The safe arrival in the United States of 7,000 photo graphs of the planet Mars is an astronomical event of no small importance. To transport a delicate lens 18 inches in diameter from New Hampshire to the Andes, to carry this lens and its mounting up to the height of 14,000 feet above sea level and there as semble the many parts into a perfect running machine, a powerful telescope, is in itself a gigantic undertak ing. But when there is added to these almost insur mountable obstacles the difficulties of existing—we can hardly call it living—for a couple of months on an arid desert so high above the sea, we can gain some small insight into the huge task undertaken by Prof. Todd of photographing Mars at its near approach to the earth. Whether these photographs prove that Mars is in habited or not, Prof. Todd- and his chief assistant, Mr. Slipher, who made all the photographs, are deserving of nothing but the greatest praise for overcoming great observational difficulties and procuring this splendid series of plates made under the greatest of bodily discomforts. One does not ordinarily go and live in a desert for the sake of having a pleasure trip; and the only motive that could have actuated Prof. Todd and Mr. Percival Lowell in financing and sending out the expedition, was the desire to arrive at the truth regarding the interesting markings on the planet Mars. Those who have never tried to photograph the stars can have no idea of the difficulty of the task; but when one attempts to portray the planets in detail, these obstacles are increased many times. Photog raphy is no new art. We all of us know how easy it is to take a snapshot of a landscape and get a pretty picture; why is it then that no serious attempt has ever been made before to observe the planets with the help of the photographic plate? However, when it is mentioned that a huge camera with a lens 18 inches in aperture and 25 feet in focal length gives an image Of Mars, when it is nearest to the earth and conse quently at its largest, only 1/25 of an inch or one millimeter in diameter, the size of a pin head, one sees the utter impossibility of showing any details on so small an image, much less to decide whether a given mark is a straight canal or some other geographical feature. A photographic plate placed at the focus of the 18-inch telescope that Frof. Todd used would give a picture but one millimeter (0.039 inch) in di ameter. After such a plate was developed, it could of course be enlarged by an ordinary camera, and a bigger Martian picture obtained. For a minute study of planetary detail this method would lead to results of little or no practical importance. It may be asked, Why not use a telephoto lens on Mars? This has led to splendid results in landscape work, giving a picture of mountains fifteen to twenty miles distant as if they were only a short way off. This is in reality the very process used in photo graphing Mars or Jupiter. A magnifying lens is plac ed in the telescope just fore the photographic plate, and an enlarged image of the planet is obtained. Prof. Todd has used a magnification of about five diameters in getting his Andes photographs. A greater enlarge ment than this is practically impossible. The much-heralded photographs of Mars taken with such an ex penditure of time, energy, and money (the original photographs cost about a dollar apiece) have the scant diameter of 3/16 of an inch, and the photo graphic work must be of the very best, and great ob servational difficulties must be overcome to procure even these tiny pictures. When the celebrated firm of Alvan Clark Co. made the Amherst lens, they did not intend it for photography, and accordingly ground and polished the objective so that the yellow and green of the spectrum, the colors that most affect the eye, should be brought to a good focus. A lens can be considered as made up of a series of prisms, and we all have seen how the ordinary prisms on a chandelier break up white light into its spectrum colors. Thus when a star is viewed through a good astronomical telescope, the red and blue rays are not sharply focused, and as a result the star is surrounded by a beautiful purple color. With this color the astronomer is always familiar, but it invariably leads the beginner on his first peep through a big glass to exclaim "What magnificent colors!" If an ordinary photograph were made with a visual lens, it would be impossible to get a sharp image, for the simple reason that the optician in mak ing the lens did not grind it with the purpose of bringing to a good focus the blue and violet light which most affects the photographic plate. This can be done by the optician, and we then have a good photographic telescope, but a poor visual one. If an astronomer happens to have a first-class visual telescope, and wishes to take photographs, he has to adopt a measure with which we are all familiar in landscape work—that of using a ray filter or color screen and isochromatic plates. This color screen must be used a little more intelligently than is done in landscape work by even the best professional pho tographers. It becomes necessary to make a careful study of the objective, and construct a color screen of a very particular kind. Mr. Wallace, of the Yerkes Observatory, has done some admirable work in inves tigating photographic plates and ray filters, and a "Wallace screen" and isochromatic plates have been found to give splendid celestial photographs with visual lenses. But consider how the light from Mars must travel after it reaches the telescope before an image 3/16 inch in diameter is obtained on the photographic plate; through the objective, through the lenses that make up the magnifier, then through the color screen to an isochromatic plate. All these several parts must be in the best of adjustment and focus, if a good sharp picture is to be obtained. But during the ten or fifteen seconds that are necessary to photograph Mars, its light has had to travel through miles and miles of the earth's atmosphere. This, we know from the twinkling of the stars, is always in a state of un rest. The disturbance in the air blurs the photo graph, and it is next to impossible to get a good im age, one in which the very fine details will stand out hard and sharp. The only feasible way of over coming this difficulty is the method adopted by Prof. Todd of making a series of exposures, twenty-five to fifty in number, all on the same plate. When the plate is developed, the few seconds of best seeing will be evident from the increased sharpness of the photo graphs. Mars was closer to the earth in 1907 than it has been at any time since 1892; but unfortunately for the observatories of America and Europe, it was very low down in the sky. At the beginning of July, when nearest the earth, Mars at the Yerkes Observatory was never more than 20 deg. above the horizon. This necessitated looking through a great thickness of the earth's atmosphere. It was to diminish the ill effects of this atmosphere that the Lowell expedition went to the top of the Andes. Prof. Barnard, with his beautiful photographs of the Milky Way and star clusters, has shown more than any other man the wonderful possibilities of photog raphy in astronomical work. With the 40-inch Yerkes telescope, he made splendid photographs of Jupiter. At the focus of this great telescope (over 60 feet in length) Jupiter when nearest the earth appears di rectly as an image 3/16 inch in diameter. When en larged five diameters, Jupiter is nearly an inch in diameter, just as if it had been photographed with a telescope 300 feet in length! But Jupiter is more easily photographed than Mars; first, because in the telescope it is about twice as big as the ruddy planet; and second, because Jupiter's surface being about two and a half times brighter than that of Mars, a shorter exposure is necessary. To get original negatives of the same size from the two planets, it would be necessary to give Mars ten times the exposure of Jupiter! The astronomical world is awaiting with great eag erness the verdict of Mr. Lowell when he examines carefully the Andes photographs. If nothing definite is proven this year regarding the canals of Mars, there will be another chance to try it again in 1909. Then the planet will be even closer than it was this summer, but what is more important, it will be 25 degrees higher in the sky, and this will permit the Lick and Yerkes telescopes to turn their full power on Mars, and make photographs which with the latter instru ment will be as large as one-half inch in diameter.
This article was originally published with the title "How the Planets are Photographed" in Scientific American 97, 20, 355 (November 1907)