"In all I made over 700 black-and-white photographs of auroras plus 20 or so in color. Most were taken with an old camera using 120-millimeter film. The exposures were 15 seconds or less at f/2.9. I used Royal-X Pan film developed in DK 60A. The pictures were so easy to make that the novelty soon wore off, and I began to cast about for something additional to do. Eventually I hit on the idea of building an aurora spectrograph. This instrument helps to distinguish among the several kinds of aurora and reveals the presence of displays that are invisible to the eye. It is a simple apparatus, but the literature disclosed scant information about its construction. With a few hints from astronomers I wound up designing my own.

"Aurora spectrographs include four basic parts: (1) a mechanical slit for admitting a thin ribbon of light to the instrument, (2) a lens for bending the diverging ribbon into a parallel beam, (3) either a diffracting or a refracting element for dispersing the collimated beam into its constituent colors and (4) a camera for recording the spectral pattern on photographic film. No telescope or other light-gathering device is used. None is needed to make auroral spectrograms. ( No illumination would be gained by bringing a small patch of the glowing area to focus on the slit. ) If desired, a telescope can be added. The instrument can then be used for photographing stellar spectra by focusing the image of a selected star on the slit.

"The structural arrangement is as follows. All of the optical parts are supported by aluminum tubing and an associated housing made of aluminum plates and sheet stock as depicted in the accompanying drawing. When in use, the instrument is mounted on a conventional camera tripod. A second camera, for making widefield pictures, is mounted on an accessory bracket. The slit assembly consists of a cylindrical base that can be made of any convenient material such as aluminum, plastic or even hardwood. Two safety-razor blades are fastened to the base with screws as shown in the second drawing [above]. The cutting edges of the blades must be parallel and separated .015 inch. A rectangular hole, somewhat larger than the slit, is made in the base for admitting light to the lens. The slit assembly is mounted in one end of an aluminum tube. An achromatic lens 1 3/4 inches in diameter fits into the other end of the tube at the distance of its focal length from the slit-20 inches in the case of my instrument. Neither the diameter of the lens nor its focal length are critical. The diffraction grating is placed immediately behind the lens and at an approximate right angle to the beam, as depicted in the third drawing [below]; The grating and the back face of the lens are enclosed by a housing. A hole in the rear of the housing, which makes a light-tight fit with the lens barrel of the camera, admits the dispersed rays to the camera lens. The focusing adjustment of the camera is set at infinity. I use a 35-millimeter Retina camera equipped with an f/2 lens. The time required for exposures depends in part on the light-gathering power of the camera lens. In general, focal ratios substantially larger than f/2 are not satisfactory.

"For the dispersing element I prefer a transmission (i.e., transparent) replica grating to a glass prism. A grating spreads the colors more than a prism does, and the dispersion is uniform throughout the spectrum. A spectrum made by a prism becomes increasingly crowded toward its red end. Spectral colors appear as parallel bands or lines that cross the ribbon-shaped pattern like the rungs of a ladder. Crowding obviously increases the difficulty of identifying interesting lines.

"With this combination of grating and camera the exposure time required to photograph the brightest aurora averages about 10 minutes. Good photographs have been made with Ansco Super Hypan film. The best results are obtained, however, with spectroscopic emulsions such as Kodak 103a-F, which is specially sensitized for the far-red region of the spectrum. Spectrograms have been made of several auroras that were not visible, in some cases by extending exposures to four hours. The outcome of such attempts is a matter of pure chance. You have no way of knowing if an aurora is up there. So you just point the instrument toward the northern sky at night, open the shutter and go away. With luck, spectral lines characteristic of the aurora will appear when you develop the film.

"For many years auroral spectra presented investigators with something of a mystery. Certain of the lines could not be reproduced by light sources then available in the laboratory, and came to be known as 'forbidden' lines. The brightest of the forbidden lines appears in the green portion of the spectrum; it is formed by light waves that measure 5,577 angstrom units in length. This line is emitted when highly ionized atoms of oxygen in the upper atmosphere release a finite part of the energy acquired by collision with particles hurled into space by the sun. The 5,577-angstrom line is observed to some extent in all auroras. When intense, it accounts for the characteristic green color of most displays; when weak, the auroras look pale white or gray. Also characteristic are two lines at 6,300 and 6,364 angstroms in the red part of the spectrum; these also are emitted by ionized oxygen. Still another relatively prominent line is found at 3,914 angstroms in the extreme violet. It is emitted by ionized nitrogen. Weaker lines also appear throughout auroral spectra. Most are associated with the emission of energy from oxygen and nitrogen, although occasional spectrograms show a well-defined line at 6,563 angstroms that has been identified with hydrogen.

"The relative intensity of the spectral pattern varies with the intensity, height and type of the display; the temperature of the upper air; the state of the earth's magnetic field, and related environmental factors. Quite often spectral lines also show variation in brightness along their length. Auroras in the early evening or morning, when part of the display is in the earth's shadow and part in the sunlit upper atmosphere, are of particular interest because not all of the accompanying effects are understood in detail. They are investigated by pointing the spectrograph toward the area in question. The techniques of analyzing spectrograms to determine the height of a display or the temperature of the air in a selected region are described in the literature [see 'Bibliography'].

"Several optional features were built into the basic instrument to facilitate its operation. A small right-angle prism was added at the lower edge of the slit so that a comparison spectrum can be photographed simultaneously as an aid in the identification of unknown lines. Light from a small neon bulb is directed against the lower face of the prism for about 10 seconds. Any miniature neon bulb can be used. Neon emits particularly strong lines at 5,400, 5,852 and 6,402 angstroms. This accessory is not too important, because the 5,577 lines can always be recognized, as is apparent in the accompanying spectrogram. Incidentally, the lines may be sharpened by making the slit narrower than .015 inch. Sharpness is gained at the cost of longer exposure, however, because the light admitted to the film varies inversely with the width of the slit. Moreover, a narrow slit emphasizes the higher-order images.

"To aid in estimating exposures and plotting variations of auroral brightness a 931-A photomultiplier tube was incorporated in the instrument. The photocathode is illuminated by a small telescope equipped with an objective lens of 1 1/2inches diameter and 12 inches focal length. The incoming light is focused on the photocathode through a diaphragm 1/2 inch in diameter that limits the illumination to a patch of aurora about five degrees in diameter. The output of the phototube actuates a microammeter. Brightness is indicated simply by the movement of the pointer. If an ultrasensitive meter is used, so that a current of 10 microamperes drives the pointer to full scale, no amplifier is necessary. If not, the circuit must include a direct-current amplifier. Warning: The 900-volt supply for the photomultiplier can be lethal if handled carelessly. The tube and all parts of the circuit must be well insulated from the metal housing, no bare wires or terminals can be exposed. The instrument is used in darkness, a condition that invites accidents.

"A meter of the pen-recording type would be preferable because it would plot a continuous graph of brightness against time automatically. I have not, however, invested in one. The accompanying graph of a typical aurora was plotted by hand from periodic meter readings. The display was of medium intensity and showed a strong peak at midnight. The spectrograph was pointed due north and about 20 degrees above the horizon. The early portion of the record doubtless includes some twilight. The photomultiplier tube draws a small amount of current even when no light falls on the photocathode, an effort that establishes the minimum illumination to which the tube is sensitive. It is called the 'dark current,' and is indicated on the graph by the horizontal broken line near the bottom.

"A second 931-A photomultiplier is being added to operate an alarm system during sleeping hours. It will be actuated by light dispersed to the side of the axis opposite the camera location. The characteristic green line at 5,577 angstroms is focused by a small lens on a second slit that excludes all other light from the photomultiplier. For mechanical compactness the beam is folded by a small front-surfaced mirror that can be rotated during initial adjustment to center the 5,577 line on the slit. The output of the photomultiplier is amplified for operating a relay that in turn triggers an alarm when the 5,577-angstrom line reaches a predetermined intensity.

"As a consequence of working with the auroral spectrograph I became interested in other optical phenomena of the atmosphere, in particular the green flash that is occasionally observed at the upper edge of the setting sun [see "The Green Flash," by D. J. K. O'Connell, S. J.; SCIENTIFIC AMERICAN, January, 1960]. Somehow I had gained the impression that observation of the green flash requires a large telescope, exceptionally good seeing and the smooth horizon of a large body of water or a desert. This proved not to be the case. The accompanying photographs were made with modest equipment through an industrial atmosphere in a hilly region.

"During the past 18 months scores of such photographs have been made from the eastern side of Pittsburgh, overlooking the downtown area about eight miles away. Green and red flashes have been observed on numerous occasions, and almost every evening the sun has a green rim at the top and a red rim at the bottom. Green flashes have been seen as long as half an hour before sunset, as I well as when the sun was partly below the horizon. In several cases green flashes have been seen and photographed even when the bottom edge of the sun was immersed in thick haze. Smoke and haze cause a pronounced reddening. Of the solar disk, however, and dilute the green flash with yellow. On exceptionally clear days the green flash appears bluish; on one occasion it was a deep violet. The phenomena are most pronounced when an abrupt drop in temperature follows several days of uniform weather. Incidentally, distortions of the solar disk at sunset can be used for predicting astronomical seeing for the night: the better the green flash, the worse the seeing will be.

"All the photographs have been made with an achromatic lens of two inches in diameter and 50 inches focal length. The lens is mounted in one end of an aluminum tube; a 35-millimeter camera (with the lens removed) occupies the other end. Originally I used a small homemade camera box equipped with a Compur shutter of 1/500 second maximum speed. Pictures made with this equipment showed a five-cornered star pattern that was traced to nonuniform motion of the five leaves of the shutter. This difficulty was solved by replacing the homemade camera with a Leica equipped with a focal-plane shutter. The complete assembly is mounted on a conventional tripod that rests against a window sill for added support.

"A reflex attachment is useful both as a view finder and when making the initial adjustment, but it is not essential. Once the focus is established at infinity, you are in business. I made a provision for inserting filters ahead of the lens, but this feature has never been used. An iris diaphragm was mounted ahead of the lens. It is useful for reducing brightness when looking at the sun through the reflex attachment. Warning: Care must be exercised when looking through the instrument at the sun, particularly when the sun is at an appreciable elevation. Serious eye injury can result. The illumination must be reduced either by means of an iris diaphragm, a dark filter or some optical arrangement that diverts most of the rays from the eyepiece.

"Most green flashes are observed when the sun is near the horizon. The accompanying photographs show that extremely low elevation is not a critical condition for the phenomenon. One of the pictures [Figure 6] shows a green flash (the narrow sliver of light at the top of the disk) just coming off the limb. Note that the bottom of the disk is immersed in smog. This picture was made on November 15, 1959. The second photograph shows a green flash at the moment of detachment from the disk and after a substantial portion of the sun has disappeared below the horizon. The silhouetted buildings in the photograph are about eight miles away.

"The third photograph shows a red flash that was detached from the bottom edge of the sun after the disk had been grossly flattened by atmospheric refraction. Such flashes separate from the disk abruptly, float free for an instant and then merge with the disk. Sometimes they resemble a suspended bubble.

"Occasionally the camera records large sunspots. One is seen in the fourth photograph above the rear of the cathedral at the right. The buildings are about 10 miles away.

"By good fortune the Greater Pittsburgh Airport happens to be situated on the west side of town, about 20 miles distant, and planes coming in for landings or taking off frequently cross the sun's disk. The exhaust gases always appear to be illuminated for a couple of seconds when the plane comes out of the sun, creating the impression that the plane has sliced out part of the disk and is dragging it away. The illusion is particularly pronounced in the case of jets. When the camera is replaced by an eyepiece, the hot exhaust gases are seen to be responsible for optical effects that are altogether comparable to the green, red and violet flashes.

"The last photograph [below] shows the phenomenon in black and white. These effects can originate quite close to the observer. One evening I watched a Constellation that was flying directly into the sun. Knowing that the wingspread of the plane is 126 feet and that the solar disk subtends an angle of about half a degree, it was easy to calculate that the plane was some 2 3/4 miles away. Suddenly, at about this distance, the plane banked and turned from the disk. As each engine crossed, the limb portions of the disk appeared to be dragged behind. Each of the turbulences Hashed red and green. Obviously these colors were of local origin."

Bibliography

THE AMATEUR SCIENTIST. C. L. Stong. Simon and Schuster, Inc., 1960.

THE AURORAE. L. Harang. John Wiley & Sons, Inc., 1951.

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