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Authors: Ronald Florence
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the sky, the mounting must be on a tall pillar. When the telescope is pointed toward targets of low altitude, the eyepiece is high off the floor, out of reach of the observer or his cameras. At Lick and Yerkes this problem was solved by having the entire floor of the observatory rise and fall around the fixed telescope mounting pillar. An early accident with the moving floor was another hint that refractors were approaching their technical limits.
Because the light can be bounced back up the tube from the primary mirror to a secondary mirror, in effect folding the focal path of the telescope, a reflector can be built with a relatively short tube, short enough in most instances for the telescope to be mounted in a movable fork with its pivot point close to the primary mirror. Without the weight of a heavy lens to support at the far end of the tube, the reflector can use an open tube, resulting in a lighter structure and greatly simplifying the construction of the instrument.
The reflector is also more versatile than a refractor. The eyepiece, or more typically for a large instrument, the cameras or spectrograph, of a reflector can be mounted at one side of the high end of the tube, in what is called the Newtonian position, after Newton’s early design. The light from the primary mirror is deflected to the Newtonian focus with a small diagonal mirror suspended inside the telescope. It is also possible to bounce the light from a secondary mirror back through a hole in the center of the main mirror so that cameras and other instruments can be mounted at the base, or supported end of the tube at what is known as the Cassegrain focus. With additional mirrors, the light can be directed to a fixed Coudé position of extreme focal length in a separate temperature and humidity-controlled room. Finally, if the telescope is big enough, the light can be deflected through the hubs of the declination axis, to Nasmyth foci on either side of the telescope. The different foci, each with different focal lengths, add up to increased versatility for the reflector.
When George Hale began thinking about a new telescope, the arguments for a reflector weren’t only theoretical. In 1895, the same year that the lenses for the great refractor at Yerkes were finally finished, Edward Crossley of Halifax, England, presented the thirty-six-inch Calver-Common reflector to the Lick Observatory. The new reflector was overshadowed in publicity by the larger telescope at Yerkes. While Yerkes’ telescope dominated the press, the mirror of what came to be known as the Crossley reflector was quietly refigured and a new mounting built for photographic work. Keeler, the director of the Lick Observatory, used the Crossley, the first large reflector in the United States, to reveal an immense number of spiral nebulae that had never before been recorded.
The Crossley was an awkward telescope to use, with a stiff mount that required a kick from time to time to get it to behave, but so manyspiral nebulae could be photographed, or even seen visually with the telescope, that it raised disturbing cosmological questions. Heber Curtis’s continuation of this study provided the background for his contributions to the great debate in Washington.
Hale kept abreast of the work with the Crossley reflector at Lick. The incentive of following up on that work, and a program to extend the detailed spectrographic studies Hale had made of the sun to distant stars, was a compelling agenda for a big reflector. And when the headstrong young director of Yerkes Observatory got an idea in his head, there was no stopping him. Before the forty-inch refractor at the Yerkes Observatory was dedicated, Hale persuaded his father to contribute the funds to have a sixty-inch glass blank—the largest piece of glass the French foundries could mold in a single pour—cast for a giant reflector. William Hale made it clear that his gift was seed money; he would pay for the glass blank and nothing
Because the light can be bounced back up the tube from the primary mirror to a secondary mirror, in effect folding the focal path of the telescope, a reflector can be built with a relatively short tube, short enough in most instances for the telescope to be mounted in a movable fork with its pivot point close to the primary mirror. Without the weight of a heavy lens to support at the far end of the tube, the reflector can use an open tube, resulting in a lighter structure and greatly simplifying the construction of the instrument.
The reflector is also more versatile than a refractor. The eyepiece, or more typically for a large instrument, the cameras or spectrograph, of a reflector can be mounted at one side of the high end of the tube, in what is called the Newtonian position, after Newton’s early design. The light from the primary mirror is deflected to the Newtonian focus with a small diagonal mirror suspended inside the telescope. It is also possible to bounce the light from a secondary mirror back through a hole in the center of the main mirror so that cameras and other instruments can be mounted at the base, or supported end of the tube at what is known as the Cassegrain focus. With additional mirrors, the light can be directed to a fixed Coudé position of extreme focal length in a separate temperature and humidity-controlled room. Finally, if the telescope is big enough, the light can be deflected through the hubs of the declination axis, to Nasmyth foci on either side of the telescope. The different foci, each with different focal lengths, add up to increased versatility for the reflector.
When George Hale began thinking about a new telescope, the arguments for a reflector weren’t only theoretical. In 1895, the same year that the lenses for the great refractor at Yerkes were finally finished, Edward Crossley of Halifax, England, presented the thirty-six-inch Calver-Common reflector to the Lick Observatory. The new reflector was overshadowed in publicity by the larger telescope at Yerkes. While Yerkes’ telescope dominated the press, the mirror of what came to be known as the Crossley reflector was quietly refigured and a new mounting built for photographic work. Keeler, the director of the Lick Observatory, used the Crossley, the first large reflector in the United States, to reveal an immense number of spiral nebulae that had never before been recorded.
The Crossley was an awkward telescope to use, with a stiff mount that required a kick from time to time to get it to behave, but so manyspiral nebulae could be photographed, or even seen visually with the telescope, that it raised disturbing cosmological questions. Heber Curtis’s continuation of this study provided the background for his contributions to the great debate in Washington.
Hale kept abreast of the work with the Crossley reflector at Lick. The incentive of following up on that work, and a program to extend the detailed spectrographic studies Hale had made of the sun to distant stars, was a compelling agenda for a big reflector. And when the headstrong young director of Yerkes Observatory got an idea in his head, there was no stopping him. Before the forty-inch refractor at the Yerkes Observatory was dedicated, Hale persuaded his father to contribute the funds to have a sixty-inch glass blank—the largest piece of glass the French foundries could mold in a single pour—cast for a giant reflector. William Hale made it clear that his gift was seed money; he would pay for the glass blank and nothing
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