How Telescope works

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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.

Telescope

Different Types of Telescope

Optical Telescope

Radio Telescope

X-ray Telescope

Gamma Ray Telescope

Optical TelescopeAn 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 TelescopeRefractors 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.

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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.

Refracting Telescope Design

Reflecting TelescopeA 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.

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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.

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.

Reflecting Telescope Design

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 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

Catadioptric Telescope DesignsThe 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

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.

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).

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Radio TelescopeA 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. Man y radio telescopes use a bowl-shaped 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 TelescopeGamma 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

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.

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