparts with a
consequently wider aberration
free field of view.
Catadioptric Telescope Designs
The first compound telescope was made by German
astronomer Bernhard Schmidt in 1930. Its optical components
are an easy-to-make spherical primary mirror and an
aspherical correcting lens located at the center of curvature of
the primary mirror. It had a glass corrector plate in the front
of the telescope to remove spherical aberration. The telescope
was used primarily for photography, and the film or other
detector is placed inside the camera, at the prime focus. The
design is noted for allowing very fast focal ratios, while
controlling coma and astigmatism.
Today, the Schmidt-Cassegrain design, which was invented in the
1960s, is the most popular type of telescope. This design uses
aspherical primary mirror and a Schmidt corrector plate to correct
for spherical aberration. In this Cassegrain configuration
the convex secondary mirror acts as a field flattener and relays the image
through the perforated primary mirror to a final focal plane located behind
the primary. Some designs include additional optical elements (such as field
flatteners) near the focal plane.
Photographic
Catadioptric lenses
Various types of catadioptric systems are
also used in camera lenses known
alternatively as catadioptric
lenses(CATs), reflex lenses, or mirror
lenses. These lenses use some form of the
cassegrain design which greatly reduces
the physical length of the optical assembly,
partly by folding the optical path, but
mostly through the telephoto effect of the
convex secondary mirror which multiplies
the focal length many times (up to 4 to 5
times).
However, Catadioptric lenses have several drawbacks. The fact that they have a central obstruction
means they cannot use an adjustable diaphragm to control light transmission.
Back
Radio Telescope
A radio telescope is a form of
directional radio antenna used in radio
astronomy. The same types of antennas
are also used in tracking and collecting
data from satellites and space probes.
Many radio telescopes use a bowlshaped reflector called a dish to collect
radio waves from space. The reflector
focuses the waves onto an antenna that
changes them into electric signals. A
radio receiver amplifies these signals and
records their strength at different
frequencies and from different
directions.
The information is analyzed by a
computer to draw a picture of the source
of the radio waves or to analyze the
chemicals found in the source.
Reber's first "dish" radio telescope Wheaton, IL 1937
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X-ray Telescope
An X-ray telescope (XRT)
is a telescope that is designed
to observe remote objects in
the X-ray spectrum. In order
to get above the Earth's
atmosphere, which is opaque
to X-rays, X-ray telescopes
must be mounted on high
altitude rockets or artificial
satellites.
X-ray telescopes collect the X-rays that are emitted from the
sun, stars, and super novas in space using a series of curved lenses
and an electronic eye. This technology permits astronomers to
produce an image of these celestial bodies which can be studied.
BACK
Gamma Ray Telescope
Gamma ray scopes use a technique known
as pair production. When a gamma ray, which is
pure energy, slams into a layer of tungsten in
the detector,
it can create a pair of subatomic particles (an
electron and its antimatter counterpart, a
positron).
The direction of the incoming gamma ray is
determined by projecting the direction of these
particles back to their source using several
layers of high-precision silicon tracking
detectors. A separate detector, called a
calorimeter, absorbs and measures the energy
of the particles.
Since the energy of the particles created
depends on the energy of the original gamma
ray, counting up the total energy determines
the energy of that gamma ray.
VERITAS - a major ground-based gamma-ray
observatory
Insight
The use of telescope distributes to the development of physics ,
astronomy and many other parts of science. With the development
of telescope, our view of the universe has been enlarged. Compared to
the ancient years, human’s knowledge of the universe has been widely
grown after telescope was invented. We discover the black whole,
determine the orbit of the planet located in the solar system and know
more facts about the galaxy. According to the picture captured by the
telescope, we know much more about stars, and we predict how the
earth change for next several decades by analyzing the data of the
other stars. Human get a chance to better understand the universe and
be able to reach the universe closer. With the guarantee of the pictures
that telescope received, human has successfully sent satellites to the
universe so that we can predict the weather which makes our daily life
much more convenient. Moreover, the history of the development of the
telescope also insight us that science is about trying, trying to know the
world. We could never reach out as far as we have in the Universe
without our telescopes.