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The venerable craft of telescope mirror grinding takes on a modern cast. All changes; all is very much the same. To some optical astronomers the beauty of their science is in the increasing sophistication with which they can manipulate and extract information from the star-born photons gathered by their giant telescopes (See "An Astronomical Revolution," Mosaic, Vol. 7, No. 4). For others, the magic is still in the great photon-catchers themselves—the painstakingly honed masses of polished glass—mirrors in today's giant reflecting telescopes—that have been astronomy's heart since Galileo turned his crude system of lenses on the heavens in the early 17th century. The quality of an astronomical instrument has always been limited by the quality of its lens or mirror. Astronomers of a century ago, hungry for more powerful, more precise instruments, would boast of and compare the skills of their favorite optical craftsmen, as if they were patrons of art comparing the merits of the world's great sculptors. And well into this century, little, if anything, had changed in the crafting of optics for telescopes; bigger did not mean fundamentally different. The big mirrors of the 20th century do transcend the capabilities of the single artisan in glass, however; they depend instead upon the collective talents of groups of opticians. Nevertheless, the big mirrors of today's major observatories have often remained on the opticians' table for five years or more as craftsmen hovered and crawled over their surfaces, seeking through an array of instruments and their craft's traditional instinct (by Zen, says one astronomer), the near-molecular-level irregularities that defocus stellar images. This forming and finishing of large optical surfaces by classical techniques has always been a tedious, costly, subjective, and not always completely successful process. It reached its peak with the magnificent 200-inch reflector for the Hale telescope on California's Mount Palomar, installed in 1948. Since then, mirror-making, with the aid of some advanced techniques, has changed. Mirrors have improved, but some of the Zen has gone out of their crafting. The optical shop at the Kitt Peak National Observatory in Tucson—where mirror blanks become optical surfaces for Earth-bound and orbiting observatories—might be considered one of the heirs of Sir William Herschel, Alvan Clark, and other legendary makers of astronomical optics. Despite the technological revolution of the last quarter century, of which the Kitt Peak opticians have been in the forefront, classical artisans in glass would probably be relatively at home with their modern counterparts. The building is modern, but the center of action is still the big grinding table. Though today's glass disk can be the size of a backyard swimming pool, grinding tools still cluster around the table. A 15-foot-diameter, perforated Hartmann test screen—something an ancient optician would not have known, though he would readily have perceived its purpose—hangs from a wall. The first of the best Proof of the skill of modern opticians and the efficacy of relatively recent innovations in techniques is the four-meter mirror Kitt Peak opticians completed in July 1974 for the Cerro Tololo InterAmerican Observatory in Chile. Astronomers who have used this mirror believe it is the finest large telescope mirror in the world. It is astronomy's most precise large scientific instrument. It represents the future of mirror-making technique; the Hale mirror is the epitome of the past. The Cerro Tololo mirror was brought to near perfection not with Zen or any secret arts, but rather with computers, better glass, and completely rational— even obvious—adaptations of the ageold optical techniques. The three basic ingredients in this great stride in mirror-making are: • The adoption of zero-temperaturecoefficient glass. • A happy marriage of optics and data processing involving, for example, computer assistance in the analysis of mirror tests and in the location of deviations from perfection in the mirror surface. • The manufacture of the mirror on the same pneumatic pads (air bags) to be used in its final installation. This simple innovation avoids the introduction of new stresses and mirror deformations during installation. The new glasses Start to finish. From the casting of the giant Cer-Vit® biank for a four-meter telescope mirror (above), to the testing of its ground, polished, and figured surface by Hartmanrt screen and computer (left), the process of telescope mirror-making is one in which modern techniques have enhanced, rather than replaced, the craftsman's art. Ordinary glass has a coefficient of thermal expansion of about 94 x 10~7 per degree Centigrade; it expands not much less than does steel. Pyrex glass expands thermally only a third as much, but still too much for comfort. A consequence is that even telescope mirrors made from Pyrex, such as the 200-inch mirror at Palomar, change their dimensions by optically significant amounts during daily temperature changes. ("Optically significant" means a dimensional change of a quarter wavelength, about five millionths of an inch or more.) In fact, the passage of a cold front may shut a large telescope down for hours until the mirror and telescope tube become adjusted to the new ambient temperatures. High resolution work is impossible during these transient periods because the carefully devised and meticulously achieved dimensions of the mirror surface change by several wavelengths of light. A second undesired consequence of a non-zero temperature coefficient arises during the final figuring and polishing of big mirrors; the mere act of polishing glass raises the average temperature of a localized area by as much as 5°—so much that opticians cannot determine the results of their work for at least several hours, the time it takes for thermal mountains several wavelengths high to subside in a 12- to 20-ton slab of glass two feet thick. The test-polishtest cycle is therefore long and frustrating with ordinary glass. With zero- or low-temperature-coefficient glass, the cycle can be reduced to a few minutes —almost "real time" in the language of radio communication. Low, lower, lowest Lower and lower coefficients of expansion have been the telescope maker's quest, ever since glass first replaced polished metal as a telescope mirror material more than 100 years ago. The first big mirror made from a truly lowtemperature-coefficient glass at Kitt Peak was the four-meter (158-inch) mirror for the now three-year-old Mayall telescope at Kitt Peak. The material employed was fused silica (quartz), which has a thermal expansion coefficient of only 5.5 x 10~7 per degree Centigrade. The 30,000-pound quartz blank was made by General Electric in 1967 by using an array of hexagonal quartz ingots at 3,300° F. The original mirror blank was 160 inches in diameter and weighed 42,000 pounds. GE reduced the weight by 9,000 pounds by trimming the outside and sawing out a 52-inch hole from the center (through which the image is received from the secondary mirror in a Cassegrain telescope). As of 1976, this is the largest quartz blank ever made. Astronomers had realized for many years that quartz was a superior material for telescope mirrors. In 1928, GE's Elihu Thomson was engaged by George Hale to make a 200-inch quartz blank for the projected Palomar telescope. MOSAIC November/December 1976 17 During the early 1930's, Thomson successfully made three 60-inch blanks. Because of financial limitations, he had to stop before he tried the 200-inch blank. The 60-inch blanks, though of relatively poor quality, ultimately ended up at Kitt Peak where they were ground into mirrors for solar telescopes. After Kitt Peak opticians finished the Mayall quartz mirror—a three-year task —they were ready to take on a tougher assignment, a four-meter mirror blank made of Cer-Vit® material, a new kind of crystallized glass that belongs to a family of glass ceramics. It is much harder—and therefore harder to work— than quartz, which in turn is harder to work than ordinary glass. The extra hardness of Cer-Vit® is a price well worth paying; its expansion coefficient of 0.5 x 10~7 per degree Centigrade is as near zero as it may be practically possible to get. Cer-Vit® was developed by OwensIllinois in the middle 1950's as an experimental material in search of an application. It had fairly ordinary use as a high-temperature material for about a decade, when it was brought to the attention of Kitt Peak astronomers ana technicians. They tested several small (8-inch to 12-inch) blanks, and the application of Cer-Vit® to astronomy began. By 1966 several American and European observatories were ordering first 42-inch and then 60-inch blanks of the material from Owens-Illinois. Then, with a British-Australian team planning a new telescope in New South Wales, a Canadian-French group planning one ultimately installed in Hawaii, and a U.S.-Chilean group planning Cerro Tololo, the company built a 150-inch blank casting facility and produced a Cer-Vit® blank for each. The 29-ton blanks were cast from special glass that had been transformed by a carefully controlled heat-treatment process that converted the normally amorphous glass into a crystalline, ceramic substance. The largest of the three —by a few inches—was the 158-inch blank for the Cerro Tololo Inter-American Observatory. It was completed in 1970, and held while the Kitt Peak opticians completed their own Mayall Telescope mirror and made room for the Cerro Tololo job. In 1972, after sawing out a 52-inch central hole and trimming the circumference, Owens-Illinois shipped the blank to Kitt Peak. The grinding and polishing task took two and a half years. This mirror was finished by an array of advanced optical techniques worthy of the new material. From band saw to polishing The surfaces of the mirror blanks arriving at Kitt Peak are rough; when they leave Kitt Peak's optical shop, they are perfect to within a millionth of an inch. The first steps taken after the mirror blank is lowered onto the grinding table are grinding the circumference to a perfect circle at the desired diameter—four meters for the Kitt Peak and Cerro Tololo mirrors—and then polishing the edge. Next, the back of the mirror blank is ground flat. This rough grinding process takes about two months and is not highly critical because the back plays no part in the optics of a reflecting telescope. The grinding table is then tilted vertically on pivots. A circumferential sling lifts the vertical mirror and swings it out so that it can be turned 180°. Thus reversed, it is returned to the table which, in turn, is rotated back to its normal position. At this point the serious mirror-making begins; its three important phases are: • Spherifying the surface; that is, grinding out enough glass to make a spherical surface of an appropriate radius. (In the four-meter mirror, approximately 3.5 inches had to be ground out at the center.) • Aspherifying the surface. A nonspherical mirror is required to focus light coming from a distant point source. In terms of grinding, only five thousandths of an inch more is removed from the center of the mirror; the surface approaches being paraboloid. • Figuring the surface. But a perfect parabolic surface, adequate for ordinary, small telescopes, is not wanted either; paraboloids are good only for "point sources," and many important astronomical targets—such as galaxies and nebula—are "extended sources"; they can be resolved into a two-dimensional image. The parabolic surface must thus be close-shaved a tiny bit more. Spherifying a mirror blank is a task that has changed little down the years. One must ensure only that the grinding tool hollows out a spherical section of the proper radius. Frequent checks with a steel tape and a spherometer monitor this process. The grinding tools employed during this phase are relatively big and powerful. The principal Kitt Peak grinder used during spherification consists of a rotating convex surface lined with pieces of ceramic tile. This tool is ten inches thick and has the same diameter as the mirror. Between the grinder surface and the mirror surface opticians inject a layer of grinding powder. At first the powder is carborundum and has the consistency of beach sand (about l / 3 0 t h of an inch in diameter). Successively finer grades of abrasive are used until something with the consistency of flour (5-micron particles) is reached. The finer powders are usually made from garnet. Opticians mix the abrasive with water and squirt it on the mirror surface with automobile battery fillers. This rough spherifying takes about six months. Finally, molded squares of pitch, two inches on a side and a quarter-inch thick, are thermally bonded to the grinding tool, and during the next month the spherical mirror surface is polished to its final finish by charges of rouge embedded in the pitch. The easy part of mirror Talcing the edge off. Shaving large telescope mirrors, such as this 127-inch (three-meter) mirror, is a technique pioneered by opticians at Kitt Peak. 18 MOSAIC November/December 1976 Testing the surface. A pattern recorded on a photographic plate (top, left) by reflections from a mirror of a rectangular Hartmartn screen leads to a computer printout like that below showing surface departure values in wavelengths. A composite print of Hartmarsn tests at four different positions of the screen can be interpreted as areas of greater or lesser density in the plate, which are produced by high and low areas in the mirror. manufacture is now complete. It has taken a year, but so far, except for some mechanization and the new glass, the process has departed little from tradition. The difference between a spherical and parabolic surface, even with a four-meter mirror, is only a few thousandths of an inch. But the precise, painstaking removal of this minute bit of glass takes almost a year and a half. And it is with the efforts to produce the exact geometric figure that the refinements in mirror-making technology become most important. The delicate stages Smaller polishing tools, about 20 inches in diameter, are the rule during the conversion of the spherical surface to a paraboloid. The thin layers of glass to be removed from selected areas of the surface vary in thickness from zero at the edge to about five thousandths of an inch at the center of the mirror. Measurements must now be made frequently to ensure that too much glass is not removed. The excavation of hollows below the level of the desired surface would be disastrous; the entire surface would then have to be ground down to compensate. This might necessitate going back 12 to 18 months to the spherifying step. Kitt Peak opticians have developed two additional techniques that improve final mirror quality: (1) they support their grinding tools from overhead so that pressures can be controlled more accurately; and (2) they replace the fallible human memory with a computer memory. Once the lore of mirror-making resided in the head of the artisan in the form of experience; viz., how long to polish a spot with a specific tool to remove so many millionths of an inch of a specific type of glass. These tables of experience now reside in a computer program and can be called up as needed for each step in the polishing and figuring process. Neither experience nor the computer, however, are any substitute for testing. During spherifying, the relatively crude radius-of-curvature measurements are made with a steel tape. These are accurate to about a sixteenth of an inch. To measure with the millionths-of-aninch precision needed during aspherification, tapes are replaced with: (1) an interferometer capable of detecting deviations from the desired surface as small as a fraction of a wavelength of light; and (2) the classical "knife-edge" test that permits a mirror-maker to "read" the surface by interpreting subjectively the shadows that play across it. The application of both tests demands a reflecting surface. Thus, each test at this stage involves quick or "flash" polishing the area of the mirror being worked. In the past, the heat from this flash polishing has slowed mirror-making to a frustrating crawl. Hours or days could be spent waiting for friction-expanded hills a few wavelengths high to cool. The near-zero expansion of the harder, and harder-to-polish, Cer-Vit® makes its greatest contributions at this stage, one well worth the trade-off in polishing time. MOSAIC November/December 1976 19 The interferometer and knife-edge tests are aided by a "null lens" placed near the mirror's focal point. The null lens compensates exactly for the change from a spherical to a parabolic mirror. In other words, when a true parabolic surface has finally been attained, all tests made with the null lens in place will indicate true sphericity; the light-flooded mirror will appear flat. Like the trick mirrors in amusement parks, the null lens twists one figure into another. The venerable Foucault knife-edge test (a wire is actually used at Kitt Peak) checks for deviations from a perfect spherical surface. (With the null lens, of course, it tests for a perfect parabolic surface.) By shining light from a pinhole source onto the mirror and moving a knife edge across the field of view, an observer sees shadows cast on the mirror's surface which can be interpreted as deviations from sphericity. The knifeedge test is hard to interpret and only qualitative. By contrast, the interferometer test is quantitative. The distance between the bands (fringes) that the viewer sees projected by the interferometer on the mirror surface is exactly one-half wavelength apart. The deviation of bands can be related directly to height deviations on the mirror surface. The inter- 20 MOSAIC November/December 1976 ferometer test is so delicate that atmospheric irregularities within the optical shop cause the bands to wiggle. High speed photography freezes the bands so that band separation can be measured. By combining the interferometer and knife-edge test results, the spherical surface is gently polished and coaxed to a paraboloid. Removing the mountains During the figuring process and final polishing, the important concerns are: (1) correcting the parabolic surface so it will handle "extended" light sources; and (2) smoothing out the microscopic hills and valleys on the glass surface. Both processes involve frequent testing because by now the mirror surface is just a few millionths of an inch away from the desired contour. The figuring stage, as distinguished from polishing, removes the peaks and valleys from the mirror surface, which now has the desired shape and looks perfect to the human eye. Even the interferometer finds difficulty in the preparation of a topographic map of surface roughness. Nevertheless, there are rough spots with peaks tenths of a wavelength (a few millionths of an inch) high that must be smoothed out if the mirror is to produce astronomical images of high quality. The standard knife-edge test for mirror surface roughness enabled a master optician to interpret shadows in terms of "too high here" and "too low there." It gave only a general idea of the location and slope of surface imperfections and required a bit of Zen to interpret. Nor is the test sensitive to larger scale topographic features with very small slopes. Consequently, the classic knife-edge test has become only supplemental in the final, most sensitive tests of important mirrors. The common modern test for largearea departures from perfection is the Hartmann screen test. For this, photographic plates are placed near the focal plane, and a large screen with holes in it is suspended just above the mirror surface. A small, intense light source located just below the photographic plate near the center of curvature directs light down through the screen holes onto the mirror, which then reflects it back toward the photographic plate. The Hartmann screen now employed at Kitt Peak has 440 holes arranged in a square array. (A radial array was used earlier, but was not as simple to handle mathematically as is a square array.) Each hole samples the mirror surface immediately below it. Effectively, 440 little circular mirrors create 440 spots on the photographic plate. The spots on the developed plate are arranged in curved lines symmetrically around a rectangular coordinate system. If the portion of mirror surface sampled by any of the 440 holes is sloped slightly from what it should be, its spot on the pattern will be shifted slightly out of line. The direction and amount of the spot's shift can be related to the slope of the high (or low) area. By carefully measuring the x-y positions of the spots and integrating the measurements along various axes of the pattern, one can compute and draw a topographic map of the mirror surface. After one such map is prepared, the screen is rotated 45° and the process repeated. A comparison of topographic maps prepared from several screen positions indicates major features that remain fixed in relation to mirror position and which, therefore, must be true surface deviations. These features, which may be only a few tenths of a wavelength high or low, are the areas that must be eliminated to perfect the mirror surface. The ultimate test The resulting topographic maps, however, show some inconsistencies in terms of high and low areas. Scientists at Kitt Peak have taken the Hartmann test one step further to make the topography it reveals more obvious to the optician. The first step is to feed the numerical topographic data into a microdensitometer with a light-emitting diode. Operating in its playback mode, the densitometer converts the information describing the high points of the surface into bright areas on a photographic plate and low points into dark areas. This is done for several (usually four) screen angles on the same plate. The result is a composite picture that emphasizes the constant features of the topographic maps; the real physical features are highlighted while the random system noise is suppressed. This so-called Mosaic Test—though it takes days as opposed to the Hartmann test's hours—represents a significant advance in the interpretation of Hartmann test results. It is believed that surface irregularities as small as l / 2 0 t h of a wavelength (0.000001 inch) can be located using Mosaic Tests. The tests alternate with periods of polishing lasting perhaps 15 to 20 minutes each. (Polishing time, of course, is tightly restricted because of the everpresent danger of going too deep.) Lengthy as these polish-test cycles may seem, they are in reality much shorter than they would be with ordinary glass, which has to cool for days before its true surface has been restored and any tests at all can be done. Such advances in the art of telescope mirror-making may seem undramatic. They permit both more rapid and more precise forming of the critical surfaces responsible for the gathering and focusing of elusive photons. But that, after all, is what astronomy is all about. Perhaps there will always be some who will regret the diminution of the individual artisan's monopoly of so venerable an activity as the crafting of a telescope mirror. Nevertheless, the proof of the telescope is in the seeing. And within very short times after the opening of each of the new four-meter telescopes—the Mayall in 1974 and Cerro Tololo's last year—each was responsible for dramatic new astronomical advances. At Mayall, one was Kitt Peak astronomer Stephen Strom's photography and study of the dust lanes in spiral galaxies, contributing to a better understanding of star formation; another was the resolution of the surface of the star Betelgeuse by Kitt Peak astronomers Roger Lynds, S. P. Worden, and Jack Harvey. At Cerro Tololo there was the location, by visiting astronomer Sidney van den Bergh of the University of Toronto, of the optical remnant of the supernova, first reported in 1006, in the constellation Lupas. Such a remnant had been sought unsuccessfully many times, even with the giant Hale Telescope. Van den Bergh gives considerable credit for the discovery to the four-meter mirror. It is unlikely that the recently installed four-meter mirrors at Kitt Peak and Cerro Tololo are the last word in telescope optics. Our experience with technology leads us to hold, as an article of faith, that in doing things faster and better there are no last words. Somewhere in the world today there inevitably is an astronomer dreaming of the things he could see, if only he had a mirror— perhaps bigger, or perhaps only better made—than the best that was possible last year or the year before. His dream will pose what might appear now to be impossible problems for the heirs of Herschel and Clark. In return for the ability to realize such dreams, perhaps a little Zen is a small price to pay. • The work reported in this article is supported through the Division of Astronomical Sciences of the National Science Foundation. MOSAIC November/December 1976 21