FORMATION OF AN IMAGE
In optical astronomy the object with which we work is the image of the light source formed by the principal image-forming part of the telescope, which is called an objective. The objective of the optical telescope is either a lens or a mirror. Light rays from the light source are refracted in passing through a lens and are reflected from a mirror. The image is produced where the light rays converge to a position known as the focus. The focal length of the objective is the distance behind the lens to the focus or the distance in front of the mirror to the focus. The image of a star is just a point of light, while that of an extended object, such as the moon, is inverted.
In telescopes using either mirrors or lenses an eyepiece magnifies the image much as a reading glass magnifies small print. Or a photographic plate may be inserted into the focal plane of the objective instead of the eyepiece, transforming the telescope into a giant camera. In this case the objective lens or mirror serves as the camera lens. The advantage of photography over observing with the eye is that the photograph is available for later study and time exposures can record fainter sources than those the eye sees.
PROPERTIES OF AN IMAGE
The image formed by either a lens or a mirror has certain properties that depend upon the diameter, or aperture, of the objective and its focal length. One property is the size of the image. Si nee the image of a star is a point, size is not an important property for it. For an extended object the image size depends upon the angular size of the light source on the sky and upon the weal length of the objective.
Image brightness is important since it determines whether the image is above the threshold of visibility or how long it would take to photograph. The brightness of an image of a point source, such as a star, depends on how much light is intercepted by the objective. Hence its brightness is proportional to the area of the objective or to the square of the aperture. Doubling the aperture but leaving the focal length the same increases the area of the objective or its lightgathering power by four times, concentrating four times as much light into the same-size image.
When we photograph an extended object, the surface brightness of the image depends on the amount of radiant energy per unit area of the image. The objective's area (or the square of its aperture) still determines the total amount of energy collected, but the total energy is distributed over the entire image. Thus the larger the image's area, the smaller the energy per unit of area. The image size of an extended object increases in proportion to the focal length; so for a given telescope aperture the surface brightness of the image decreases as the focal length is made longer.
How well a telescope discriminates between two adjacent objects or shows fine details is called its resolving power. Because of the wave natu re of light, the image of a point source produces a diffraction pattern; it appears as a bright central spot, called a diffraction disk, surrounded by progressively fainter rings. When the diffraction patterns of two stars that are close together no longer overlap, we can see separate stellar images. The larger the telescope's aperture, the smaller is the diffraction disk of each image. A large aperture therefore improves the resolution of closely adjoining features by making the diffraction effect of adjacent objects overlap less. We define resolving power as the smallest angle between two close objects whose images can just be separated by a telescope. This critical angle is directly proportional to the wavelength of the observed radiation and inversely proportional to the aperture of the objective.
VIEWING PROBLEMS
The theoretical resolving power of any optical telescope is never fully realized. The lower layers in the earth's atmosphere are unsteady and turbulent; this turbulence blurs and distorts the star's image and makes it twinkle, or scintillate. The rapid scintillations break the starlight into many dancing specks of light, which in long exposu res merge to form the fuzzy stellar images we see in photographs. When the atmospheric turbulence is low, the stars twinkle, or scintillate, less, and the so-called seeing is improved. A planet, on the other hand, shines with a steady light because each point on the tiny disk twinkles out of step with neighboring points; we see an average of all the twinkling points.
A technique called speckle photography, which can be used with large telescopes, can get around the smearing and wiggling of the image that comes from atmospheric turbulence. In the exposure of the photographic plate for an extremely short time (less than 0.01 second), each star image appears as a cluster of sharp specks of different brightness. Then the photograph is run through a high-speed light-sensing device which measures the variations in brightness across each speck. The information from the assemblage of specks in each of many photographed images is fed into a computer that is programmed to analyze and reassemble the information into the unsmeared image of the star.
Other nuisances hamper our observation of the heavens. The night sky's transparency varies as smog, dust, and atmospheric haze cloud it. The upper atmosphere is also suffused with a faint light called airglow. Atmospheric atoms and molecules absorb the ultraviolet photons in sunlight and reradiate the energy in a few wavelengths of the green, red, and infrared spectral regions. On long exposures airglow fogs a photograph and reduces the contrast between the faintest images and the sky background.
Another problem is that starlight entering the atmosphere is bent increasi ngly toward the vertical so that we see a star slightly closer to the zenith (the point directly above the observer) than it really is. This atmospheric-refraction effect is greatest near the horizon (about OS), for there the light's path through the air is the longest. When we observe the rising or setting sun, it is really below our horizon,
but refraction raises the sun's image above the horizon by an amount equal to its apparent diameter, which is OS.
Other viewing problems are related to the geographical location of the observatory. An ideal site for an optical observatory is a mountaintop where the air is dry, transparent, and steady, and the sky is dark. An observatory also needs a minimum amount of wind and relatively easy access. The southwestern part of the United States satisfies most of these conditions and has many clear days and nights. Kitt Peak National Observatory is located there, about 65 miles southwest of Tucson, Arizona.
REFLECTING AND REFRACTING TELESCOPES
Telescopes that use lenses for the objective are known as refracting telescopes, while those that employ a mirror are called reflecting telescopes. The objectives of the early refracting telescopes could not form sharp images because of a condition known as spherical aberration; single lenses also failed to bring all colors to a common focus, a failure called chromatic aberration. A compound lens, or two lenses of different types of glass cemented together was invented to minimize these aberrations in refracting telescopes.
Spherical aberration also occurs in a reflecting telescope. If the surface of the mirror is parabolic rather than spherical, that aberration is eliminated although some minor deficiencies still remain.
Why are the big modern telescopes of the reflecting type? Reflecting telescopes have many advantages over refractors: The reflecti ng telescope is free from chromatic aberration, making it ideal for allpurpose photography and spectroscopy. Also, since a lens must be supported by its edges, there is a material limit to how large a lens system can be. But a mirror can be held both at its edges and from the back, the supports allowing a wide range of sizes for mirror systems. The largest refractor has an aperture slightly larger than 1 meter, butthe largest reflector is 6 meters in diameter.
There are other advantages to reflectors: The glass for the mirror in a reflecting telescope need not be so optically pure and homogeneous as that required for a large lens because the light reflects off the front surface and does not pass through the mirror, as it does through a lens. And the mirror has only one surface that must be painstakingly ground-the achromatic lens has four. To counter changes in temperature that would affect the focal length of the reflector, large mirrors are constructed of fused quartz or of a zero-expansion pyroceramic material. The mirror's surface is coated with a thin layer of highly reflecting aluminum that is replaced many times during the life of the telescope.
FOCAL POSITION FOR REFLECTING TELESCOPES
Reflecting telescopes can be designed for many kinds of astronomical work through choice of the focal arrangement to suit the type of observation. For photography, photometry, and spectroscopy of faint objects the prime focus is best because its small focal length lessens the exposu re time required. The Newtonian focus, most useful for small telescopes, is now little used by professional astronomers. In both these arrangements the observer works at a considerable distance above the observatory floor since both focal positions are near the entrance of the telescope.
I n the Cassegrain focal arrangement a convex secondary mirror positioned at the top in front of the focus slows the rate at which light rays converge, effectively increasing the telescope's focal length. The secondary mirror reflects the converging rays to the bottom of the telescope and through a hole in the objective mirror to focus behind the objective. This is a much more convenient observing position since it is near the floor and behind the telescope. Of all the observations made with the 5.1-meter Hale telescope on Palomar Mountain 75 percent are from the Cassegrain focus.
We might think that putting the secondary mirror and its supports or the observer's cage for the prime focus into the path of the light rays would obscure part of the image; but the only effect is to cut down the amount of light reaching the objective; the loss is small, and the quality of the image is not affected.
Equipment that is too heavy and bulky to be attached to the back of the primary mirror or is sensitive to changing gravitation as the telescope moves can be placed in a room below the observatory floor. An auxiliary flat mirror diverts the long converging beam down the hollow polar axis around which the telescope rotates, and with this coude focal arrangement the focus can remain stationary no matter which way the telescope points.
TELESCOPE MOUNTINGS
An optical telescope, in order to follow an object as the earth's rotation carries it across the sky, must be free to move. To track stars accu rately and to permit a telescope to be conveniently pointed in any direction, the equatorial mounting system is used for most telescopes. This system has two axes of rotation: The telescope can rotate in an east-west direction, called hour-angle, around its polar axis, which is aligned with the earth's axis of rotation; another allows the telescope to swing in a north-south direction about the declination axis, which is perpendicular to the polar axis.
Large telescopes are usually positioned by a computer from an operating console and guided to the exact location with hand controls. Once a large telescope is properly set, the computer operating a clock drive slowly turns it westward around its polar axis at the same rate as the earth turns eastward, keeping the stellar images locked in position in the field of view. The great simplicity in equatorial mounting is that tracking requires continuous motion about only one of its two axes. The disadvantage, which obtains in the largest telescopes now in operation and planned for the future, lies in the stresses on the polar axis due to gravity. The polar axis is inclined in the earth's gravitational field and must rotate on one edge of its end. For a very large telescope that is a difficult engineering problem.
One means of removing some of the stress from the primary axis is to align it with gravity. Such a mounting is known as altazimuth mounting; with it a telescope rotates about a vertical axis and about a horizontal axis. This mounting's disadvantage is that, to track a star, it must turn continuously about both axes at the same time. When the telescope approaches the area of the sky directly overhead, continuous tracking becomes virtually impossible. Even with this disadvantage the altazimuth mounting will be the primary mounting for very large telescopes to be constructed in the future.
OTHER APPROACHES TO MAKING TELESCOPES
The principal problems in building very large telescopes on the earth's surface today are costs and construction time. A new 5-meter Hale telescope would now cost about 25 million dollars and take 10 years to build, while a 10-meter telescope would cost 200 million dollars and take 20 years to build, and a 25-meter telescope would require about 3 billion dollars and 50 years to construct. Clearly some dramatic changes in design are needed to lower cost and construction time.
A new telescope design, called the Multiple Mirror Telescope (MMT), which is well suited for infrared observations, has been installed on Mount Hopkins in Arizona. It uses a mosaic of independent mirrors of small size to collect and focus light in order to simulate the collecting ability of a largeaperture single mirror. The MMT consists of a circular array of six identical 1.8-meter mirrors on an altazimuth mounting; the array has light-gathering power equivalent to that of a 4.5-meter single mirror.
The six mirrors are not thick solid ones but are of a new lightweight design. They are partially hollow, which requires a smaller mechanical structure to move them; thus they save money and construction time. The six images from the six mirrors may be either superimposed to form a single image or aligned along a spectrographic slit, one on top of the other, to take full advantage of slit geometry. The pointing directions of the six mirrors are locked together by laser beams. This instrument has been successful in demonstrating the practicality of the multiple-mirror concept, and it may be the forerunner of telescopes that are equivalent to a 25-meter (82-foot) telescope. Under consideration is an MMT consisting of eight 5-meter lightweight mirrors, having the light-gathering power of a 14-meter telescope, the angular resolution of a 22-meter telescope, and (it is hoped) the cost of a 4-meter telescope.
The MMT is not the only new design which shows an artist conception of these new designs for future large telescopes, but it is not now certain whether any of them will ever be built. The success of the Space Telescope, a 2.4-meter conventional reflector that is to be put into orbit in early 1985, will not lessen the need for a mammoth new telescope on the ground but will probably increase it. Since Space Telescope does not have to contend with light losses produced by the atmosphere, a very large telescope will be required on the ground to observe in visible wavelengths what Space Telescope is able to observe at shorter wavelengths, where it will primarily operate.
In optical astronomy the object with which we work is the image of the light source formed by the principal image-forming part of the telescope, which is called an objective. The objective of the optical telescope is either a lens or a mirror. Light rays from the light source are refracted in passing through a lens and are reflected from a mirror. The image is produced where the light rays converge to a position known as the focus. The focal length of the objective is the distance behind the lens to the focus or the distance in front of the mirror to the focus. The image of a star is just a point of light, while that of an extended object, such as the moon, is inverted.
In telescopes using either mirrors or lenses an eyepiece magnifies the image much as a reading glass magnifies small print. Or a photographic plate may be inserted into the focal plane of the objective instead of the eyepiece, transforming the telescope into a giant camera. In this case the objective lens or mirror serves as the camera lens. The advantage of photography over observing with the eye is that the photograph is available for later study and time exposures can record fainter sources than those the eye sees.
PROPERTIES OF AN IMAGE
The image formed by either a lens or a mirror has certain properties that depend upon the diameter, or aperture, of the objective and its focal length. One property is the size of the image. Si nee the image of a star is a point, size is not an important property for it. For an extended object the image size depends upon the angular size of the light source on the sky and upon the weal length of the objective.
Image brightness is important since it determines whether the image is above the threshold of visibility or how long it would take to photograph. The brightness of an image of a point source, such as a star, depends on how much light is intercepted by the objective. Hence its brightness is proportional to the area of the objective or to the square of the aperture. Doubling the aperture but leaving the focal length the same increases the area of the objective or its lightgathering power by four times, concentrating four times as much light into the same-size image.
When we photograph an extended object, the surface brightness of the image depends on the amount of radiant energy per unit area of the image. The objective's area (or the square of its aperture) still determines the total amount of energy collected, but the total energy is distributed over the entire image. Thus the larger the image's area, the smaller the energy per unit of area. The image size of an extended object increases in proportion to the focal length; so for a given telescope aperture the surface brightness of the image decreases as the focal length is made longer.
How well a telescope discriminates between two adjacent objects or shows fine details is called its resolving power. Because of the wave natu re of light, the image of a point source produces a diffraction pattern; it appears as a bright central spot, called a diffraction disk, surrounded by progressively fainter rings. When the diffraction patterns of two stars that are close together no longer overlap, we can see separate stellar images. The larger the telescope's aperture, the smaller is the diffraction disk of each image. A large aperture therefore improves the resolution of closely adjoining features by making the diffraction effect of adjacent objects overlap less. We define resolving power as the smallest angle between two close objects whose images can just be separated by a telescope. This critical angle is directly proportional to the wavelength of the observed radiation and inversely proportional to the aperture of the objective.
VIEWING PROBLEMS
The theoretical resolving power of any optical telescope is never fully realized. The lower layers in the earth's atmosphere are unsteady and turbulent; this turbulence blurs and distorts the star's image and makes it twinkle, or scintillate. The rapid scintillations break the starlight into many dancing specks of light, which in long exposu res merge to form the fuzzy stellar images we see in photographs. When the atmospheric turbulence is low, the stars twinkle, or scintillate, less, and the so-called seeing is improved. A planet, on the other hand, shines with a steady light because each point on the tiny disk twinkles out of step with neighboring points; we see an average of all the twinkling points.
A technique called speckle photography, which can be used with large telescopes, can get around the smearing and wiggling of the image that comes from atmospheric turbulence. In the exposure of the photographic plate for an extremely short time (less than 0.01 second), each star image appears as a cluster of sharp specks of different brightness. Then the photograph is run through a high-speed light-sensing device which measures the variations in brightness across each speck. The information from the assemblage of specks in each of many photographed images is fed into a computer that is programmed to analyze and reassemble the information into the unsmeared image of the star.
Other nuisances hamper our observation of the heavens. The night sky's transparency varies as smog, dust, and atmospheric haze cloud it. The upper atmosphere is also suffused with a faint light called airglow. Atmospheric atoms and molecules absorb the ultraviolet photons in sunlight and reradiate the energy in a few wavelengths of the green, red, and infrared spectral regions. On long exposures airglow fogs a photograph and reduces the contrast between the faintest images and the sky background.
Another problem is that starlight entering the atmosphere is bent increasi ngly toward the vertical so that we see a star slightly closer to the zenith (the point directly above the observer) than it really is. This atmospheric-refraction effect is greatest near the horizon (about OS), for there the light's path through the air is the longest. When we observe the rising or setting sun, it is really below our horizon,
but refraction raises the sun's image above the horizon by an amount equal to its apparent diameter, which is OS.
Other viewing problems are related to the geographical location of the observatory. An ideal site for an optical observatory is a mountaintop where the air is dry, transparent, and steady, and the sky is dark. An observatory also needs a minimum amount of wind and relatively easy access. The southwestern part of the United States satisfies most of these conditions and has many clear days and nights. Kitt Peak National Observatory is located there, about 65 miles southwest of Tucson, Arizona.
REFLECTING AND REFRACTING TELESCOPES
Telescopes that use lenses for the objective are known as refracting telescopes, while those that employ a mirror are called reflecting telescopes. The objectives of the early refracting telescopes could not form sharp images because of a condition known as spherical aberration; single lenses also failed to bring all colors to a common focus, a failure called chromatic aberration. A compound lens, or two lenses of different types of glass cemented together was invented to minimize these aberrations in refracting telescopes.
Spherical aberration also occurs in a reflecting telescope. If the surface of the mirror is parabolic rather than spherical, that aberration is eliminated although some minor deficiencies still remain.
Why are the big modern telescopes of the reflecting type? Reflecting telescopes have many advantages over refractors: The reflecti ng telescope is free from chromatic aberration, making it ideal for allpurpose photography and spectroscopy. Also, since a lens must be supported by its edges, there is a material limit to how large a lens system can be. But a mirror can be held both at its edges and from the back, the supports allowing a wide range of sizes for mirror systems. The largest refractor has an aperture slightly larger than 1 meter, butthe largest reflector is 6 meters in diameter.
There are other advantages to reflectors: The glass for the mirror in a reflecting telescope need not be so optically pure and homogeneous as that required for a large lens because the light reflects off the front surface and does not pass through the mirror, as it does through a lens. And the mirror has only one surface that must be painstakingly ground-the achromatic lens has four. To counter changes in temperature that would affect the focal length of the reflector, large mirrors are constructed of fused quartz or of a zero-expansion pyroceramic material. The mirror's surface is coated with a thin layer of highly reflecting aluminum that is replaced many times during the life of the telescope.
FOCAL POSITION FOR REFLECTING TELESCOPES
Reflecting telescopes can be designed for many kinds of astronomical work through choice of the focal arrangement to suit the type of observation. For photography, photometry, and spectroscopy of faint objects the prime focus is best because its small focal length lessens the exposu re time required. The Newtonian focus, most useful for small telescopes, is now little used by professional astronomers. In both these arrangements the observer works at a considerable distance above the observatory floor since both focal positions are near the entrance of the telescope.
I n the Cassegrain focal arrangement a convex secondary mirror positioned at the top in front of the focus slows the rate at which light rays converge, effectively increasing the telescope's focal length. The secondary mirror reflects the converging rays to the bottom of the telescope and through a hole in the objective mirror to focus behind the objective. This is a much more convenient observing position since it is near the floor and behind the telescope. Of all the observations made with the 5.1-meter Hale telescope on Palomar Mountain 75 percent are from the Cassegrain focus.
We might think that putting the secondary mirror and its supports or the observer's cage for the prime focus into the path of the light rays would obscure part of the image; but the only effect is to cut down the amount of light reaching the objective; the loss is small, and the quality of the image is not affected.
Equipment that is too heavy and bulky to be attached to the back of the primary mirror or is sensitive to changing gravitation as the telescope moves can be placed in a room below the observatory floor. An auxiliary flat mirror diverts the long converging beam down the hollow polar axis around which the telescope rotates, and with this coude focal arrangement the focus can remain stationary no matter which way the telescope points.
TELESCOPE MOUNTINGS
An optical telescope, in order to follow an object as the earth's rotation carries it across the sky, must be free to move. To track stars accu rately and to permit a telescope to be conveniently pointed in any direction, the equatorial mounting system is used for most telescopes. This system has two axes of rotation: The telescope can rotate in an east-west direction, called hour-angle, around its polar axis, which is aligned with the earth's axis of rotation; another allows the telescope to swing in a north-south direction about the declination axis, which is perpendicular to the polar axis.
Large telescopes are usually positioned by a computer from an operating console and guided to the exact location with hand controls. Once a large telescope is properly set, the computer operating a clock drive slowly turns it westward around its polar axis at the same rate as the earth turns eastward, keeping the stellar images locked in position in the field of view. The great simplicity in equatorial mounting is that tracking requires continuous motion about only one of its two axes. The disadvantage, which obtains in the largest telescopes now in operation and planned for the future, lies in the stresses on the polar axis due to gravity. The polar axis is inclined in the earth's gravitational field and must rotate on one edge of its end. For a very large telescope that is a difficult engineering problem.
One means of removing some of the stress from the primary axis is to align it with gravity. Such a mounting is known as altazimuth mounting; with it a telescope rotates about a vertical axis and about a horizontal axis. This mounting's disadvantage is that, to track a star, it must turn continuously about both axes at the same time. When the telescope approaches the area of the sky directly overhead, continuous tracking becomes virtually impossible. Even with this disadvantage the altazimuth mounting will be the primary mounting for very large telescopes to be constructed in the future.
OTHER APPROACHES TO MAKING TELESCOPES
The principal problems in building very large telescopes on the earth's surface today are costs and construction time. A new 5-meter Hale telescope would now cost about 25 million dollars and take 10 years to build, while a 10-meter telescope would cost 200 million dollars and take 20 years to build, and a 25-meter telescope would require about 3 billion dollars and 50 years to construct. Clearly some dramatic changes in design are needed to lower cost and construction time.
A new telescope design, called the Multiple Mirror Telescope (MMT), which is well suited for infrared observations, has been installed on Mount Hopkins in Arizona. It uses a mosaic of independent mirrors of small size to collect and focus light in order to simulate the collecting ability of a largeaperture single mirror. The MMT consists of a circular array of six identical 1.8-meter mirrors on an altazimuth mounting; the array has light-gathering power equivalent to that of a 4.5-meter single mirror.
The six mirrors are not thick solid ones but are of a new lightweight design. They are partially hollow, which requires a smaller mechanical structure to move them; thus they save money and construction time. The six images from the six mirrors may be either superimposed to form a single image or aligned along a spectrographic slit, one on top of the other, to take full advantage of slit geometry. The pointing directions of the six mirrors are locked together by laser beams. This instrument has been successful in demonstrating the practicality of the multiple-mirror concept, and it may be the forerunner of telescopes that are equivalent to a 25-meter (82-foot) telescope. Under consideration is an MMT consisting of eight 5-meter lightweight mirrors, having the light-gathering power of a 14-meter telescope, the angular resolution of a 22-meter telescope, and (it is hoped) the cost of a 4-meter telescope.
The MMT is not the only new design which shows an artist conception of these new designs for future large telescopes, but it is not now certain whether any of them will ever be built. The success of the Space Telescope, a 2.4-meter conventional reflector that is to be put into orbit in early 1985, will not lessen the need for a mammoth new telescope on the ground but will probably increase it. Since Space Telescope does not have to contend with light losses produced by the atmosphere, a very large telescope will be required on the ground to observe in visible wavelengths what Space Telescope is able to observe at shorter wavelengths, where it will primarily operate.
