Large Telescopes

Conclusive evidence that would settle the great debate was obtained through observations made with a new generation of powerful telescopes constructed on high stations in the American West. Astronomy had entered a stage where advanced technology and large-scale science would dominate the frontiers of research. Although theorists continued to play an important role, they were overshadowed by the continuous stream of stunning findings coming from the mountaintop observatories. The primary place of advanced technology in astronomical research has continued up to the present and appears to have become a permanent feature of the science of astronomy.

During the nineteenth century the refracting telescope was the instrument of choice for astronomical investigation at the professional observatory. Although the refractor provided very good resolution and was excellent for precision measurement, it was subject to several limitations. The passage of light through a large, primary, objective lens resulted in a loss in the blue end of the spectrum. The objective lens needed to be supported around its perimeter, and there was a tendency for it to sag, a fact that limited its size to less than 100 centimeters in diameter. The largest refractor ever to be built was a 40-inch (100 centimeter) telescope installed at the Yerkes Observatory in Wisconsin. A telescope that would play a critical role in the history of modern cosmology was the 24-inch (60 centimeter) refractor at the Lowell Observatory in Flagstaff, Arizona. Perhaps more than anything else, the establishment of the Lowell Observatory indicated a recognition by researchers that high-altitude and dry locations were necessary to secure ideal seeing conditions.

Until the end of the nineteenth century the reflecting telescope was viewed as an instrument for amateurs, limited by difficulties in keeping the primary speculum-metal mirror polished and properly figured. However, reflecting telescopes could be built much larger than refractors since one entire surface of the primary mirror could be supported structurally within the telescope assembly. With advances in casting technology, plate glass replaced speculum metal in the construction of the primary mirror. The invention of improved mountings and the shorter focal length of reflectors also contributed to their competitiveness.

In the period 1900—1920, large reflectors were built on mountaintops in California and several other locations around the globe. The 36-inch (90 centimeter) Crossley reflector was installed in 1898 at Lick Observatory on Mount Hamilton and proved to be very effective for stellar photography. James Keeler (1857-1900) initiated a program to photograph many thousands of spiral nebulae. His successor, William Campbell (1862-1938), became a vigorous advocate of the island-universe theory. Curtis cited observations of nebulae made with the Lick Crossley reflector in his debate with Shapley.

George Ellery Hale (1868-1938) was an American astronomer who arranged to have three powerful solar telescopes built on Mount Wilson overlooking Pasadena, California. Hale was also very active in promoting the construction of large telescopes for stellar and nebular astrophysical work. He recognized the superiority of the reflector for such work and was able to convince philanthropists and scientific foundations to support some very ambitious projects. In 1908 a 60-inch (150 centimeter) reflector was built on Mount Wilson; this was followed in 1917 at the same location by a larger 100-inch (250 centimeter) reflector, the Hooker telescope (figure 7.2), an instrument of unprecedented size and power. Financial support for the Hooker telescope was provided by the philanthropist J. D. Hooker and by the Carnegie Institution. The construction of large reflectors was not limited to California. In 1918 a 74-inch (190 centimeter) reflector was built at the Dominion Astrophysical Laboratory near Victoria, British Columbia, in Canada. The large reflectors were well suited to deep-space observation, in which the objects are very faint, and by the middle of the twentieth century had achieved complete dominance in astro-physical research. This dominance was furthered by the Hamburg astronomer Bernhard Schmidt's (1879—1935) invention in 1931 of a special reflector and camera with a wide field of observation. gress.

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