Radio age (Jan 1927-Jan 1928)

28 RADIO AGE for September, 1927 Great nebula in Orion, photographed with the 100-inch telescope. Such objects as these would be shown in far greater detail with a still larger telescope, such as Mr. Pease has planned is renewed occasionally. However, in the large size contemplated there might be some defects of a block of glass as huge as would be required. Glass transmits heat slowly. When the temperature goes down, the great mirror would cool on its surface sooner than inside. The result would be that the outside would contract a little and the mirror would be slightly twisted until it reached the same temperature throughout. Though very minute, the twisting would be enough to be serious in accurate observing. So it may be that some metallic alloy, which transmits heat quickly to its interior, will prove better than glass. Faster Photographic Plates But astronomy doesn't want merely bigger telescopes. Even more welcome to the world of star-gazers would be better and faster photographic plates. Most astronomical observations today are made with the aid of photography. If you visit the modern astronomer at an observatory, you are not likely to find him peering through a telescope. Instead, you will probably find him looking through a microscope at a photographic negative made with the telescope. In a single night at a big observa tory enough photographs might be made to keep the astronomers busy for a month. The plate has one great advantage over the eye because it doesn't get tired. If you look through a telescope, you see as much in thefirst second as you will see if you looksteadily for an hour. Of course, if there is a lot of fine detail, it may take time to give it careful scrutiny. But long gazing doesn't make details visible which were at first invisible. In fact, the eye gets tired, and really sees less after prolonged looking than at first. The photographic plate is untiring. If a star of a certain brightness can be photographed in five minutes, one half as bright can be photographed in ten minutes, or one a quarter as bright in twenty. Some nebulae are so faint that even in the great Mt. Wilson telescope they can not be seen with the eye. But when a photograph of one is made with a long exposure, it is revealed in all its glory. Sometimes exposures as long as twenty or thirty hours are made, on several nights. All night long the plate is exposed, and then covered at the approach of dawn. Then the next night it is again uncovered, and it is kept pointed at the object for all of that night. In this way things are seen in the sky that without photography would have remained ever beyond our ken. But photographic plates are not perfect. Some are more sensitive to light than others. The fast plates that the newspaper photographers use in their cameras record a scene even in poor light in a fraction of a second. The "wet plates" that the photoengraver used in making the illustrations for this article require long exposures with brilliant arc lights. Fast Plates Show "Grain" It might then seem that the astronomer should merely use the same kind of plates as the newspaper camera man. However, as soon as you begin to magnify the picture on one of these plates, the "grain" appears. It is like looking at a halftone reproduction of the photograph of Mr. Pease on this page. As soon as you look at it through a magnifying glass, the dots that make up the picture become so evident that the picture is no longer recognizable. In the plate, the grain is irregular, unlike the uniform rows of dots, but it is no less troublesome. The plate of the photoengraver does not suffer from this defect. Even (Continued on page 37) Francis G. Pease, builder of the 100-inch telescope, who has planned one three times as large, to cost an estimated total $12,000,000