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Posts Tagged ‘optics’

Ten Random Astromony Questions

January 21, 2010 Leave a comment

1. What are the steps in the scientific method?

The scientific method is the method used to explore observations and answer questions within the scientific community. It is a step by step process that has evolved since the earliest days of the analysis of observation. The first step in the scientific method is to ask a question about something that is observed. After a question is proposed, one must research in order to find the most appropriate process to find the answer and to ensure that mistakes that have been documented in the past are not repeated. Next, the researcher must propose a hypothesis, which is an educated guess constructed as an if-then statement, which can be easily measured and interpreted. Then the researcher must plan and implement a scientific experiment in order to test the hypothesis. After the experiment is concluded, the researcher must interpret the measurements and draw conclusions which either support or oppose the hypothesis. If the hypothesis is proven false, a new hypothesis must be formulated and the process continues from there; however, if the hypothesis is supported and methods are repeatable, the researcher must communicate his results to be verified by the rest of the scientific community.

http://www.sciencebuddies.org/science-fair-projects/project_scientific_method.shtml

2.   How is a light year defined?

A light year a unit a measure used in astronomy due to the vast distances between astronomical bodies. Our Sun, for example, is approximately 150 million km away from the Earth. It is impractical to define such large distances by miles or kilometers, so scientists developed several different units of measure for astronomy. One such measure is the light year. It is defined as the distance that light travels in one year. Light travels at 300,000 kilometers per second. So in a year, light travels 9,460,800,000,000 kilometers. What this means for astronomers is that if a star is 100 light years away, the light that we are seeing from the star represents what the star looked like 100 years ago not what it looks like at the present time.

http://www.howstuffworks.com/question94.htm

3.  Describe what happens during the two kinds of eclipses?

There are two categories of eclipses. These are solar and lunar. In a solar eclipse the moon passes directly between the Sun and the Earth obstructing the path of the Sun’s light to the Earth. Whether or not the viewer sees a partial or total eclipse depends on what part of the moon’s shadow falls on the Earth. The total eclipse in only visible in the umbra and this part of the shadow is very small on the Earth. A partial eclipse is observable in the penumbral shadow which covers a larger part of the Earth’s surface. The second eclipse category is called a lunar eclipse. This phenomenon occurs when the Earth passes between the Sun and the Moon during a full moon and the moon passes with in the umbral shadow of the Earth.

http://www.mreclipse.com/Special/SEprimer.html

http://www.mreclipse.com/Special/LEprimer.html

4. What is surface gravity?

Surface gravity is defined as the gravitational acceleration on the surface of an astronomical object such as a planet or a star. It is measured in units of acceleration, which is meters per second squared. Each astronomical body has a unique surface gravity which is determined by the product of the gravitational constant, G, and the mass of the object divided by radius of the object squared. The relative surface gravity of the Earth is 9.81 m/s squared. This means, that the gravitational pull of Earth exerts enough force to pull every object that is caught in its gravitational field toward itself at a speed of 9.81 m/s squared. Further, two objects that are accelerating toward the Earth’s surface will do so at this speed barring any outside interference. This outside interference is measured by multiplying the gravitational constant or G. For example, an F-16 fighter can withstand up to nine Gs. Within the equation, the number nine becomes the coefficient to measure the final modified surface gravity when taking into account the outside interference.

5. What is the difference between reflecting and refracting telescopes?

Every optic telescope falls in to one of two classifications, either refracting or reflecting. The telescopes are classified according to the method that they use to focus the image into the viewing device.  A refracting telescope uses lenses to gather and focus light, while a reflecting telescope 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. 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).

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

6. What are the Oort cloud and Kuiper Belt?

The Kuiper belt is a disk shaped region of icy debris about 30-50 AU from the Sun, which is outside the orbit of Neptune. It is similar in organization to the asteroid belt although it is far larger being 20 times as wide and 20-200 times as massive. Although similar in organization, the make up of the individual bodies is markedly different. The asteroid belt is similar to terrestrial planets being made mostly of rock and metal while the Kuiper Belt Objects (KBOs) share a similarity with the Jovian planets being made principally of frozen volatiles such as methane, ammonia, and water. The Kuiper belt is also the home of the dwarf planets Pluto, Haumea, and Makemake. Another organized structure of astronomical bodies has been theorized to exist called the Oort cloud, named for Jan Oort who originally theorized its existence in 1950. Light is so scarce in the far reaches of the proposed solar system that it is extremely difficult to identify the existence the cloud. The main evidence for the belt is the passage of long-period comets that pass through the inner solar system only once. The Oort cloud is home to astronomical bodies that vary in size from 50km to the size of Pluto. It has been theorized that there might be larger bodies within the Oort cloud as well, but no conclusive proof has yet been presented to confirm or deny this presumption.

http://solarsystem.nasa.gov/planets/profile.cfm?Object=OortCloud

7. What are the advantages of a telescope in space?

The main advantage of using a telescope that is based in space rather than on Earth is simply that the space telescope does not have to compete with the Earth’s atmosphere for light. The Earth’s atmosphere can distort the imaging ability of the earthbound telescope. It also blocks x-ray and infrared light so that those spectrums cannot be studied from Earth. Also, a telescope based in space does not have to deal with light pollution as do observatories on Earth.

8. What is a dwarf planet?

A dwarf planed it s a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape, has not cleared the neighborhood around its orbit, and is not a satellite. The classification was created for objects that are not quite large enough to be considered planets, but are larger than asteroids. There are currently five celestial bodies that are defined as dwarf planets.

9. What is meant by the resolution of a telescope?

The resolution of a telescope is defined as how clearly a telescope is able to view objects. The higher resolution yields a better ability to make out fine details in the celestial bodies being observed. Resolution is based highly on the quality of the optical components within the telescope, but the aperture, the hole that the light enters the telescope, of the telescope is also critical when dealing with resolution. For this reason, astronomers build bigger telescopes to allow more light in the aperture, increase the resolution, and create a finer more precise picture.

10. What is the difference between the geocentric and heliocentric model of the solar system?

The difference between the geocentric and heliocentric models of the universe hinges on Earth’s role in universal organization. The earliest thinkers believed that the Earth was the center of the universe and all things revolved around it, which was the central idea in the geocentric model of universal organization. This was refuted in 1530 when Copernicus presented a mathematical model in his book De Revolutionibus. Copernicus’ theory upset the religious order of the time so his work was refuted and suppressed, but eventually, with the invention of the telescope, Copernicus’ theory of the heliocentric  model of universal organization became provable scientific fact.

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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