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Stargazing as done by computers

By Trevor C. WeekesTrevor C. Weekes is an astronomer at the Mt. Hopkins Observatory near Amado, Ariz., a facility of the Smithsonian Institution Astrophysical Observatory. / February 27, 1980

To entertain his dinner guests at his home in Birr Castle, it was the custom of the third Earl of Rosse to demonstrate his 72-inch telescope, the largest in the world in 1845. The massive wooden tube of the telescope was housed between two large masonry walls just outside the castle and was moved only by an ingenious arrangement of pulleys and weights which were manipulated by workers from the earl's estate. If the cloudy skies of an Irish winter permitted, the earl would direct his telescope to the heavens, then to the accompaniment of his guests' social chatter, the shouts of his workmen, and the creaking and groaning of his wooden telescope, he would laboriously sketch the nebula, many of them seen through his telescope for the first time.

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One hundred and thirty-five years later, I study these same nebula (now known to be galaxies) using a computer controlled telescope (of similar size to Lord Rosse's) on an Arizona mountaintop. While my television camera, at the focus of the telescope, integrates the light from the galaxy, stores it in a computer, and displays it on a screen so that I can do my observing in a heated room, I wonder what Lord Rosse would think of modern astronomical techniques. Mechanical genius that he was, Lord Rosse would quickly have understood and appreciated the gadgetry that characterizes modern astronomy. Perhaps the most startling advance he would see would be the fast compact digital computer, which enables the astronomer to control the telescope from a terminal in another room or another building.

The human eye (Lord Rosse's detector) has long been replaced as an astronomical detector by the photographic plate; this, in turn, is now being replaced by the CCD (charge- coupled device) camera. This camera, which has only recently been adapted for astronomical studies, consists of 10,000 tiny detecting elements packed into an area less than one tenth that of my thumbnail; each of the elements detects photons of light with high efficiency and gives a digital output that can be immediately processed by computer. The device can be used either to take a direct image of the sky or to register the output of a spectrograph. In either case, the strength of the CCD lies in its high efficiency, wide dynamic range, and computer compatible output.

Although the final data processing must use the large facilities at central laboratories, it is essential that the observer get some instant feedback from his data so that he can plot the future course of his observations. Hence, most observing programs now require the availability of computers on site. These have the function not only of analyzing the data, but of controlling the instrument, the telescope mount, and perhaps the telescope mirrors. The astronomer thus becomes one step removed from the actual observing and his major function is the operation of a computer terminal.

One of the unique features of modern astronomical research is that it requires the use of very sophisticated equipment in some of the most remote and inaccessible corners of the globe. Driven by limitations of atmospheric "seeing" (essentially, the requirement that the atmosphere above the telescope be stable) optical observatories are almost invariably located on high mountains. To escape the effect of back-scattered city lights, these mountaintops must be as far as possible from civilization. Some of the best sites are on high mountains in the Andes in Chile, on the Baja California peninsula in Mexico, in Hawaii, in Arizona, or in the Canary Islands. None of these remote sites at first sight would seem likely places for modern research.