Optical Telescopes

Type of physical science: Astronomy; Astrophysics

Field of study: Observational techniques

Optical telescopes are instruments designed to collect light from distant objects so that images may be produced or the light analyzed by a detector. The most powerful of these are reflector telescopes, which focus light with curved mirrors. Telescopes that use multiple primary mirrors are becoming popular as a means of circumventing size limitations on older designs.

Overview

Light collectors are generally known as telescopes (from the Greek meaning "to see from afar"). Although they are the most visible portion of an observatory, they are really only photon buckets and conduits that intercept and concentrate enough light to effect a change in a detector (that is, to cause a reading, make a film image, and the like). Telescopes operate by two phenomena, refraction and reflection, to concentrate light.

In refraction, light is bent as it passes through a material; the degree to which it is bent depends on the angle at which the light strikes the surface and the material's own index of refraction. If the material's surfaces are curved to match a spherical surface, then the incident light can be brought to a point known as the lens's focus. Nevertheless, the index of refraction varies with the wavelength of light (longer wavelengths are bent less than short wavelengths), which results in the projected image having its colors smeared (chromatic aberration). This can be compensated for by using multiple sets of lenses with different indices of refraction and varying focal lengths to compensate for the effect. Refractor telescopes are useful but have a number of limitations, including a maximum aperture of 1 meter, because at that point deformation caused by gravity becomes noticeable. Further, glass will absorb portions of the incoming light so that its spectral range is limited.

Reflector telescopes have proved to be far more versatile and powerful for astronomy.

Reflectors are based on the simple law of the angle of incidence equaling the angle of reflection.

If the mirror is shaped to match the curve from a conic section (such as a parabola or hyperbola), then the light from a source at infinity will be brought to a focus regardless of where on the mirror it strikes. Light from a source slightly removed from the centerline of the telescope (off axis) will be focused at a point proportionally distant from the center, and an image will be formed. In practice, the design of a telescope is far more difficult, since reflectors have their own problems. The focal plane is actually a curved surface and design efforts attempt to make that curve as flat as possible so that aberrations are of little effect, especially in the center.

Few reflector telescopes work in the "prime focus mode," where a camera or detector is placed directly at the focal plane, because of the distortions that normally result. The most widely used reflector telescope types are Newtonian and Cassegrain. The Newtonian reflector is the original reflector developed by Sir Isaac Newton. It has a parabolic primary mirror and a flat secondary mirror at the focus of the primary. The secondary mirror reflects light at a right angle to an eyepiece (normally refractive) or instrument at the side of the telescope barrel. Newtonians are not widely used outside amateur astronomy.

The Cassegrain telescope and its variants have become the most widely used in professional astronomy because they greatly reduce obstructions in the field of view and allow instruments to be mounted behind the primary mirror for easier servicing. The classic Cassegrain uses two parabolic reflectors, a concave primary to collect light and a convex secondary to reduce aberrations and refocus light through an opening in the center of the primary mirror and to instruments mounted behind the mirror. A popular form of the Cassegrain is the Ritchey-Chretien (named for George Ritchey and Henri Chretien), which has hyperbolic primary and secondary mirrors (other variations use ellipsoidal mirrors, depending on the field of view or other desired effects). A variation possible with any Cassegrain telescope is the coude focus, in which a third, flat mirror (between primary and secondary) reflects the incident light through an aperture in the telescope's structural axle. This allows heavy instruments to be mounted on the floor by the telescope and to receive a stable view, despite the telescope's rotation to stay pointed at the stars. The coude focus, though, is of less value in an era of lightweight electronic systems, which can be easily mounted on the telescope itself.

The laws of reflection work at any wavelength and without regard to color (that is, there is no chromatic aberration). As a result, the basic layout of visible light reflector telescopes can be seen in radio and X-ray telescopes (the principal difference is the smoothness of the reflective surface compared to the wavelength being focused). Reflectors suffer from a different problem called "coma," which smears an image into a comet shape pointing away from the focus.

Coma is greatest the farther one moves from the focus and can be corrected by complex optical systems.

A hybrid telescope design is the Schmidt, which combines elements of reflector and refractor designs. Spherical rather than parabolic primary mirrors are ideal for making wide-field surveys of the sky but suffer from extreme chromatic aberration. This is corrected in the Schmidt design by a large refractive corrector plate at the telescope aperture, which introduces chromatic aberration in the reverse of what the spherical reflector would cause, thus producing a correctly colored image.

Reflector telescopes, like refractors, have been limited by the bulk of the primary mirror. Besides the great difficulty in manufacturing a large glass blank to great precision, there also are problems in supporting so massive a structure; it sags of its own weight, even when hollowed in the back. The atmosphere poses additional limitations in that the resolution possible from the ground is limited to about 1 arc second. A full circle encompasses 360 degrees; there are 60 arc minutes in 1 degree and 60 arc seconds in 1 arc minute. The apparent diameters of the sun and Moon, for example, are about 30 arc minutes. Air has its own index of refraction, which can be enhanced where cold layers meet warm layers of air. The atmosphere thus acts as a variable lens that misshapes starlight (causing stars to twinkle) even before it enters a telescope.

As a result, the best "seeing" that can normally be achieved on the ground is about 1 arc second and can be achieved with a telescope aperture of 40 to 50 centimeters. Telescopes larger than 50 centimeters may see fainter objects, but not to enhance resolution (or, stated differently, magnification). Further, the background glow contributed by the atmosphere also serves as a limit on how faint an object can be seen by a telescope. Isolated, mountaintop locations can provide occasional seeing of 0.4 arc second or better, but this is only a partial solution.

Telescope mirrors are made from glass because it is stable and can be shaped by grinding with specialized materials. Metal mirrors are not widely used in reflector telescopes because they are not as resistant to changes in shape with temperature shifts. The rule of thumb in polishing a mirror is that its surface must match a theoretically perfect curve to within a quarter wavelength of light. A surface that is bumpier than this will have the same effect as a funhouse mirror. Achieving the required smoothness can be a phenomenal challenge for large mirrors, especially those that will observe the shorter wavelengths in the optical window. After polishing, the telescope mirrors are "silvered" with a thin layer of aluminum and a thin coating to protect the aluminum from oxidation. "Silvering" is an archaic term from the earliest days of reflector telescopes, when silver was actually used.

The advent of advanced computerized control systems in the 1970's raised the possibility of building large telescopes that would not be perfect when finished but could be made so during an observation. Basically, the concept was to use actuators to adjust the shape of the mirror several times a second in response to changes in the atmosphere. Two approaches are employed. In the first, the images from several telescopes are joined to yield the same effect as a larger mirror. This allows construction of a telescope using several conventional primary mirrors but requires the use of a complex, precision optical alignment system to keep the multiple telescopes pointed at the same target, then to merge their images accurately. The alternate design is to fabricate a single primary mirror as a series of segments, which are mounted on a support structure. Laser rangefinders then measure the position of each mirror and feed that information to computers that calculate how far each must be adjusted to align the collection. (Some proposals have been made for using both approaches--joining the images of several segmented-mirror telescopes.)

Another important element for a telescope is the mounting and pointing system. The two principal mounts are equatorial and altazimuth. Each mount can rotate the telescope in two axes. In the equatorial design, the main axis is perpendicular to the earth's equator (parallel to the axis of rotation) so the telescope, once pointed at a target, can be rotated opposite the earth and thus keep the view stable. In the latter situation, the telescope is in a fork mount somewhat like a surveyor's transit. While mechanically simpler, it is far more complex to operate and was not widely used until the advent of modern computers, which allowed precision automated tracking.

Applications

Optical reflector telescopes have served as the primary instruments through which much of the universe has been explored. Optical astronomy is conducted from the surface of the earth and from satellites in orbit, where the blurring and absorptive effects of the atmosphere are largely eliminated. Most space-based astronomy has involved those regions of the spectrum that are obscured by the atmosphere, so the first optical astronomy telescope to be launched was the 2.4-meter Hubble Space Telescope in 1990.

Although one often thinks of the 5-meter Hale telescope (Mount Palomar Observatory) when talking about astronomy, such instruments are the exception rather than the rule. Most large observatories have primary mirrors between 2 and 4 meters in diameter not only because resolution is sharply limited but also because the cost of telescopes increases with the size of the main mirror. A 4-meter telescope will cost about six times as much as a 2-meter telescope, all other things being equal.

Observatories may be classed as old technology and new technology. New technology telescopes, a generic term applied in the late 1970's, are those that use multiple or segmented mirrors to gain the effect of a single larger observatory. Old technology telescopes are those that use a single main mirror, although many were built in the 1970's and 1980's and took advantage of new advances in materials, computers, and the like. Space-based telescopes that operate in orbit and make use of the broader spectrum that is available there have been orbital imitations of ground telescopes.

Despite post-launch problems, the Hubble Space Telescope represents the greatest advance of old technology optical astronomy (even despite the flaw accidentally built into its mirror). The Hubble uses a Ritchey-Chretien variation of the Cassegrain design: Its 2.4-meter paraboloid primary mirror focuses light onto a hyperboloid secondary mirror that projects the image into a focal plane shared by a total of eight instruments behind the primary mirror.

The Multiple Mirror Telescope (MMT) atop Mount Hopkins in Arizona was the first of the new technology telescopes. Its design arose when the Air Force offered six 43-centimeter paraboloid mirrors to the Smithsonian Astrophysical Observatory in the 1970's. The Smithsonian decided to take advantage of design work in multimirror telescopes and built an observatory with the six mirrors arranged in a hexagonal pattern.

Following the success of the MMT, a number of New Technology Telescope (NTT) designs were suggested, although funding became a major issue. The first of the NTTs to be built was the 10-meter Keck Memorial Telescope atop Mauna Kea, Hawaii. The Keck mirror encompasses thirty-two hexagonal 0.9-meter segments mounted atop electrically driven actuators. Instruments on the telescope sense the state of the wavefront entering the telescope and adjust the position and tilt of each mirror to compensate. In effect, the mirror becomes an optical complement to the atmosphere's distorting effects and thus presents a clean wavefront to the secondary mirror.

Speckle-cell interferometry, a computational technique, has been applied to enhance the resolution from ground-based telescopes. This approach treats the atmosphere as the first optical element in the telescope. All that is needed, then, is to understand the optical figure of that element and adjust the optics of the telescope itself to cancel those effects.

This would be quite simple if the atmosphere were a static layer of gas, but in fact it roils and shimmers like a desert mirage (an apt comparison, since it is caused by layers of cold air over warm). In speckle-cell interferometry, a laser is beamed into the atmosphere and the return signal is analyzed to determine how much the perfect spot beam has been distorted.

An alternate technique is to measure distortion in a star's image. From this technique, a corrective algorithm is derived and used by a computer to adjust the primary mirror of the telescope or to adjust the image from the charge-coupled device (CCD). Because the atmosphere changes rapidly, this technique became possible only with modern computers and electromechanical actuators that could keep up with the atmosphere.

Context

Optical telescopes were first used for astronomy in the 1600's, when Galileo manufactured his own telescope and pointed it at the heavens. His discovery of mountains and valleys on Earth's moon and of smaller moons orbiting Jupiter inspired generations of observers who quickly discovered the rings of Saturn and many other phenomena. Development of advanced mathematics also allowed Newton to formulate the design of the first reflector telescope using curves from conic sections. Problems in polishing and silvering the mirrors, however, caused it to lag behind refractors in size and capabilities for some years. Refractor and reflector designs were competitive into the late 1800's.

Modern reflector telescope history may be dated from the early 1900's, when the Hooker 2.5-meter telescope was built atop Mount Wilson, California, in response to the 1-meter limit on refractors. The success of the Hooker telescope led to construction of an array of telescopes through the first half of the twentieth century and reached its zenith with the completion of the 5-meter Hale telescope in 1950. The Schmidt telescope was developed in the 1930's and a 1.2-meter Schmidt was used in the 1950's to produce the Palomar-National Geographic sky survey that mapped the entire heavens down to 30 degrees south latitude. In the 1950's and 1960's, though, it seemed that optical telescopes had reached their peak because of cost and limits on detectors. After the Hale telescope, for example, the next biggest observatory was the 3-meter Lick Observatory at Mount Hamilton, California, in 1959.

New technologies in mechanical engineering and electronics soon made possible more sophisticated telescope design and highly advanced detectors, respectively. The result was new life for optical astronomy as the 1970's and 1980's saw many 3- and 4-meter class telescopes being built. Of special note are the 4.1-meter Cerro Tololo Interamerican Observatory at La Serena, Chile, and the 4-meter Kitt Peak National Observatory at Kitt Peak, Arizona, built as a complementary pair to provide similar capabilities in Northern and Southern Hemispheres. As an outgrowth of this boom, the 2.4-meter Hubble Space Telescope was designed and built. In some respects, it is the ultimate optical telescope, since its design allows use of the entire electromagnetic spectrum from 121 nanometers (where dust obscures much of ultraviolet light) to 1 million nanometers (where the telescope "glows" from its own body temperature). The Hubble Space Telescope was built to excruciating tolerances in order to take full advantage of the "perfect" view afforded in space. As a result, thermal expansion and surface smoothness problems that are of little consequence on the ground (because atmospheric turbulence causes greater distortion) became of paramount importance. Unfortunately, a manufacturing error produced a primary mirror with a slight case of spherical aberration that holds the telescope short of perfection. Nevertheless, in its early months, the telescope still showed the promise of performing better than ground-based telescopes, and an in-orbit repair is expected to provide corrective optics to fix the problem.

The 1970's also saw the rise of multimirror telescope technologies, led by the construction of the Multiple Mirror Telescope. Its six 1.8-meter primary mirrors operate to the same effect as a single 4.5-meter telescope. Although the concept has been around for some years, it had not been feasible because of the dynamic computer controls necessary to maintain alignment of the mirrors. The success of the Smithsonian Astrophysical Observatory and the University of Arizona with the MMT encouraged other designers to proceed with more ambitious multimirror designs. First among these is the 10-meter Keck Memorial Telescope. A number of other multimirror telescope designs have been proposed and may be built into the twenty-first century.

The rise of advanced electronic detector systems gave existing telescopes a new lease on life and encouraged the construction of new telescopes. By the late 1960's scientists were approaching the limits of what could be done with instrumentation then available for ground use.

As a result, many of the advances in astronomy were coming from smaller telescopes observing in space (on satellites, rockets, or balloons). The invention of the charge-coupled device, a sort of electronic retina, revolutionized optical astronomy. In 1976, the first astronomical CCD views, through the 1.5-meter Mount Bigelow Observatory in Arizona, produced images of Jupiter, Saturn, and Uranus--the latter revealing "limb brightening" of the planet's methane atmosphere for the first time. Although the resolution of CCDs was crude compared with film, they allowed direct brightness measurements (especially when used in spectrographs) and produced images that could be directly manipulated by computers. It also became apparent that the greater sensitivity of CCDs meant that many more targets could be observed in the time that one would require with film or conventional instruments and that a series of quick images could be computer corrected to compensate for atmospheric distortion and thus improve the telescope's resolution.

Principal terms

INDEX OF REFRACTION: the degree to which a material will bend a ray of light as it enters or leaves; the change is calculated with the angle of incidence

OPTICAL WINDOW: the region of the electromagnetic spectrum (295 to 1,100 nanometers) that is passed by the atmosphere and that is easily manipulated by lenses and mirrors; "visible" light (400 to 700 nanometers) lies near the center of this window

REFLECTOR TELESCOPE: a telescope that works principally by using curved mirrors to focus light

REFRACTOR TELESCOPE: a telescope that works principally by using lenses to focus light

RESOLUTION: a measure of the level of detail that a telescope can "see" in a particular scene; resolution depends on the design of the telescope and on "seeing" conditions between it and the object

SPACE TELESCOPE: generically, any astronomical telescope that operates in space rather than on the earth

SPECTROSCOPY: measurement of the intensity of light at specific wavelengths (energy levels) in the spectrum

TELESCOPE: a device that permits detailed inspection of a distant object; originally, an instrument that used lenses or mirrors to collect large quantities of light and focus an image from a tiny area

WAVELENGTH: the distance between the crests of two successive waves of electromagnetic radiation; longer wavelengths correspond to lower energy levels and shorter wavelengths to higher energy levels

Bibliography

Field, George. THE SPACE TELESCOPE. Chicago: Contemporary Books, 1989. Description of the origins of the Hubble Space Telescope and the problems of ground-based and space-based astronomy. Includes descriptions of how optics and detectors work.

Krisciunas, Kevin. ASTRONOMICAL CENTERS OF THE WORLD. Cambridge, England: Cambridge University Press, 1988. Survey of the major ground-based observatories and a description of their capabilities.

Kuiper, Gerard P., and Barbara Middlehurst, eds. TELESCOPES. Chicago: University of Chicago Press, 1977. Technical description of telescope design, with the Hale and Lick telescopes used as examples.

Moore, Patrick. ASTRONOMICAL TELESCOPES AND OBSERVATORIES FOR AMATEURS. New York: Norton, 1973. Introduction for the amateur astronomer to the basics of optical astronomy. Most details apply, though, to large-scale observatories.

U.S. National Research Council. Astronomy Survey Committee. ASTRONOMY AND ASTROPHYSICS FOR THE 1980'S. 2 vols. Washington, D.C.: National Academy Press, 1982-1983. Recommendations of the National Research Council for new astronomy facilities; addresses major astronomical questions. Written as an official report, but contains much useful information.

Converging and diverging light

Hubble Space Telescope