ON JULY 10 the sun will be totally eclipsed along a path 110 miles wide extending from eastern Asia across Alaska and Canada to a point in the southern Atlantic about 1,000 miles off the coast of Africa. The partial phase of the eclipse will be visible in the northern half of the Western Hemisphere, the northern coast of South America and western Europe from Spain through the Scandinavian peninsula. Only one more total eclipse of the sun will be visible in North America in the 20th century. For this reason many people from the U.S., Canada and Mexico are planning to observe the event either in Alaska, where the moon's elliptical shadow will pas just inside the Arctic Circle, or in the Canadian provinces of Quebec, New Brunswick and Nova Scotia.

George G. Manning of New York, who has had the privilege of observing many eclipses, points out that observing on can be hazardous, particularly for an in experienced layman viewing with a improvised filter. Discussing technique for observing the sun safely and for making photographs of either the partial o the total phase, Manning writes:

"The trick of observing and photographing the sun successfully lies in reducing the intensity of the light by a factor of at least 100,000 without degrading the quality of the image. The reduction can be accomplished in several ways, including the use of various filters. Some filters consist of a transparent sheet of gray glass or plastic. They transmit more or less light depending on the tone of the gray. They are known as neutral density filters because they transmit all colors equally. Their neutrality, however, does not necessarily extend beyond the visible spectrum. For example, filters made of gelatin or plastic (such as the Eastman Kodak Wratten No. 96) are dense in the visual and photographically active portions of the spectrum but are relatively transparent to infrared rays that can permanently damage the eye. Many clear, thin materials are also transparent to infrared. During past eclipses laymen have suffered eye damage by looking at the sun through such improvised filters as blackened photographic film and sheets of glass coated with soot.

"Most filters that are safe for visual observation operate on the principle of reflection. Some types consist of a highly reflective metallic coating deposited on an optically flat sheet of glass. The metal reflects both visible and infrared rays, and the glass absorbs any ultraviolet rays transmitted by the metallic coating.

"Neutral-density filters are rated in terms of density, which is defined as the logarithm of the reciprocal of transmittance. For example, assume that a filter transmits .00001 of the light, a reduction of 100,000 in brightness. The reciprocal of the transmittance is 1/.00001, or 100,000. The logarithm (to the base 10) of 100,000 is 5. The filter is designated No. 5. Similarly, a filter that transmits a tenth of the light is designated No. 1, a filter that transmits a hundredth of the light is designated No. 2 and so on. The numbering system enables the experimenter to determine by simple addition the optical result of combining two or more filters. A No. 4 and a No. 2 filter combined have a density of 6. The transmittance of the combination is 1/1,000,000. (Note that the number of zeros in the divisor is equal to the numeral that designates the density-6 in this case.) The combination would reduce the brilliance of the light a millionfold. The sun is about a million times brighter than the full moon. Hence when the sun is viewed through a No. 6 neutral-density filter, it looks about as bright as the full moon.

"A number of schemes have been devised for reducing the brightness of sunlight that emerges from the eyepiece of a telescope. The simplest and perhaps the safest system consists in fully covering the front of the telescope with a mirror that reflects the unwanted light and heat directly toward the sun. The mirror is coated with a metallic film equivalent in density to a No. 5 filter. The mirror consists of a metallic coating deposited on a sheet of glass whose surfaces are parallel and polished to within a fraction of a wavelength of optical flatness. Filters of this kind have recently become available at reasonable prices. They are made by the Bausch & Lomb Company (Box 938, Jamestown, N.Y. 14701). The filters are manufactured in three sizes (for telescopes of 4-1/4, six and eight inches) and are currently priced at $20, $35 and $50 respectively.

"Assume that the intensity of the sunlight has been satisfactorily reduced by this filtering technique or by another of the several systems that will be discussed. The photographer who is intent on photographing an eclipse next confronts the problem of determining the size of the sun's image at the focal plane of the camera. When the sun is photographed with an ordinary camera that has the customary 50-millimeter lens the solar image is about half a millimeter in diameter, comparable to the width of the period at the end of this sentence. The size can be increased by an objective lens of greater focal length.

"The diameter of the sun's image can be calculated roughly by dividing the focal length of the objective by 100. For example, an image of the sun approximately half an inch in diameter is focused in the plane of the film by a lens of 48 inches focal length. The exact diameter of the image is calculated by multiplying the focal length of the lens (or mirror) by the angular diameter of the sun in minutes of arc and dividing the product by 3,438. On July 10 the angular diameter of the sun will be 31.4633 minutes of arc. The diameter of the sun's image when the sun is photographed on that date with an objective lens of 48 inches focal length will be 48 x 31.4633/3,438 = .439 inch.

"For photographing the partial phases of the eclipse a focal length of between 1,000 and 2,000 millimeters is suggested. Achromatic lenses of adequate size for photographing a solar eclipse are fairly inexpensive: a lens with a focal length of 1,270 millimeters and an aperture of 51 millimeters is available from the Edmund Scientific Co. (300 Edscorp Building, Barrington, N.J. 08007) for $13.50 (Catalogue No. 30,190). Six-inch reflecting telescopes of f/8 focal ratio, which are made by thousands of amateur telescope-makers, are also adequate. The observer can make reasonably good photographs of the eclipse by coupling a 60-millimeter spotting telescope to a 35-millimeter reflex camera of the single-lens type. An apparatus of this kind has the advantages of variable focal length, compact size and general utility.

"Observers who photograph the total phase must perform several operations within a rather short time. The maximum period of totality during the July eclipse will be only 155.6 seconds. On the basis of my own experience I recommend the following procedure. Just before totality remove the filters from the camera. If the objective is adjustable, set the instrument to f/8. Adjust the shutter for minimum exposure. As totality begins, make an exposure at every possible shutter setting, for example 1/1,000, 1/500, 1/250, 1/125 and so on. As totality ends be prepared to photograph the 'diamond ring' effect. Replace the filters as quickly as possible. The corona, which becomes visible at totality, will probably be more extensive than it was during the 1970 eclipse. For this reason those who plan to photograph the full corona should use a lens of not more than 600 millimeters focal length.

"All equipment needed for the project should be assembled well in advance of the expedition. Practice operating the camera and inserting the filters until the routine becomes automatic. The proper exposure interval for the partial phase should be determined experimentally before the eclipse by photographing the sun. First, make a record of the filter combination and focal ratio that will be used during the eclipse. I suggest a focal ratio of f/16. Make a series of exposures covering the entire range of available shutter speeds. Keep a record of the shutter speed at which each frame is exposed. Select the most pleasing photograph and note the shutter speed at which it was taken. Use that shutter speed for photographing all partial phases of the eclipse. A rigid but adjustable tripod is essential for supporting the camera or the telescope. Operate the shutter with a cable or an air-pressure release to avoid jiggling the camera.

"If the film is to be developed by a commercial laboratory, include a normal pictorial scene at one end of the roll as a guide for the technician who will cut the film into individual frames. As a precaution against the accidental loss of the film I ordinarily make one exposure of poster that includes my name and address. I usually buy several rolls of film at a time and make sure that they have the same emulsion number, which is printed on the box, usually above the expiration date. I use one roll for determining the proper exposure time for the partial phase and store the remaining rolls in a freezer until the day before the eclipse. Refrigeration minimizes change in the response characteristics of the film. I recommend Kodak Kodachrome X and High Speed Ektachrome film for color pictures. Do not try to increase the speed by forced processing. It is not needed and tends to degrade the quality of the pictures. For black-and-white photographs I recommend film of moderate grain size with an exposure speed rating of ASA 50 to 100. Your result will be best if you process the films your self."

AS Manning warns, never look at the sun through an absorption filter such as a sheet of smoked glass or darkened photographic film. People who have done so have reported that they felt no discomfort even though the infrared rays cooked their retina. Unlike the cornea, the retina has no sensory nerves.

The partial eclipse can be observed safely, but indirectly, by projecting the image of the sun on a white screen. To observe the projected image use two sheets of cardboard about a foot square. Make a pinhole in the center of one of the sheets. Hold it up to the sun. Support the second sheet a foot or so behind the perforation. The pinhole will act as a small lens. The projected rays cast an image on the second cardboard that can be examined harmlessly.

A larger and sharper image can be projected by a telescope. Support the cardboard screen a few inches beyond the eyepiece. Focus the solar image on the cardboard by pulling the eyepiece a little way beyond the position at which distant objects are normally seen most clearly. Do not look into the objective end of the telescope when the telescope is pointed at the sun; the reflected image can damage the eye.

The classical technique of viewing the sun directly through a telescope involves the use of a Herschel wedge, which is a thin prism of glass with faces that make an angle of about 10 degrees. One face of the wedge is polished optically flat. The prism is installed in the telescope at an angle such that light is reflected from the flat face into the eyepiece. The flat face reflects only about 5 percent of the incident light. The remaining light proceeds through the prism to the second face, where 5 percent is again reflected, but at an angle such that these rays do not enter the eyepiece. The rest of the light, about 90 percent, proceeds through the prism and falls on the inner wall of the telescope, where it is absorbed. Although the light reduction thus achieved is insufficient, a No. 4 neutral-density filter can be inserted between the wedge and the eyepiece for safe and comfortable viewing.

Reflecting telescopes of the Newtonian type can be modified for viewing the sun by substituting the wedge for the diagonal mirror that normally diverts the converging rays into the eyepiece. The wedge and the neutral-density filter are available from the Edmund Scientific Co. (Catalogue Nos. 30,266 and 2,729, price $5.50 and $2 respectively). Do not attempt to use an absorption filter at the eyepiece without the wedge. Heat developed by concentrated sunlight can melt plastic and shatter glass.

An excellent telescope for viewing the sun that amateurs can make in a matter of hours has been developed by John A. Dobson of San Francisco. The instrument is essentially a reflector of the Newtonian type. The intensity of the light is reduced in three steps. Light enters the telescope through a beam splitter, or reflecting filter, that is set at an angle of 45 degrees with respect to the optical axis of the objective mirror. The filter occupies the normal position of the diagonal mirror and is supported by the end of the tube, which is cut at an angle of 45 degrees [see illustration at left]. Dobson explains details of the construction as follows:

"I mount a piece of welder's glass over the lower end of the eyepiece tube to serve as an absorption filter. The objective mirror is unsilvered. The beam splitter covers the entire front opening of the telescope tube. The tube is cut off at an angle of 45 degrees at a point such that the projected axis of the eyepiece tube intersects the center of the lower surface of the beam splitter. For small-aperture telescopes (three to four inches) the beam splitter is cemented with blocks of leather to three equidistant points around the end of the tube after the welder's glass has been cemented over the eyepiece tube. For larger telescopes the beam splitter is supported by an elliptical ring of plywood glued to the front of the tube.

"For the beam splitter I use a partially transmitting mirror of evaporated chromium deposited on twin-polished plate glass 1/4 inch thick. 'One way' mirrors of this kind are used by casinos and supermarkets to discreetly keep an eye on the customer. They are available from dealers who specialize in mirrors. The beam splitter is cemented to the tube with the metal side down, that is, with the reflective side facing the objective mirror. The metallic film reflects most of the light and heat away. About 95 percent of the transmitted light passes through the objective mirror About 2 percent of the total sunlight is reflected from the objective mirror to the beam splitter, where the rays converge through the welder's glass to the focal plane of the eyepiece. I have found by experiment that if the beam splitter transmits about half of the light, a No. 8 welder's glass transmits an image of the sun that can be viewed with comfort. When I use beam splitters of higher or lower transmission, I choose welder's glass of greater or lesser density, such as a No. 6 or a No. 7. The glasses are available from dealers in welding supplies. (The buyer should be aware that the numbering scale for welder's glass differs from the one for neutral-density filters.)

"Telescopes of this design are unobstructed reflectors. Resolution is limited only by the quality of the optical surfaces and the turbulence of the sky. Turbulence in the tube of the telescope remains low because most of the heat that would generate convection currents is diverted by the beam splitter."

Those who do not own an objective mirror for a telescope can make one for observing the sun in the course of a weekend. The job is fairly easy because the figure of the mirror becomes essentially spherical at a focal ratio of f/10 or more. If abrasive powder is sandwiched between a pair of glass disks that are equal in diameter, the surfaces will automatically grind to a spherical figure when one glass is stroked back and forth on top of the other disk. By using abrasives of successively finer particle size and finishing with rouge applied to a disk of pine pitch, the glass can be polished to a circular figure. Even a beginner can make an acceptable three-inch mirror of 30-inch focal length within four working hours [see "The Amateur Scientist," SCIENTIFIC AMERICAN; November, 1959].

It has often been observed that about half of the labor of making a reflecting telescope is spent on the mirror and half on the mounting Dobson has developed a mounting that is exceptionally easy to make, highly functional and inexpensive. The accompanying illustrations depict the details of the construction. The mirror assembly, its adjustable mounting cell, the diagonal beam splitter and the eyepiece assembly are supported by a cardboard tube. The tube is clamped at its balance point by a rectangular box made of wood. The trunnions turn in sockets atop a cradle. For the smoothest action tack pads of Teflon spaced about 120 degrees apart to the inner face of the sockets. A hole in the bottom of the cradle makes a sliding fit with a centered pin in the baseplate. The telescope turns in azimuth around the pin on three blocks of Teflon.

Inexpensive telescopes of relatively long focal length have become available in recent years. They can be adapted for observing the eclipse by those who may not feel up to making their own instrument. Representative is a three-inch Newtonian reflector that is available from the Edmund Scientific Co. (Catalogue No. 80,162) for $59.50. A Herschel wedge that is interchangeable with the diagonal mirror is available from the same source, as is a companion sun filter. In addition a special bracket can be obtained for mounting a camera rigidly to the telescope. Edmund also has a fixture for displaying a magnified image of the sun on a small self-contained screen.

Those who would like to observe at the professional level should not overlook the Questar, a compact telescope that has an equivalent focal length of 1,245 millimeters when it is coupled closely to a camera. Specially designed accessories for this instrument include a sun filter, a camera and a power supply of variable frequency for tracking the sun automatically. The instrument is available from the Questar Corporation (Box 120, New Hope, Pa. 18938).

Bibliography

THE SCIENTIFIC AMERICAN BOOK OF PROJECTS FOR THE AMATEUR SCIENTIST. C. L. Stong. Simon and Schuster, Inc., 1960.

AMATEUR TELESCOPE MAKING: BOOKS I-III, edited by Albert G. Ingalls. Scientific American, Inc.

SOLAR ECLIPSE PHOTOGRAPHY FOR THE AMATEUR. Eastman Kodak Company, 1969.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.