IN RECENT YEARS A NUMBER of astronomers have said that the 200-inch Hale telescope is the biggest that will ever be built. This has not discouraged the proponents of still larger instruments. Two European astronomers have notably continued to experiment with an unconventional method of constructing very large mirrors. They propose making the large mirrors out of smaller ones united on a single fixed support of great rigidity.

The pessimists do not mean that engineers could not build larger telescopes, or that if built these would necessarily be defective in an instrument-engineering sense. The outer limit would be set not by engineering limitations but by irregularities in the earth's atmosphere which blur the images of stars: the larger the telescope, the greater the blurring.

It has almost been forgotten that in 1926 F. G. Pease of the Mount Wilson Observatory published a design for a proposed 300-inch telescope. His design called for a 25-foot mirror in a mounting carried on a horseshoe-bearing similar to the one that now carries the 200-inch, a feature proposed by Russell W. Porter in 1918. Pease, an astronomer, precision optician and engineer who with G. W. Ritchey designed the 100-inch telescope, was not frightened by magnitude; he stated that "anything up to 100 feet in aperture can be built provided one wants to pay for it." But the financing of a conventional telescope larger than the 200-inch would be a very serious problem. The 200-inch cost $6,550,000, and might cost twice as much today, and a 300-inch would be theoretically three times as bulky and costly. These difficulties have been tackled in a radical way by Guido Horn-D'Arturo of Italy and Y. Vaisala of Finland.

In 1932 Horn-D'Arturo, an astronomer at the University of Bologna who had worked at the Lick Observatory in California, was meditating upon methods of building very large telescopes. He recalled an experiment made a century earlier by William Parsons, otherwise known as Lord Rosse (the same Lord Rosse who in 1845 built a six-foot telescope and became one of the great figures in the history of the telescope). In 1828 Lord Rosse had described an experiment with a composite mirror of speculum metal. As shown at the upper left-hand corner of Roger Hayward's drawing on the left, the mirror consisted of only two pieces: an inner circle and a ring around it. The purpose was not to attain great size but to cope with spherical aberration without the necessity of parabolizing the mirror. By adjusting the screws in the brass plate beneath the mirror, Lord Rosse was able to alter the sphere to a two-step approximation of a paraboloid.

The article by Lord Rosse gave Horn-D'Arturo an idea for a method of making gigantic mirrors. They could be built up of identical smaller mirrors that were spherically concave, relatively thin and inexpensively mass-produced. These would be mounted on an extremely rigid common backing. By means of adjusting screws, each ring of mirrors could be raised a little above the ring within it, and each mirror could be tilted to focus on the same spot.

Such a composite mirror could also be constructed of individually figured paraboloids. Obviously these would be more expensive than identical spheres. Their resolving power would moreover only equal that of a single mirror, as explained in Amateur Telescope Making, page 317. This would not be true of spheres.

Horn-D'Arturo calls his mosaic mirror a tessellated mirror, and each piece of the mosaic a tessera. His experiments were performed in the 160-foot high, 30-foot square stone tower of the University of Bologna, built in 1712. In its upper part is a large room with a 88-foot ceiling. Here Horn-D'Arturo erected a solid timber table with a thick marble top, on which he assembled the 10 tesserae then in his possession. Each was approximately trapezoidal and about four inches across. When they were assembled as shown at left center in the drawing on the opposite page, they could be used to test a hypothetical 41inch telescope. The focal length of the tesserae averaged 410 inches or about 34 feet.

In the ceiling directly above this assembly was an opening 50 inches in diameter. Across this Horn-D'Arturo mounted a spider and testing equipment for collimating the tesserae. Later there would be a motor to move a photographic plate horizontally as the earth rotated.

Here is revealed the major feature of the Horn-D'Arturo telescope: only the plate is moved, while the mirror remains fixed firmly, immovably and permanently in the horizontal position. It is backed, braced and held rigidly to the earth, while a tiny motor moves the plate to compensate for the earth's rotation.

Thus the Horn-D'Arturo telescope "sees" only the relatively tiny area of the sky directly above it. As the earth rotates the telescope sweeps out around the sky a band or torus of less than two degrees in width. A chain of these telescopes, however, could cover a relatively broad belt. Six of them spaced at 115-mile intervals could photograph the heavens above all of Italy.

At the bottoms of six pits 200 feet deep would be six composite mirrors of 230-inch diameter. At the surface a 3 1/2-by-12-inch photographic plate would be moved horizontally westward as earth turned eastward. This would afford exposures of 6 minutes and 15 seconds before the whole width of the mirror was used. Other fields along the band of sky would then be photographed, but the same one could not be photographed again until another night.

SIX - huge telescopes of the Horn-D'Arturo type should not cost as much as a single one of the conventional type. Each telescope would consist only of a hole in the earth, a composite mirror, and a spider with motor drive and plate holder. Because we are so familiar with conventional telescopes many of us unconsciously take it for granted that a telescope must include all the standard mechanisms, and thus we accept cost and complexity as inevitable. To emphasize this complexity, here is a description taken from an article in the August, 1948, issue of this magazine:

"The optical essence of the two-million-pound telescope weighs less than a quarter of an ounce. Light from the stars reaches a paraboloidally curved aluminum mirror 200 inches in diameter and 1/200,000-inch thick-an amount of metal approximately the bulk of a nickel-which reflects it to a light-sensitized sheet of photographic emulsion a thousandth of an inch thick. All the remainder of the telescope consists of accessories: the glass plate that supports the emulsion; the glass disk that supports the aluminum, the 36 levers with counterweights that support the glass disk and the platform or cell that supports both; the tube that carries the cell; the yoke the carries the tube; the base frame that carries the yoke.... The function of all these accessories is to support mirror and emulsion in correct geometrical relation and to move them precisely and controllably."

The key to the argument is in the final six words of this quotation, with heavy emphasis on the word move. If we can subtract from our telescope this necessity for moving the large mirror without deforming it, there remains a need for only a very small mechanism. This is why the Horn-D'Arturo telescope is inexpensive. The mechanism that contributes the motion is free: it is the rotating earth.

The limited view of the tubeless, mountingless telescope might be corrected by adding a coelostat of two mirrors to bring the light from any part of the sky to the composite mirror. Unfortunately each of these two mirrors would then have to be as large as the main mirror and to have elaborate equipment for maintaining its rigidity as it moved. This would require almost as much paraphernalia as a conventional telescope or, rather, twice as much, since there would be two mirrors. Roger Hayward suggests a group of telescopes all in the same place with mirrors attached to the earth at differing slants. This might cost little less, but would concentrate the observing staffs.

In 1986 Horn-D'Arturo obtained 10 additional tesserae and with 20 now in hand he was able to assemble a quadrant of his 41-inch mirror. Then anti-Semitic persecutions in Italy delayed his experiments until 1945. In a new publication of the Observatory of Bologna dated 1950 and entitled "Further Experiments with the Tessellated Mirror," Horn-D'Arturo has described his last five spherical aberration. This elevated the first circle of 6 tesserae about 1/50-inch above the central hexagon, and the second circle of 12 tesserae about 1/16-inch above the first.

A more precise problem was to bring all the 19 foci to coincidence at the same spot. This required adjustment in parallel rays of light, and such rays were manufactured successively above each tessera by means of a small collimating telescope. In each instance the collimator was accurately adjusted perpendicular to the same reference plane-the surface of a basin of colored water-by means of interference fringes. The basin was then removed and the image reflected by the tessera was brought to the common focus. Two persons, one manipulating the screws under the marble slab, the other 34 feet above in the next story of the tower, watching the focused image and directing its movement by electric signals, were able to complete the collimation in two hours.

A small mechanism compensates for the rotation of field during exposure of the plate. An actual photograph of a star field made by the 41-inch telescope is shown in the Horn-D'Arturo report and reveals that "all the small images at the edges and in the corners are equally round." Coma is "lost in the diffusion disk."

In two other papers, publications of the Astronomical Observatory of Bologna, Horn-D'Arturo analyzes "The Deformation of Stellar Images Due to Coma, Resolved into its Elements" and "Extra-axial Stellar Images Generated by Paraboloidal, Spherical and Tessellated Mirrors." These are available for loan to advanced amateur telescope makers who can brave mathematical optics in Italian. The last-named paper demonstrates "that the form and dimension of the images theoretically obtained from the tessellated mirror are practically identical with those generated by a paraboloid of revolution."

THE SECOND European proponent of the large composite mirror is Professor Y. Väisälä of the University of Turku in Finland. In optical literature Väisälä's name has often been linked with that of the American Franklin B. Wright. As George Z. Dimitroff and James G. Baker state in Telescopes and Accessories, both Väisälä and Wright have independently investigated the possible positions of the correcting plate of the Schmidt camera, and both have found that by varying these positions a spherically corrected and coma-free system may be had for each distance of the correcting plate along the axis.

As in the Wright camera, and as in the drawing above, Väisälä 's correcting lens is not at the center of curvature of the spherical primary but is a little closer to the mirror. A field-flattening lens lies in front of the plate, which is therefore plane instead of curved as in the Schmidt.

In 1984 Väisälä built a 6 3/4-inch f/l.9 instrument of this type. It worked so well that he next built a 20-inch f/2 of the same type. These were the first wide-field telescopes with flat plates and the first anastigmats in the world. Väisälä has been optician, engineer, architect, carpenter and bricklayer of the Turku observatory; now he is its director. In Urania, an astronomical journal published in Madrid and Barcelona, Väisälä has described in Spanish his proposal for fixed composite-mirror telescopes with moving photographic plates, to be erected in deep pits in the earth. It appeared under the title "A New Procedure for Constructing Gigantic Telescopes." The following is a translation of the major portions of this article. The amateur telescope maker may discover a hint for a way to build a large mirror at less expense than that of a single blank.

"I shall present here a new procedure for building large telescopes. At first glance it seems audacious, but I believe that it is worthy of much experimentation. The mirror of the telescope, instead of having a continuous surface, is a compound of separate mirrors solidly united by a rigid support of skeleton construction made of steel bars or tubes. By this method it is not necessary to make the Component mirrors very large, for by increasing their number a gigantic mirror is obtained.

"In the spring of 1949 I began to build a trial telescope, a miniature model for immense telescopes. Its spherical mirror is composed of seven individual mirrors of 12 1/2-inch diameter and 1 1/5-inch thickness. Their total surface corresponds to that of a single mirror of 33 1/2-inch diameter. Each one of the individual mirrors rests on three adjustable screw points above the steel framework. In the extreme top of the tube, which is made of heavy slats of wood, will be placed the spherically corrected lens of 35 1/2-inch diameter.

"We intend to use the telescope as a simple Schmidt, or as an anastigmat with the correcting lens in the neighborhood of the focus, in such a way that it will have a wide field on a flat plate. Perhaps we can equip the telescope in addition with a flat removable secondary, inclined at 45 degrees, to perform visual observations.

"The mirrors with their support weigh 187 pounds. The skeleton construction should be capable of resisting flexures and should follow well the temperature changes by freely circulating air between the mirrors and the steel bars of the support. It is not necessary to locate the individual mirrors exactly in the same spherical surface; that is, not with 'interference' precision. To obtain near enough to point images it is sufficient that the light rays reflected by the various mirrors fall on the same point on the photographic plate with a precision of some hundredths of a millimeter since, as experience shows, the star images always measure some hundredths of a millimeter on the plate.

"In dealing with immense telescopes we need take into account only the reflecting type. To reduce the difficulties of fabrication and adjustment it will be best to make the component mirrors spherical. In such a case we shall have to provide the telescope with a correcting plate. There are other solutions if one is content with a narrow field. The fabrication of the correcting lens, even in large dimensions, does not offer any insuperable difficulties. It may be made very thin, so that flexure will cause only a minimum deviation of the light ray. Thus it should be possible to make a correcting lens of a single piece of glass, even with a diameter of 16 to 33 feet, thereby avoiding the inconveniences which arise from constructing it in separate parts.

"If the practical trials show that the principle of composite mirrors is useful, the construction of a telescope of 200 to 400 inches will become considerably easier and more economical than was the telescope at Palomar Mountain. To avoid excessively high towers, large relative apertures will be chosen, perhaps one to two or one to three. For example, with relative aperture of 1:2.5 the tube of a Schmidt telescope of 16 1/2-foot diameter will become 82 feet long.

"When we discuss a super-giant telescope the difficulties of building the tube, mounting and dome increase with each increase of dimensions. But we can assume that supergiant telescopes will be built only for observing limited regions of the heavens; for example, the neighborhood of zenith; to compensate for this, such instruments will be built in different latitudes. The spherical mirror can then be mounted fixed in the earth. Let us suppose that an f/2 super-Schmidt is 66 feet in aperture. The correcting lens will be 262 feet above the earth's surface, supported by appropriate towers. At 131-foot height will be the cabin of the observer, who will move the photographic plate on a surface whose

center of curvature falls at the center of the correcting plate. But the mirror, whose center of curvature is the same, may remain fixed. During the exposure a new part of the mirror will be use each time. Even if the exposures cannot be very long, perhaps a maximum of half an hour, that will be enough time given the enormous aperture.

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