EXCEPT FOR TWO OBSTACLES YOU could raise your thumb to the sky on a clear day, line up the nail with the edge of the sun and see tongues of blood-red flame lashing thousands of miles into space. You are cheated out of this exciting spectacle by your heterochromatic eyes and the dense, dirty atmosphere of the earth. Glare refracted by dust, water vapor and the molecules of the air masks the relatively faint rays of these solar prominences. The problem of suppressing the glare and sorting the red rays from the white residue constitutes one of the most interesting challenges to the amateur astronomer.

Prominences show up clearly during a total eclipse of the sun, when the glare is masked by the moon. Why not equip a telescope with an artificial moon-an opaque disk just large enough to blot out the sun? Such a disk would be merely an elaboration of your thumbnail, and it would fail for the same reason that the thumbnail fails. The disk of the real moon is located in airless space: hence it casts a knife-sharp shadow. Your artificial moon would cut off the direct rays of the sun but could not stop the rays reflected from all angles by the dust-laden atmosphere. You must equip your telescope not only with an artificial moon but also with a filter which can distinguish between direct rays from the disk of the sun and those from the prominences. This requirement implies a difference between the two kinds of light, because you cannot sort things which are identical.

It turns out that prominences are largely composed of hydrogen, which emits a deep red light. The sun as a whole, on the other hand, is composed of all elements, each of which contributes one or more colors to the visible spectrum. White light is a mixture of all these colors. Thus the problem of seeing prominences comes down to dimming the white light as much as possible and filtering it out of the desired red of hydrogen.

A limited solution of the problem was devised in 1868. By attaching a spectroscope to a telescope, bringing the slit of the spectroscope tangent with the edge of the sun and moving the slit in a circle around the edge, prominences could be detected. Then the slit could be opened wide enough to see an entire small prominence or part of a large one. Even though this method distorts the image of a prominence, it is still used on special occasions.

In 1890 George Ellery Hale and Henri Deslandres independently invented the spectrohelioscope. This instrument utilized the red light of hydrogen to produce an image of the entire disk of the sun. Thus prominences were visible not only at the edge of the sun but also on the face of it. For 40 years the spectrohelioscope was the major source of knowledge about solar prominences.

The sun is entirely gaseous. No solid surface lies beneath its outer layers. From the outside in, the outer layers are the corona, the chromosphere, the reversing layer and the photosphere (the layer we see when we look at the sun with the naked eye or a conventional telescope). An orderly mind instinctively seeks to arrange these layers in a diagram resembling the cross section of an onion. Illustrations of this sort do not appear in textbooks, largely because they would convey the misleading impression that the layers have sharp boundaries.

The corona, the pale yellow and pearly white outer aura seen during a total eclipse of the sun, is by far the thickest of the four layers. It extends beyond the visible disk of the sun for about one third of its diameter, and sometimes much farther. It is a diaphanous thing, fainter than moonlight. The scarlet clouds of hydrogen that comprise the prominences appear to shoot up from the chromosphere, which contributes only 12,000 miles to the 864,000-mile diameter of the visible solar disk.

Although the prominences take various flamelike forms, they are not flames in the ordinary sense because the sun does not burn. The astronomer Edison Pettit classified and named the prominences according to their behavior. There are three active types: "interactive," "coronal active" and "common eruptive." The latter are subdivided into "quasi-eruptive," "common eruptive," and "eruptive arch." Then there are the sunspot types: "cap," "common coronal sunspot," "looped coronal sunspot," "active sunspot," "surge," "ejection," "secondary" and "coronal cloud." The tornado types are "columnar" and "skeleton." Finally there are the, "quiescent," and "coronal" types, which have no variants. Sometimes 20 prominences of these various types are simultaneously visible at the edge of the sun. Sometimes there is no prominence for days. Prominences erupt at velocities up to 451 miles per second; a common velocity is 100 miles per second. They often change from one type to another.

The natural fascination of watching things that move may largely explain the amateur astronomer's dream of owning an instrument that makes prominences visible. Because the prominences are so large by terrestrial standards, the appear to move lazily. But like the slow movement of the hour hand of a clock, their transformations can easily be perceived over a matter of minutes. When recorded by time-lapse photography and then projected as a motion picture, prominences are an awesome spectacle of nature. Henry Paul of Norwich, N.Y., chemist and amateur astronomer, has written: "Spine-tingling excitement tinged with awe usually accompanies the first viewing of an eruptive prominence. These millions of tons of glowing gas often stream back in graceful arcs to the surface as if drawn by a huge magnet."

A good spectrohelioscope requires 17 optical surfaces and an original defraction grating (a replica grating will not work). This doubtless explains why so few amateurs have built these instruments. This department knows of only five spectrohelioscopes made by amateurs-four in Great Britain and one in the U. S. The American instrument was destroyed in a fire.

Paul's exciting experience came not out of a spectrohelioscope but from a newer instrument called the coronagraph. Where the spectrohelioscope sorts out the red light of hydrogen by means of a diffraction grating, the coronagraph does so with a remarkably selective filter known as the quartz polarizing monochromator.

In general the coronagraph is the better instrument. One astronomer who began as an amateur telescope maker and has used both instruments says that the views with the coronagraph are enough to make a spectrohelioscope man feel that his life has been wasted. Another calls this an understatement. The older instrument does enjoy some advantages. It is superior, for example, in revealing details of the chromosphere, unless the design of the coronagraph is carried to the extreme limit of its capability-at greatly added cost. Moreover the spectrohelioscope may be adjusted to filter light of any color.

The coronagraph was independently invented by several astronomers: first the late Bernard Lyot of France and later Yngve Ohman of Sweden and John S. Evans of the U. S. (who began as an amateur telescope maker). Three amateurs are known to have built coronagraphs using the quartz polarizing monochromator: David Warshaw of Brooklyn, N.Y., who spent several years at the task and accomplished it in a kitchen without machine tools; Paul, who gained some insights in personal visits with Warshaw; and Walter J. Semerau of Kenmore, N.Y., who was aided by Paul's published instructions in Amateur Telescope Making-Book Three and directly by Paul.

After using his instrument for a year, Semerau writes: "My coronagraph performs well beyond all my expectations. Building it was the most fascinating fun I ever had." The prominence photographs on page 195 were made with this instrument.

Semerau mounted his coronagraph on the same telescope axes that support a 12.5-inch astrographic camera he had made earlier. At the bottom of the coronagraph tube a pair of right-angle prisms jacknifes the light beam from the long-focus objective [see drawing in Figure 2]. Hence what appears to be parallel tubes is in effect a very long single tube folded for the sake of convenience. In one part of the tube the light passes through a rectangular box containing the quartz polarizing monochromator. The monochromatic light emitted by the prominences is passed upward into the eyepiece by a reflex mirror. Light of other colors is absorbed by filter and dissipated in the form of heat. The light can be directed into either of two cameras by moving a small lever attached to the mirror. The instrument can thus serve as either a coronascope or a coronagraph. The arrangement enables the observer to view prominences up to the instant when he wishes to make a photograph. The drawing shows a 35-millimeter camera in place. It may be replaced by a 16-millimeter time-lapse camera.

The business end of the coronagraph is a quartz polarizing monochromator. Though it is a filter, the term is misleading because it suggests simple glass or gelatin filters. Even the best of these would transmit a band of color much too broad. The filter must exclude all light waves which do not fall within a single hydrogen line-a band of about three to seven Angstrom units in the 3,750 Angstroms of the visible spectrum. To accomplish this the quartz polarizing monochromator sends the light through a stack of six or more quartz plates. At the top and bottom of the stack and between each pair of plates are sheets of polaroid. In addition to this filter the coronagraph requires a cone of metal (the circular bottom of which eclipses the sun in the instrument), a field lens, a diaphragm, a lens to send the rays parallel through the quartz plates and another lens to focus the emerging rays.

Since the filtered light of a prominence is narrowly monochromatic, the objective lens of the coronagraph need not be corrected for color. In fact, a single-element lens works better than an achromat. The lens must be superlatively free of bubbles, striae, dirt, dust and even microscopic scratches which would diffract light. Single-element lenses may also be used beyond the eclipsing cone; these need not be so free from defects. A reflecting telescope cannot be used for a coronagraph because irregularities in the metal reflecting surface diffract light.

The quartz polarizing monochromator has other demands. It is an incurable thermometer watcher. It misbehaves unless the temperature is held constant within about one degree of a predetermined value, usually between 100 and 125 degrees Fahrenheit. This is usually done by heater bulbs, a fan and a thermoswitch.

The principle by which the quartz polarizing monochromator works has been explained many times but never so lucidly as by Evans in a leaflet issued by the semiprofessional Astronomical Society of the Pacific. The following is abstracted from this leaflet:

"The birefringent filter," Evans explains, "is a very simple and elegant device which, when attached to a small telescope, performs the same function as the spectroheliograph and spectrohelioscope; better in some respects, and not so well in others.

"The basic function of these three instruments is to permit the formation of a well-defined image of an extended object like the sun in the light from a very narrow band in the spectrum. A radio set is a good analogy. Observation of the sun in white light is equivalent to trying to listen to a radio which receives all wavelengths simultaneously. All stations and static would come in together, and the result would be most unsatisfactory. To avoid this difficulty we use tuned radio filters, which receive only the narrow band of wavelengths put out by a particular station, excluding all other wavelengths. We then hear that station alone.

"The various kinds of atoms in the sun are like the radio stations. The hydrogen atoms broadcast only particular wavelengths of light which we can see as lines in the spectrum. If we can look at the sun through a filter which transmits only the wavelength of a hydrogen line, we can see the hydrogen on the sun to the exclusion of everything else, and its appearance is very different from the white-line picture.

"The elements 1 through 6 [see drawing above] are birefringent plates of crystal quartz. Each crystal is ground to a thickness one half that of the preceding element. Both surfaces are parallel to the crystal's optic axis.

"Each element receives plane-polarized light through a sheet of Polaroid film sandwiched between the quartz plates and splits it into two components of equal intensity which traverse the crystal at slightly different velocities. When the component rays emerge from the crystal, their combined polarization is altered in a manner depending on the wavelength. Certain wavelengths vibrate at right angles; intermediate wavelengths vibrate in ellipses. The next Polaroid transmits at each wavelength only that portion of the vibration parallel to the original direction, absorbing the rest. The resulting spectrum is represented graphically for the first plate [top curve in Figure 4]. Transmitted intensity [vertical scale] is plotted against wavelength [horizontal scale]. The other five elements of the filter yield similar curves. Curves for different elements differ only in the spacing between the peaks. The width of the spacing is inversely proportional to the thickness of the corresponding crystal. Since each crystal is one half as thick as its predecessor, successive curves have peaks twice as far apart.

"The transmission of any combination of elements is simply the product of their individual transmissions. Hence the alternate peaks emerging from the first plate which coincide with zeros of the second are absorbed, and the combination gives the solid transmission curve shown [second curve from the top]. Similarly, alternate peaks resulting from this combination coincide with zeros of the next pair of plates and are absorbed. The combination of the three elements, then, transmits a series of widely separated sharp bands. The separation of the bands can thus be increased to any desired amount by adding elements.

"A six-element filter, ideal for observing prominences, has a total thickness about 4 1/2 inches. It is best used on a refracting telescope, preferably with simple lens objective, with an occulting disk covering the image of the sun in the focal plane. A small lens projects the image of the prominences around the occulting disk on an eyepiece or camera. Filters with band width greater than one Angstrom do not work well with reflecting telescopes because of the strong scattering of light by metallic reflecting surfaces."

The construction of the filter and optical system of a coronagraph is well within the capabilities of an experienced amateur telescope maker. The essential details of design and construction are given by R. B. Dunn in Sky and Telescope for April-October, 1951, and by Paul in Amateur Telescope Making Book Three.

Writes Semerau: "The coronagraph can be built at a cost for materials of $150 to $200. Time of construction will be longer than for a telescope because it will take extra time to become acquainted with the special techniques of grinding, polishing, testing and orienting crystal quartz.

"If the close tolerances frighten the amateur who has made no telescope at all but has done fine mechanical work, the feeling is understandable, but Paul tells how the close work may be done with the aid of optical tests. With these aids the very close standards of optical work are no more difficult than those fine mechanical work. Nevertheless apprenticeship in optical work gained making a telescope is almost a 'must.'

"If the builder will accept the crystal quartz with chipped corners or other defects outside the clear round aperture of the instrument, which result in actual harm, the cost for quartz will be greatly reduced. To make the least quartz go the farthest it is expedient cut a potato to match the crystal in dimensions. Study the potato model very carefully and mark the direction of the optical axis as it would be found in the quartz. Cut the potato with as much care and love as if it were quartz. If then a mistake has been made, you may either change the dimensions of the plate or throw the potato away and start all over again.

"Just because some amateur telescope makers have no advanced degree in physics, they imagine they can't do the job and thereby miss a front seat at one of nature's best shows. A number of experts have commented favorably on the performance of my instrument. It certainly cannot be credited to my formal education. My schooling stopped with the ninth grade in 1925.

"In the seven years that I have worked as an instrument maker I have built three large spectrographs, a recording densitometer, many special types of cameras, a vibrating-reed electrometer, X-ray diffraction equipment, three schlieren apparatuses, a double-beam infrared spectrometer and many other pieces of equipment. I have accumulated one of the finest one-man instrument shops in the country. I am treated with as much respect as any Ph.D. on the property.

"I have no plans of my coronagraph or, for that matter, of anything I have ever built. Throughout the years I have trained myself to visualize how I want an object to be built. I make a few simple sketches-and I mean simple-to determine sizes and so on, and proceed to build. It usually turns out O.K. My employer likes it because it requires no engineering before an instrument is built. The drawings are made afterward.

"Since most solar observatories are perched on high mountains in clean air far from the smoky industries, many have the belief that very little can be done at a solar observatory at low altitude among industries. It is true that the higher the observatory and cleaner the air the better, but the amateur astronomer will be amazed at what he can see and learn of our sun from such a locality.

"A year of observing experience in the heavily industrialized area of Buffalo, N. Y., at an altitude of only 600 feet, has taught me the following facts about the coronagraph. A clear blue sky, preferably after a rain, with temperature not above 72 degrees, will give finest observing and photography. From mid-July to mid-September observing must be done before 10 a.m. while the earth is still cool. After mid-morning the heat from city streets and industry makes observing impossible. Observing is generally bad during December, January and February, when the sun is low and the light must pass through a thicker layer of smoke and haze. Prominences are just barely visible on a cool day if the sky is gray-blue or hazy."

Bibliography

AMATEUR TELESCOPE MAKING. Edited by Albert G. Ingalls. Scientific American, Inc., 1952.

AMATEUR TELESCOPE MAKING-ADVANCED. Edited by Albert G. Ingalls. Scientific American; Inc., 1952.

Suppliers and Organizations

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