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Reflecting Telescopes vs. Refracting Telescopes

December 3, 2009 1 comment

Telescopes are the window to the universe. They allow the universe to be observed and studied from the vantage point of Earth. Without the amazing insight provided by these instruments, humanity would still be living in naivety of our place in the universe and the universal picture as a whole. Beginning in the 17th century, telescopes have provided great insight into the space around us and continue to provide amazing imagery and data that allow us to understand the universe as a whole in much more definite detail.

Every optic telescope falls in to one of two classifications, either refracting or reflecting. They are classified according to the method that is used to focus the image into the viewing device.  A refracting telescope (refractor) uses lenses to gather and focus light, while a reflecting telescope (reflector) uses a mirror. The refractor telescope gathers a greater amount of light into the lens than is possible to gather with the naked eye. This presents the observer with a brighter, clearer, and magnified image of the object being observed. This is accomplished by focusing the parallel light onto a focal point while the peripheral light is focused onto a focal plane.

Refracting Telescope Optics

The refracting telescope was the first telescope to be invented and used. The first apparitions of the refractor telescope were created in the early 17th century. The inventor was a Dutch lens maker named Hans Lippershey who intended to use the device for military purposes. He applied for the patent for the refractor in 1608. Galileo was the first person credited with applying the use of the telescope to the study of the sky. With this, he was able to see the craters on the moon, the four major moons of Jupiter, and the rings of Saturn.

There are several problems with the refracting telescope. First and foremost, refractors lend themselves to what are called chromatic and spherical aberrations. Chromatic aberration occurs when a lens fails to focus all the color to the same focal point. This defect shows as a fringe of color along the boundaries that separate dark and bright parts of the image. This was dealt with originally by increasing the focal length of the lens which led to extremely long telescopes. Spherical aberration occurs due to the increased refraction of light rays when they strike a lens near its edge. This causes the outer rays of light to be focused more tightly away from the focal point which causes the image to be imperfect. Another issue with refracting telescopes is lens sagging. This occurs in telescopes with large lenses. This is a result of gravity deforming the glass since the lens can only be held in place on the edges. This also produces an imperfect image. Further, there is a problem with lenses themselves. Lenses are flawed with small air bubbles trapped within the glass, which is also opaque to certain wavelengths of light. Even visible light is dimmed by the reflection and absorption when the light passes through the glass.

Spherical Aberration

Due to the issues with the refracting telescope, ideas for developing a telescope that used curved mirrors instead lenses began to circulate in the 17th century. This idea had been introduced in the 11th century by Alhazen in his widely read work, Book of Optics. Although the idea had been present for several centuries, no practical application of the theory occurred until 1673 when Robert Hooke created the first working reflecting telescope. Isaac Newton has been credited with the creation of the first practical reflecting telescope using a spherically ground metal primary mirror and a small diagonal mirror. This design is now known as the Newtonian telescope, and is still popular for amateur telescope builders. Although the theoretical advantages of the reflector design compensated for many of the disadvantages of the refractor, it took over 100 years for the reflector to become popular due to the poor performance of speculum metal which was being used as the reflective surface at the time. With the perfection of parabolic mirror fabrication of the 18th century, silver coated mirrors of the 19th century, and long-lasting aluminum coated mirrors of the 20th century the reflecting mirror has become the telescope of choice for astronomers world-wide.

Reflecting Telescope Optics

A reflecting telescope uses a combination of curved mirrors that reflect light and form an image into a viewing device. A curved primary mirror is the basic optical element and creates an image at the focal plane. A viewing device such as film or a digital sensor may be located at the focal plane to record the image or an eyepiece might be present for viewing the image. The mirror in most modern telescopes is composed of solid glass that has been ground into a parabolic or spherical shape with a thin layer of aluminum deposited on the front which provides a highly reflective metal surface to reflect the images. The light from the image enters the end of the tube and reflects off the primary mirror, to the secondary mirror, and finally to the viewing device. Reflectors are not only useful for measuring visible light, but they can also detect shorter and longer wavelengths (e.g. ultraviolet and infrared light).

Mirrors within the reflecting telescope eliminate the risk of chromatic aberration, but this type of telescope may still produce other types of aberrations, namely spherical. This was the design flaw within the Hubble Space Telescope’s mirrors originally. There are other types of aberrations as well, but these have been corrected with more advance telescope design. The design most common in professional telescopes is the Ritchey-Chrétien telescope, which is a specialized Cassegrain telescope that utilizes two hyperbolic mirrors instead of a primary parabolic that is common in the standard Cassegrain design.

As technology continues to advance, the future for optics is bright. With further advance in the science of optics, we will continue to garner a greater understanding of the universal picture as a whole and where our place is within that ever expanding picture.

References

http://csep10.phys.utk.edu/astr162/lect/light/refracting.html

http://www.enchantedlearning.com/inventors/page/t/telescope.shtml

http://abyss.uoregon.edu/~js/glossary/reflecting_telescope.html

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