Top Question about telescope
About light, color, and optics
1. What is the electromagnetic spectrum?
The electromagnetic spectrum consists of all the different wavelengths of light (also known as electromagnetic radiation), including visible light, radio waves, and X-rays. Light is a continuum of wavelengths. We name regions of the spectrum a bit arbitrarily, but the names give us a general sense of the energy; for example, infrared light has shorter wavelengths than radio light. The only region in the entire electromagnetic spectrum that our eyes are sensitive to is the visible region.
Gamma rays have the shortest wavelengths, < 0.001 nanometers (about the size of an atomic nucleus). This is the most energetic region of the electromagnetic spectrum. Gamma rays can result from high-energy reactions taking place in objects such as pulsars, quasars, and black holes.
X-rays range in wavelength from 0.001–10 nanometers (about the size of an atom). They are generated, for example, by superheated gas from exploding stars and quasars, where temperatures can reach more than 10 million degrees Celsius.
Ultraviolet radiation has wavelengths of 10–400 nanometers (about the size of a virus). Young, hot stars produce a lot of ultraviolet light that bathes interstellar space with this energy and causes nearby gas to glow as nebulae.
Visible light covers the range of wavelengths from 400–700 nanometers (from the size of a molecule to a protozoan). Our Sun emits most of its radiation in the visible range, which our eyes perceive as the colors of a rainbow. Our eyes are sensitive only to this small portion of the electromagnetic spectrum.
Infrared wavelengths span from 700 nanometers – 1 millimeter (from the width of a pinpoint to the size of small plant seeds). Infrared radiation is associated with heat. At a temperature of 37 degrees Celsius, our bodies radiate with a peak intensity near 900 nanometers.
Microwaves have wavelengths between 1 millimeter and 1 meter. The radiation resulting from the "big bang" is detected in the microwave region and is often referred to as the microwave background radiation. Microwave ovens use a specific microwave frequency to cause water molecules to absorb the energy and thus heat food.
Radio waves are longer than 1 meter. Because these are the longest waves, they have the lowest energy and are associated with the lowest temperatures. Radio wavelengths are found many places: in a variety of stars (especially binary, X-ray, and other active stars), in interstellar clouds, in pulsars, and in the cool remnants of supernova explosions. Radio stations use radio wavelengths to send signals that our radios then translate into sound.
2. How does light travel (and what causes light to change directions)?
Light travels in straight lines unless something causes it to change direction. When light encounters matter, it can change its direction through a process of reflection, refraction, or diffraction. Telescopes operate on the principals of either reflection or refraction. These topics are discussed in the questions listed below. Diffraction is the bending of a wave around an obstacle. Buildings in a city cause sound waves to diffract, making it difficult to locate the source of sirens. A diffraction grating can be used to disperse light into a rainbow of colors.
3. What is refraction?
Refraction is the bending of light as it enters a new material. The degree of bending depends on the differences in the densities of the materials and the angle at which the light encounters the matter. When the matter is encountered head on (normal to the surface), the light will not refract as it enters the material.
Refraction occurs because the speed of the wave changes when it enters a new material with a different density. Light travels at a maximum speed in a vacuum and is slowed when traveling through matter. The speed of light in air is almost the same as in a vacuum. Diamond, the hardest material, retards the speed of light the most. The ratio of the speed of light in a vacuum to the speed in a material is called the index of refraction for that material. As the index of refraction increases, the speed of light in that material decreases.
4. What is dispersion?
Dispersion is the splitting of light by wavelength that occurs when certain materials refract light. Different wavelengths of light are refracted, or bent, by slightly different amounts as the light enters and leaves the material. The material refracts violet light more than red light, causing white light to be split into a rainbow of colors. A prism shows white light being dispersed by glass.
5. What is a lens?
A lens is a transparent material shaped so that parallel rays of light either converge to a point or appear to diverge from a point after passing through the lens. If the parallel rays of light converge to a point, the light passed through a convex lens. If the parallel rays of light appear to diverge from a point, the light passed through a concave lens.
6. What is the difference between a concave and convex lens?
Concave lenses are thinner in the center than at the edges and make parallel rays of light diverge. Concave lenses are used to correct the vision of near-sighted people. Convex lenses are thicker in the center than at the edges and make parallel rays of light converge. Convex lenses are used as magnifying glasses and are found in binoculars and cameras as well as telescopes.
7. What is a focal point and how is it measured?
The focal point is the point where parallel rays of light converge after passing through a converging lens or being reflected from a converging mirror. The distance from the center of the lens or mirror to the point where the rays converge is called the focal length.
8. What is reflection?
In reflection, a wave traveling through a medium, such as air, bounces off a new medium — for example, the surface of a mirror — and remains in its original medium, the air. The orientation of the incoming wave determines how the wave will be reflected. According to the law of reflection, the angle of incidence is equal to the angle of reflection.
9. What are the different types of mirrors, and how do they compare?
There are three types of mirrors: plane, concave, and convex. Plane mirrors are flat, like most mirrors found in homes. They do not alter the special relationship between parallel rays of light; they neither cause the parallel rays to converge nor diverge. Concave mirrors are curved inward, like the bowl of a spoon or the inside of an umbrella. Concave mirrors cause parallel rays of light to converge and are used as cosmetics mirrors and primary mirrors in telescopes. Convex mirrors are curved outward like the outside of a sphere or the back of a spoon. Convex mirrors cause parallel rays of light to diverge and are frequently used to look around corners.
10. What is absorption?
Absorption occurs when an electromagnetic wave interacts with matter. Some or all of the wave’s energy is added to the matter in the form of heat. Dark-colored materials tend to absorb more visible light that light-colored ones. For telescopes, this loss of energy means that less light reaches the detectors. Light is absorbed when it travels through the glass lens or when reflected from the mirrors of telescopes.
11. What causes chromatic aberration in lenses?
Chromatic aberration is caused by the dispersion of light as it passes through a lens. White light will split into a rainbow of colors as it passes through the lens, because the longer wavelengths of red light are refracted less than the shorter wavelengths of blue light. When used in a telescope, lenses produce an image surrounded by rings of color. Chromatic aberration can be corrected by passing the light through two lenses. The first lens disperses light into colors, and the second lens brings the separate colors back together into white light.
12. What causes spherical aberration in lenses or mirrors?
The shape of the lens or mirror causes the defect known as spherical aberration. A spherical lens or mirror is a portion of a sphere. This shape causes light passing through various areas of the lens or mirror to converge at slightly different focal lengths. Light passing through the edges of a lens, for example, would have a shorter focal length than light passing through the central regions of the lens. The result is a blurry image. To correct spherical aberration in mirrors, the mirror’s shape is altered to parabolic rather than spherical. To correct spherical aberration in lenses, the shape of a single lens could be altered to parabolic, but this would leave the lens with chromatic aberration. The accepted method of correcting spherical aberration in lenses is to use a lens doublet, which is a combination of a convex lens made of one type of glass and a concave lens made from a different type of glass. Spherical and chromatic aberrations are corrected as the light passes through both lenses.
13. What parts of the electromagnetic spectrum can be detected from the ground?
Visible light and radio waves can be detected from the ground. Some infrared and ultraviolet radiation can also be detected. Much of the infrared radiation is absorbed by water vapor in the lower atmosphere, so infrared telescopes are built on high mountains. The ozone layer absorbs much of the ultraviolet radiation, and the upper atmosphere is responsible for stopping X-rays and gamma rays from reaching the ground.
ABOUT TELESCOPES
14. Who invented the telescope?
Based on written records, the invention of the telescope is credited to Hans Lippershey of the Netherlands. Galileo is the first person to use a telescope to look at astronomical objects and record his observations. Galileo built about 30 telescopes but used only 10 to observe the sky. Through his careful observations and dedication, Galileo found support for the Copernican view of the solar system.
15. How does a telescope work?
A telescope collects light from a distant object and focuses it to form an image of the object. When the image is recorded, an observation is made. Originally, the only way to record the image was by hand — astronomers would make a drawing of what they saw through their telescopes. In the 1800’s, photography was invented and astronomers experimented with making photographs through their telescopes. In the early 20th century, astronomers started specifically designing and building telescopes to record the image on photographic plates. The 1980’s saw the invention of charge-coupled devices (CCDs) that allow the image to be recorded digitally. By the end of the 20th century, all research telescopes would use CCDs to make observations.
16. By what qualities is a telescope judged?
The factors used to determine the quality of a telescope are its ability to gather light, its resolution, its magnification, and the quality of its instruments. Of these, the magnification is the least important for research telescopes.
The light-gathering power is a measure of how much light the telescope captures and is a function of the diameter of the primary mirror or lens. The bigger the lens or mirror, the more light the telescope can gather.
Resolution is the ability to see detail. A telescope with high resolution can separate two closely spaced objects, whereas a telescope with low resolution will reveal a single object that may be misshaped.
Using a different eyepiece can change the magnification of today’s backyard telescopes. As the magnification increases, the telescope focuses on increasingly smaller parts of the sky, reducing the field of view of the telescope. Research telescopes don’t have eyepieces, so they can’t change the magnification and therefore have a fixed field of view for the telescope. Researchers are more interested in the instruments that are used to record and analyze the light. A large telescope with high resolution and quality instruments is desirable for research.
17. How does a reflector compare to a refractor?
The first telescope used a lens to gather light, so it was a refractor. Early refractors suffered from chromatic and spherical aberrations. After understanding the process that creates these aberrations, astronomers were able to make achromatic lenses by combining a convex lens made from one type of glass with a concave lens of another type. The resulting lens doublet could also correct spherical aberration. Meanwhile, reflectors made their debut. Although the first reflectors eliminated chromatic aberration, they suffered from spherical aberration and had dimmer images due to poor reflection from the metal mirrors. New technology brought glass-backed mirrors into the picture and soon reflectors surpassed the 40-inch-diameter technological limit of refractors. Today, research telescopes are built as reflectors, although many of the largest refractors continue to be used.
18. Why are telescopes mounted?
Galileo mounted his telescope to minimize the shaking that results from holding the telescope. As technology progressed and telescope tubes became longer, mounting was necessary to support the weight of the telescope. The invention of photography and its application to telescopes made mounting more important.
19. What is tracking and why do telescopes need to track objects?
The motion of Earth makes the stars appear to move across the sky. To observe a celestial object for a period of time, the telescope must be able to follow, or track, the star as it moves. In 1850, the first photographs (actually they were daguerreotypes) of the Moon and the star Vega were taken through a telescope using very short exposures. After clockwork devices were attached to telescopes to allow them to track the stars, longer-exposure photographs yielded images of more stars than can be seen with the unaided eye.
20. What are the types of telescope mounts?
There are two types of mounts. Each is designed to work with a particular sky-mapping system. One uses horizontal and vertical axes, and the mounting is called alt-az (for altitude and azimuth). The alt-az mounting resembles a camera tripod or a gun turret and uses horizontal and vertical axes to position the telescope. Many inexpensive amateur telescopes use this mounting. The other mounting type uses axes parallel and perpendicular to Earth’s axis of rotation (called right ascension and declination) and is called an equatorial mount.
21. What are the advantages and disadvantages of an altitude-azimuth mounting?
The advantages of an alt-az mounting are that it is very simple to build and it is easy to initially position the telescope on an object. For research telescopes, this type of mounting puts less stress on the mirror and requires less space in which to house the telescope.
The disadvantage becomes apparent in tracking the celestial object. Because the axes of the telescope don’t align with Earth’s axis, the position of the telescope must change in both the horizontal and the vertical directions to follow a star as it moves across the sky. Further, the orientation of the astronomical object appears to rotate over time in the telescope’s view. These problems make it unsuitable for amateur astrophotography. For astronomical research telescopes, a computer controls the tracking, making it seem smooth, and can correct for the rotation of the image.
22. What are the advantages and disadvantages of an equatorial mount?
An equatorial mount is a traditional type of mounting for research telescopes but it is challenging to build. The fact that the whole telescope must be supported at an angle makes it difficult to mount very large, heavy telescopes. The telescope is always fighting gravity. It needs massive bearings and other mechanical devices to keep it steady as the telescope turns at odd angles. Because it must achieve these odd angles, there is a need to counter balance it with additional weights and to provide room for the telescope to swing. This means the dome needs to be very large for large telescopes. Also, the mounting must be specific to a given location. For example, a mounting built for a telescope in Alaska can’t be used to mount the telescope in Florida, because since the two locations have different latitudes and will align with Earth’s axis at different angles.
The advantages of an equatorial mounting become apparent in the operation of the telescope. Once the telescope is pointed at a particular celestial object, it is easy to track that object. Because one axis is parallel to Earth’s axis of rotation, the telescope can track a celestial object across the sky by changing only one axis. This can be done using a simple clock-drive device to follow the smooth, arc-like motion of a star across the sky. The ability to track celestial objects was of paramount importance to the application of photography to astronomy. People have been making clocks for centuries, so it follows that telescope makers would design their telescopes for ease in tracking rather than for ease of building.
23. Why do radio telescopes need to be so big?
Radio telescopes need to be big for two reasons. One reason is that there isn’t much radio radiation reaching Earth, so big reflectors will capture more light. The second reason is the large wavelength of radio waves. To resolve closely spaced sources, the radio dish needs to be very large. One alternative to building very large radio telescopes is to build arrays of telescopes.
24. What are arrays and how are the signals combined?
An array is a group of telescopes that are operated together to simultaneously observe the same target. This serves to increase the resolution of the telescopes. Astronomers combine the signals of two or more telescopes through a process called interferometry, which relies on the principles of interference to combine the signals into one. Interferometry is only possible with the aid of computers to process the signal.
25. Why are solar telescopes different from other telescopes?
Solar telescopes are specialized to observe the hot, bright Sun, the nearest star to Earth. The heat and brightness of the Sun make it difficult to use equipment that was designed to look at dim sources of light like distant stars and nebulae. The effects of heating tend to change the shape of the mirrors and cause the air to move, which blurs the image.
26. Why are telescopes put into space?
Telescopes are put into space to get above the distorting effects of Earth’s atmosphere. For visible light, pockets of air in the atmosphere act like tiny lenses, bending the light from celestial objects in random, unpredictable directions. By placing a visible-light telescope above the atmosphere, these distortions are not encountered. Also, the atmosphere absorbs most other wavelengths of light. Placing a telescope in space is the only way to view celestial objects in those wavelengths.
