This chapter was copied with permission from Nick Strobel’s Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.
Covers refractors, reflectors, radio telescopes, light-gathering power, resolving power, interferometers, magnification, and atmospheric distortion such as seeing, reddening, and extinction.
Men and women have looked up at the sky and wondered about the things they see up there for as long as humans have lived on our Earth. Long ago, the Sun and Moon were mysterious objects that could be seen in the day and night. But the planets and stars were even more mysterious probably because they are so far away that we could only see them as points of light. Unlike the things on the Earth that we can study up close, handle, listen to, smell, and taste, the only thing ancient watchers of the sky had to learn about things in space was their eyes and imaginations. Only very recently in the history of humanity have astronomers been able to extend the reach of our eyes (and our imaginations!).
Galileo pioneered modern explorations in the early 1600’s by using a device originally invented for naval operations to explore the heavens. The device he used, of course, was the telescope, an instrument used to gather and focus light. Our atmosphere prevents most of the electromagnetic radiation from reaching the ground, allowing just the visible band, parts of the radio band, and small fractions of the infrared and ultraviolet through. Our eyes can detect the visible (optical) band, so the early telescopes were all built to observe in that part of the electromagnetic spectrum. It wasn’t until the 1930’s that astronomers began observing with another part of the electromagnetic spectrum—the radio band. The development of space technology has enabled astronomers to put telescopes above the atmosphere and explore all of those places out there using the full range of the electromagnetic spectrum. This chapter covers the basics of telescopes and the effects of the atmosphere on images. Vocabulary terms are in boldface.
8.2 Types of Telescopes
There are two basic types of telescopes, refractors and reflectors. The part of the telescope that gathers the light, called the objective, determines the type of telescope. A refractor telescope uses a glass lens as its objective. The glass lens is at the front of the telescope and light is bent (refracted) as it passes through the lens. A reflector telescope uses a mirror as its objective. The mirror is close to the rear of the telescope and light is bounced off (reflected) as it strikes the mirror.
8.2.1 Refractor Telescopes
The refractor telescope uses a lens to gather and focus light. The first telescopes built were refractors. The small telescopes sold in department stores are refractors, as well as, those used for rifle scopes.
- Refractor telescopes are rugged. After the initial alignment, their optical system is more resistant to misalignment than the reflector telescopes.
- The glass surface inside the tube is sealed from the atmosphere so it rarely needs cleaning.
- Since the tube is closed off from the outside, air currents and effects due to changing temperatures are eliminated. This means that the images are steadier and sharper than those from a reflector telescope of the same size.
Though excellent refractors are still made, the disadvantages of the refractor telescope have blocked the construction of very large refractors for use in astronomical research.
- All refractors suffer from an effect called chromatic aberration (“color deviation or distortion”) that produces a rainbow of colors around the image. Because of the wave nature of light, the longer wavelength light (redder colors) is bent less than the shorter wavelength light (bluer colors) as it passes through the lens. This is used in prisms to produce pretty rainbows, but can it ruin an image!
There a couple of ways to reduce chromatic aberration. One way uses multiple compensating lenses to counteract chromatic aberration. The other way uses a very long objective focal length (distance between the focus and the objective) to minimize the effect. This is why the early refracting telescopes were made very long.
- How well the light passes through the lens varies with the wavelength of the light. Ultraviolet light does not pass through the lens at all.
- How well the light passes through decreases as the thickness of the lens increases.
- It is difficult to make a glass lens with no imperfections inside the lens and with a perfect curvature on both sides of the lens.
- The objective lens can be supported only at the ends. The glass lens will sag under its own weight.
Because of these disadvantages, the largest refractor telescope built is the one at Yerkes Observatory. It has an objective 1.02 meters (40 inches) across at one end of a 19.2-meter (63 feet) tube. The two largest refractors are shown below. The first picture is the 40-inch refractor at Yerkes Observatory. Clicking on the image will lead you to more information about this telescope (and better pictures!) in another window. The second picture shows an astronomer (Kyle Cudworth) next to the objective to give you an idea of the size of the telescope. Notice the size of the people in the first picture! The third picture is the 0.91-meter (36-inch) refractor at Lick Observatory.Notice the astronomer at the lower left. Clicking on the image will take you to the Lick Observatory webpage about the telescope in another window. The last picture is E.E. Barnard at the eyepiece of the Lick 36-inch.
8.2.2 Reflector Telescopes
The reflector telescope uses a mirror to gather and focus light. All celestial objects (including those in our solar system) are so far away that all of the light rays coming from them reach the Earth as parallel rays. Because the light rays are parallel to each other, the reflector telescope’s mirror has a parabolic shape. The parabolic-shaped mirror focuses the parallel lights rays to a single point. All modern research telescopes and large amateur ones are of the reflector type because of its advantages over the refractor telescope.
- Reflector telescopes do not suffer from chromatic aberration because all wavelengths will reflect off the mirror in the same way.
- Support for the objective mirror is all along the back side so they can be made very BIG!
- Reflector telescopes are cheaper to make than refractors of the same size.
- Because light is reflecting off the objective, rather than passing through it, only one side of the reflector telescope’s objective needs to be perfect.
Alas! Despite the advantages of the reflector telescope, astronomers must contend with some minor annoyances.
- It is easy to get the optics out of alignment.
- A reflector telescope’s tube is open to the outside and the optics need frequent cleaning.
- Often a secondary mirror is used to redirect the light into a more convenient viewing spot. The secondary mirror and its supports can produce diffraction effects: bright objects have spikes (the “Christmas star effect”).
Two famous reflector telescopes are shown below. The first picture is of the 5-meter (200-inch) Hale Telescope at Palomar Observatory. The number refers to the diameter of the objective (almost 17 feet across!). Clicking on the image will send you to a gallery of images of the telescope in another window. The telescope is the vertical piece in the middle with the mirror close to the floor. The huge diagonal piece is used to balance the telescope.
The second picture shows the path light travels in the 10-meter Keck Telescope at the W.M. Keck Observatory. The objective is composed of 36 hexagonal mirrors put together to act as one large mirror 10 meters across. Clicking on the image will give you more information about this telescope in another window. The small image next to it shows the 10-meter objective. The person in the red clothing at the center gives you a sense of scale.
In both the reflector and refractor telescopes, the focus is before the eyepiece, so the image in astronomical telescopes is upside down. Telescopes used to look at things on the Earth’s surface use another lens to re-invert the image right-side up. Most reflector telescopes will use a smaller secondary mirror in front of of the large primary mirror to reflect the light to a more convenient viewing spot. Isaac Newton used a flat secondary mirror at a 45° angle to reflect the light to an eyepiece at the side of the telescope tube near the top. Such an arrangement, called a Newtonian design is used by many amateur telescopes.
Many reflector telescope use another light path design called the Cassegrain design to reflect the light back through a hole in the primary mirror, so that detectors or the eyepiece can be conveniently placed behind the telescope. Most of the large telescopes used for research, including the Hubble Space Telescope, are of this design. Some of the largest telescopes like the Hale Telescope and the Keck Telescope have places to put detectors at the prime focus, where the light from the primary mirror first comes to a focus. The images in reflector telescopes do not have holes or shadows in them because the light rays from the unblocked parts of the primary mirror are all added together when they are focused together. Even though part of the primary mirror is blocked or missing, there is still plenty of usable primary mirror space to gather the light.
Both types of telescope can suffer from a defect called spherical aberration so that not all of the light is focused to the same point. This can happen if the mirror is not curved enough (shaped like part of a sphere instead of a paraboloid) or the glass lens is not shaped correctly. The Hubble Space Telescope objective suffers from this (it is too flat by 2 microns, about 1/50 the width of a human hair) so it uses corrective optics to compensate. The corrective optics intercept the light beams from the secondary mirror before they reach the cameras and spectrographs. Fortunately, the Hubble Space Telescope’s spherical aberration is so perfect, that it is easy to correct for!
Even before the servicing mission that installed the corrective optics 2.5 years after the Hubble Space Telescope was put in orbit, astronomers were able to get significant results from the telescope. The images were computer-enhanced to correct for the spherical aberration to produce sharper images than from any ground-based telescope. Also, astronomers were able to observe ultraviolet light from celestial objects and fainter objects than could be seen from the ground. However, the computer processing took a long time and the aberration prevented the focusing of most of the light. This meant that astronomers could not see the very faint (and distant) objects they were looking for. Astronomers and the public were very pleased after the corrective optics were installed.
M 100 a few days before (left) and after (right) the corrective optics (COSTAR) were installed in December 1993.
Ground-based telescopes larger than Keck that will outperform even the Hubble Space Telescope (in the optical and infrared bands using adaptive optics) are being built now. Here are links to some of these future very large telescopes. Selecting any of the links will bring up the site in a new window (in front of or behind this window).
- The Large Binocular Telescope
- Large Synoptic Survey Telescope
- OWL (a 100-meter telescope)
- Thirty Meter Telescope
8.2.3 Radio Telescopes
Radio astronomy has its roots back in the 1930’s when Karl Jansky accidentally detected radio emission from the center of the Milky Way as part of his research on the interference on transatlantic phone lines. The British advanced radio antenna technology in their development of radar technology to fight the German warplanes in World War II. After the war, astronomers adapted the technology to detect radio waves coming from space.
The Robert C Byrd Green Bank Telescope (GBT) is the largest fully steerable radio telescope in the world; a much-improved replacement for the former 100-meter radio telescope. Although it has an unusual off-axis design to improve sensitivity and reduce distortions, the basic principles are the same as described in the cartoon at left. This image shows the underside lattice supports of the main reflector dish. Select the image to bring up an enlargement of the image. A zoomed-in image of the GBT is also available at this link (ask for a full-res version of the zoom image). A photo tour of Green Bank NRAO is available by selecting the tour link.
A radio telescope uses a large metal dish or wire mesh, usually parabolic-shaped, to reflect the radio waves to an antenna above the dish. An example of a mesh is shown at left. This was the mesh of the parabolic dish for the former 100-meter radio telescope at Green Bank, West Virginia (photo courtesy of National Radio Astronomy Observatory). Looking from underneath a radio telescope, a person can see the clouds in the sky overhead but to the much longer wavelength radio waves, the metal mesh is an excellent reflector. See also images from the Parkes Radio Telescope. Radio telescopes designed to also receive smaller wavelengths, such as the GBT pictured above, have solid metal dishes. The GBT’s metal surface is made up of 2004 panels, each roughly the size of a queen-sized bed, mounted on actuators to fine-tune the shape as the telescope is tilted and wind speed and direction changes.
The signal from the antenna is sent to an amplifier to magnify the very faint signals. At the last step, the amplified signal is processed by a computer to turn the radio signals into an image that follows the shape of the radio emission. False colors are used to indicate the intensity of the radio emission at different locations. An example is shown below for Jupiter. Charged particles in its magnetic field produce a large amount of radio energy in donut-shaped regions around its center. A visible band image of Jupiter is shown below the radio image.
Radio telescopes are much larger than optical telescopes because radio wavelengths are much longer than optical wavelengths. The longer wavelengths means that the radio waves have lower energy than optical light waves. In order to collect enough radio photons to detect a signal, the radio dishes must be very large. Both optical and radio telescope reflectors use a parabolic shape to perfectly focus the light to a point. Increasing the size of the radio dish is also necessary in order to improve the clarity of the radio images. I will discuss the issue of image clarity further in the next two sections.
Radio telescopes detect the emission from cool clouds of hydrogen in the space between the stars. Hydrogen atoms are the most common type of atoms in the universe and much of the hydrogen gas is too far away from any star to produce emission in the optical wavelength band. In addition, there are cold clouds made of over a hundred different types of molecules including organic molecules. Stars and planetary systems form in these molecular clouds. Therefore, radio telescopes are a vital tool in understanding the universe. I will discuss further the use of radio waves to explore the material between the stars and the structure of our galaxy in the interstellar medium chapter.
- chromatic aberration
- spherical aberration
Review Questions 1
- What are the two basic types of telescopes? What are their advantages and disadvantages?
- Why are the large modern telescopes reflector telescopes?
- How is the rainbow of colors produced around the images in refractor telescopes?
- How will spherical aberration affect an image?
- How does the size and shape of a radio telescope compare to an optical telescope? Why is there such a difference in their sizes?
- What are the parts of a radio telescope?
- What kinds of things can be seen with a radio telescope that cannot be detected by an optical telescope?
8.3 Powers of a Telescope
There are three features of a telescope that enable them to extend the power of our vision: a telescope’s superior light-gathering ability enables us to see faint objects, a telescope’s superior resolving power enables us to see even the tiniest of details, and the magnification power enables us to enlarge tiny images. Department stores and camera shops which do not know anything about telescopes, loudly proclaim their telescope’s magnifying power. Magnification is the least important power of a telescope. Amateur and professional astronomers know that the light-gathering power and resolving power are the most important. These two abilities depend critically on the objective, so they make sure the optics of the objective are excellent.
8.3.1 Light-Gathering Power
The ability of a telescope to collect a lot more light than the human eye, its light-gathering power, is probably its most important feature. The telescope acts as a “light bucket,” collecting all of the photons that come down on it from a far away object. Just as a bigger bucket catches more rain water, a bigger objective collects more light in a given time interval. This makes faint images brighter. This is why the pupils of your eyes enlarge at night so that more light reaches the retinas.
Very far away, faint objects can be seen only with BIG objective telescopes. Making faint images brighter is critical if the light is going to be dispersed to make a spectrum.
The area of the objective is the determining factor. Since most telescope objectives are circular, the area = π × (diameter of objective)2/4, where the value of π is approximately 3.1416. For example: a 40-centimeter mirror has four times the light-gathering power as a 20-centimeter mirror [( π 402/4) / ( π 202/4) = (40/20)2 = 4].
The light from a glowing object becomes more dilute as it spreads out from the object in a predictable way. The light covers larger and larger concentric spheres centered on the object, so that the object appears dimmer with the square of the distance. An object will appear four times dimmer when it is twice as far away, it will appear nine times dimmer when it is three times as far away, etc. Since the light-gathering power depends on the square of the diameter of the objective, the distance that an object can be detected increases as just the diameter of the objective (no square). Some examples are given in the figure below. Consider another example of how much further one could see with the 10-meter Keck telescope than with the human eye. The largest the pupil of the eye can dilate is 9 mm (for a young eye—older eyes do not dilate as much). So the Keck telescope at 10 meters x 1000 mm/meter = 10,000 mm across can see an object at least 10,000/9 = 1111 times further away than with the unaided, dark-adapted eye.
8.3.2 Resolving Power
Another important power of a telescope is its ability to make us see really small details and see sharp images. This is its resolving power. Objects that are so close together in the sky that they blur together into a single blob are easily seen as separate objects with a good telescope. The resolving power is measured in the absolute smallest angle that can be resolved. The absolute minimum resolvable angle (smallest visible detail) in arc seconds = 252,000 × (observation wavelength) / (objective diameter). The wavelength and diameter must be measured in the same length units (i.e., both wavelength and objective diameter given in meters or both in nanometers). A telescope with one arc second resolution would be able to see a dime from about 3.7 kilometers (2.3 miles) away. Modern telescopes are able to count the number of lines in President Roosevelt’s hair on a dime at that distance.
The desire is to make as small as possible. This can be done by making the observation wavelength small (e.g., use UV instead of visible light) or by making the objective diameter large. Another way to understand it is the more waves that can be packed on the objective, the more information the telescope detects and, therefore, the more detailed the image is. A 40-centimeter telescope has twotimes the resolution of a 20-centimeter telescope at the same observing wavelength ( for the 40-centimeter telescope is one-half the for the 20-centimeter telescope). However, fluctuations in the atmosphere will usually smear images into a fuzzy blob about one arc second or more across so the resolution is usually limited to the resolution from a 12.5-centimeter telescope on the ground. I will discuss the atmosphere’s effect on images further in the another section and ways you can compensate for it.
|The desire for greater resolving power is the main reason why radio telescopes are so enormous compared to their optical counterparts. Radio wavelengths are LARGE so the radio telescope must be LARGE to get decent resolving power (and also to increase the signal strength of the low-energy radio waves—light-gathering power!). The Keck 10-meter telescope is considered a very large optical telescope. However, it is easily dwarfed by the HUGE 305-meter Arecibo Radio Telescope at the Arecibo Observatory. A picture of this telescope is shown at left. This telescope was built in a natural bowl-shaped valley in Puerto Rico. Clicking on the image will show the telescope from other perspectives.|
Another way to increase the resolution is to connect telescopes together to make an interferometer. Radio waves from an object reach each telescope in the interferometer at slightly different times, so the waves are out of sync with one another. Knowing the distances between the telescopes and how out of sync the waves are, the signals can be combined electronically to create an image of exceptional resolution. The image has the same sharpness as one taken by a single instrument that would extend from one end of the interferometer to the other. The light-gathering power is equal to the sum of the light-gathering powers of each individual telescope. To learn more about the principles of interferometry, see NASA/JPL’s Origins Explorers cartoon.
|A spectacular example of such a system is the Very Large Array shown here. This telescope is made of 27 radio dishes, each 25 meters in diameter, on a Y-shaped track. Fully extended, the Very Large Array is 36 kilometers across and has a resolution of around one arc second (depending on the radio wavelength). It has the light-gathering power of a 130-meter telescope. Aerial views are shown below.Another example is the Australia Telescope Compact Array outside of Narrabri. Six 22-meter dishes can be placed in an array 6 kilometers across. A photo tour of the site is available here.|
The Very Long Baseline Array is a huge interferometer that uses ten telescopes placed in sites from Hawaii to the Virgin Islands (see map below). This telescope is the 8,600 kilometers across and has a resolution as good as 0.0002 arc second! With a resolution about 50 times better than the Hubble Space Telescope, it is able to detect features as small as the inner solar system at the center of our galaxy, about 27,000 light years away. Astronomers are constructing radio telescopes out in space that work in conjunction with ground-based radio telescopes to make interferometers much larger than the Earth (see also the Orbiting VLBI web site). Other huge radio telescope arrays include Australian Square Kilometre Array Pathfinder (ASKAP) made of 36 identical antennae, each 12 meters in diameter in western Australia and the Atacama Large Millimeter/submillimeter Array (ALMA) at over 16,500 foot (5000 meters) elevation in the Atacama Desert in Chile. ALMA is made of 33 antennae so far and will increase to 66 total antennae in the year 2013 with 54 of them 12 meters in diameter and 12 of them 7 meters in diameter in an array 14 kilometers across. Both ALMA and ASKAP are large international projects.
Astronomers are also now connecting optical telescopes to increase their resolving power. Two nice examples are the Keck Interferometer on Mauna Kea, Hawaii and the Very Large Telescope Interferometer of Paranal Observatory on Cerro Paranal in the Atacama Desert, northern Chile.
Sites for the Very Long Baseline Array—an array 8600 km across!
8.3.3 Magnifying Power
The ability of a telescope to enlarge images is the best-known feature of a telescope. Though it is so well-known, the magnifying power is the least important power of a telescope because it enlarges any distortions due to the telescope and atmosphere. A small, fuzzy faint blob becomes only a big, fuzzy blob. Also, the light becomes more spread out under higher magnification so the image appears fainter! The magnifying power = (focal length of objective) / (focal length of eyepiece); both focal lengths must be in the same length units. A rough rule for the maximum magnification to use on your telescope is 20 × D to 24 × D, where the objective diameter D is measured in centimeters. So an observer with a 15-centimeter telescope should not use magnification higher than about 24 × 15 = 360-power.
The set of four figures below shows the effect of a larger objective size. They have the same magnification. These are ideal images of two stars separated by 0.5 arc seconds which would be the angular separation for stars at the Sun and Jupiter’s positions if the system was 33 light years from us. The frames are 1.5 arc seconds square and are at the observation wavelength of 500 nanometers. The resolving power is given by θR and they all have the same brightness—the light in the bottom images from the large telescopes is just much more concentrated than for the small telescopes. The image from the 0.1524-meter telescope (image A) would take 30 minutes to make, but the image from the 5.08-meter telescope (image D) would take only 1.6 seconds! The exposure times for the other telescopes are given.
The pictures clearly show the increase in sharpness as the objective size is increased. The size of each of the blobs is the size of the smallest detail that can be seen with that telescope under ideal conditions. Atmospheric distortion effects (smearing of the binary star images to a blob the size of the entire frame) and obscuration and diffraction by the secondary and its supports are NOT shown here.
- light-gathering power
- magnifying power
- resolving power
- Light-Gathering Power = π × (diameter of objective)2/4.
- Resolving Power θR = 252,000×(observation wavelength/diameter of objective). Better resolving power has smaller θR.
- Magnifying Power = (objective focal length) / (eyepiece focal length).
Review Questions 2
- Of the three powers of the telescope (light-gathering power, resolving power, magnification) which is least important? Which depend on the size of the objective mirror or lens?
- How many times brighter will a 60-centimeter telescope make a 10-second exposure image than a 12-centimeter telescope?
- How many times better resolution does a 48-centimeter telescope have than a 12-centimeter telescope?
- Will a shorter or longer wavelength enable us to see smaller details?
- Why do radio telescopes have to be so large?
- How can an interferometer be used to improve resolution?
- What is the maximum magnification that should be used with a 20-centimeter telescope?
- Would a 30-power telescope with lens 4 centimeters across be better for observing a faint, faraway object than a 60-power telescope with lens 3 centimeters across? Why or why not?
8.4 Atmospheric Distortion
Many people believe that astronomers want to build telescopes on tall mountains or put them in space, so they can be “closer” to the objects they are observing. This is INcorrect! The nearest star is over 41,500,000,000,000 kilometers (26 trillion miles) away. If you ignore the 300-million kilometer variation in the distances due to the Earth’s motion around the Sun and the 12,756-kilometer variation due to the Earth’s rotation, being 4 kilometers closer on a tall mountain amounts to a difference of at most 1 × 10-11 percent. Telescopes in space get up to 1 × 10-9 percent closer (again ignoring the much larger variations of the Earth’s orbit around the Sun and the telescope’s orbit around the Earth). These are extremely small differences—the distances to the even the nearest stars are around 100,000’s times greater than the distances between the planets in our solar system. The reason large telescopes are built on tall mountains or put in space is to get away from the distortion of starlight due to the atmosphere. The atmospheric distortion is poor seeing, reddening, extinction and the adding of absorption lines to stellar spectra.
The famous observing site at the Kitt Peak National Observatory has many large telescopes including the 4-meter Mayall telescope (top right) and the McMath Solar Telescope (triangular one at the lower right). Although it is over 60 kilometers from Tucson, AZ, light pollution from the increasing population of that city has stopped the construction of any more telescopes on the mountain.
The Mauna Kea Observatory is probably the best observing site in the world. Many very large telescopes are at the 4177-meter summit of the extinct volcano. Because of the elevation, the telescopes are above most of the water vapor in the atmosphere, so infrared astronomy can be done. Kitt Peak’s elevation of 2070 meters is too low for infrared telescopes.
The James C. Maxwell Telescope is in the center front of this picture of the Mauna Kea Observatory. The two white Keck 10-meter telescopes are to the left on the ridgetop next to the silver 8.3-meter Subaru telescope. Select the image to go to the Mauna Kea Observatory homepage (will display in another window).
The air is constantly in turbulent motion and light from celestial objects is bent randomly in many ways over time periods of tens of milliseconds. Images dance about (twinkle) and images are blurred. The atmospheric blurring distorts the view of astronomical objects much as ripples in water distort the objects below the surface. This atmospheric effect is called seeing.
The short animation shows how light from a star is refracted in random directions by rapidly-moving pockets of air of varying densities and temperatures. The light reaches different parts of the telescope’s objective from slightly different directions and different times. A highly magnified view shows multiple images of the star that dance about in the field of view many times a second, so a second or longer exposure picture will show a fuzzy blob (called the “seeing disk”) the size of the entire distribution of dancing images. An actual observation shows much more rapid variation and more images of the star. A movie (mpeg format) of a binary star is available by selecting this link. The data is from the William Herschel 4.2-meter Telescope in La Palma, Canary Islands, Spain and is courtesy of the Applied Optics Group now part of the Photonics Group at Imperial College. Links to other movies are available there.
Good seeing is when the air is stable (little turbulence) and the twinkling is small. Details as small as 0.5 arc seconds can be seen when the seeing is good (still much larger than the theoretical resolving power of large research telescopes). Poor seeing happens when the air is turbulent so the images dance about and details smaller than 2 to 3 arc seconds cannot be seen. The more atmosphere there is above a telescope, the greater is the turbulent motion and the poorer is the seeing. This is one reason why research telescopes are located on very high mountains.
Speckle interferometry can get rid of atmospheric distortion by taking many fast exposures of an object. Each fraction-of-a-second exposure freezes the motion of the object. Extensive computer processing then shifts the images to a common center and removes other noise and distortions caused by the atmosphere, telescope, and electronics to build up a distortion-free image. See Steven Majewski’s lecture notes on speckle for further exploration of speckle interferometry. Another technique called adaptive optics makes quick changes in the light path of the optics to compensate for the atmospheric turbulence. Before the focused light from the objective reaches the camera, it bounces off a thin deformable mirror that can be adjusted thousands of times a second to reposition the multiple images back to the center. An excellent site to explore this topic further is the Center for Astronomical Adaptive Optics web site.
Telescopes in orbit like the Hubble Space Telescope are above the turbulent effect of the atmosphere and can achieve their theoretical resolving power. The Hubble Space Telescope has a 2.4-meter objective, making it the largest telescope ever put in orbit. One major drawback to satellite observatories is the large cost to build and maintain them. Adaptive optics on ground-based telescopes now can remove the seeing effects on small patches of the sky at a time and enable the huge research telescopes to take even sharper pictures than those from the Hubble Space Telescope.
8.4.2 Reddening and Extinction
The air also absorbs and scatters electromagnetic radiation by an amount that varies with the wavelength. Redder (longer wavelength) light is scattered less by atmosphere molecules and dust than bluer (shorter wavelength) light. This effect is known as reddening. This effect explains why the Sun appears orange or red when it is close to the horizon. The other colors of sunlight are scattered out of your line of sight so that only the orange and red colors make it through the atmosphere to your eyes. This effect also explains why the sky is blue. Since blue light is scattered more, you will see more blue light scattered back to your eyes when you look in a direction away from the Sun.
All wavelengths of light are scattered or absorbed by some amount. This effect is called extinction. Some wavelength bands suffer more extinction than others. Some of the infrared band can be observed from mountains above 2750 meters elevation, because the telescopes are above most of the water vapor in the air that absorbs much of the infrared energy from space. Carbon dioxide also absorbs a lesser amount of the infrared energy. Gamma-rays and X-rays are absorbed by oxygen and nitrogen molecules very high above the surface, so none of this very short wavelength radiation makes it to within 100 kilometers of the surface. The ultraviolet light is absorbed by the oxygen and ozone molecules at altitudes of about 60 kilometers. The longest wavelengths of the radio band are blocked by electrons at altitudes around 200 kilometers.
The atmosphere also scatters light coming from the ground to wash out a lot of the fainter stars and planets in what is called light pollution. As more people move to the cities and the cities get larger, an increasing percentage of people are missing out on the beauty of a star-filled night sky. The increasing light pollution is also threatening the amount and quality of research that can be done at many of the major astronomical observatories. The image below shows how much of the world is now cut off from the night sky. Select the image to bring up a larger version from NASA’s Visible Earth website. Visit the International Dark-Sky Association website for more about light pollution and ways to bring back the night sky.
The Hubble Space Telescope (HST) is able to observe in the ultraviolet, something that ground-based research telescopes cannot do. This is one advantage that HST will always have over ground-based telescopes, even those with adaptive optics. Even though HST has a smaller objective than many ground-based telescopes, its ability to observe in shorter wavelengths will keep its resolving power very competitive with the largest ground-based telescopes with the best adaptive optics. The Hubble Space Telescope can also observe in a broader swath of the infrared than can be done from the ground. Furthermore, HST has no sky glow background so it is able to easily detect very faint objects against a truly black background. Another ultraviolet space telescope is the Galaxy Evolution Explorer (GALEX).
Telescopes used to observe in the high-energy end of the electromagnetic spectrum, like the Chandra X-ray Observatory and XMM-Newton above, must be put above the atmosphere and require special arrangements of their reflecting surfaces. The extreme ultraviolet and X-rays cannot be focused using an ordinary mirror because the high-energy photons would bury themselves into the mirror. But if they hit the reflecting surface at a very shallow angle, they will bounce off. Using a series of concentric cone-shaped metal plates, high energy ultraviolet and X-ray photons can be focused to make an image.
Gamma rays have too high an energy to be focused with even the shallow angle reflecting technique, so gamma ray telescopes simply point in a desired direction and count the number of photons coming from that direction. Some examples of gamma-ray space observatories are shown below. Clicking on the images will take you to sites describing the telescopes in greater detail.
On the long wavelength end are the infrared space telescopes such as Spitzer Space Telescope and the Wide-field Infrared Survey Explorer (WISE). They are cooled to very low temperatures (just a few degrees above absolute zero) and they observe from behind a sun shield so that the telescope’s own internal heat will not interfere with the observations. Infrared observations are especially good for studying objects hidden behind thick dust clouds such as forming stars and planets, cool objects such as asteroids, dim stars, and exoplanets, and very distant galaxies.
8.4.3 Atmospheric lines
Gases in the Earth’s atmosphere can introduce extra absorption lines into the spectra of celestial objects. The atmospheric spectral lines must be removed from the spectroscopy data, otherwise astronomers will find a hot star with molecular nitrogen, oxygen and water lines! Such lines are only produced by gases much cooler than that in stars.
- adaptive optics
- speckle interferometry
Review Questions 3
- The distance to the nearest star is 4.3 light years = 4.3 × 9.7 trillion kilometers = 41,800,000,000,000 kilometers. Does Mauna Kea’s elevation of 4177 meters (=2.6 miles) put it significantly closer to even the nearest star than something at sea level? Explain your answer. (1 kilometer = 1000 meters.)
- What causes stars to twinkle? What would make good seeing?
- Even with perfectly clear skies free of human-made pollution, the seeing on Mauna Kea (4177 meters elevation) is much better than at sea level. Why is that?
- What absorbs infrared light in our atmosphere and up to what height above sea level is most of this infrared absorber found?
- Even with perfectly clear skies free of human-made pollution, infrared observations can be made at Mauna Kea but not at Kitt Peak (2070 meters elevation). Why is that?
- Why are all ultraviolet, X-ray, and gamma ray telescopes put up in orbit?