The Era of the Giant Reflectors
Once Common’s technique became known, telescope makers began working to construct larger mirrors. They also set out to improve on Newton’s reflector design.
One improvement was the Cassegrain. In this design, Newton’s angled secondary mirror is replaced with a hyperbolic secondary that directs the light back through a small hole in the primary. The Cassegrain design puts the focal point at a more easily accessible point than it would be in a standard Newtonian. It also makes a more compact instrument for a given fl. The design actually goes back to Newton's time, but there is some disagreement on who 'Cassegrain' was. Most believe it was Laurent Cassegrain, a Frenchman. It was however not until the 20th century that mirror grinding became sufficiently developed to make the design viable.
The major improvement came around 1910 when George Ritchey and Henri Chretien designed a Cassegrain reflector with two hyperbolic mirrors. Hyperbolic mirrors eliminate coma and spherical aberration. Many of today’s major instruments, including the Hubble, are Ritchey-Chretien telescopes. Significantly missing from the list are the 100-inch Mt Wilson scope and Mt. Palomar’s 200-inch, for reasons we will discuss later.
Those two giants were preceded by a 60-inch mirror at Mt. Wilson made by Ritchey in 1908, before he and Chretien developed their new design. George Hale, now Director at Mt. Wilson, however wanted an even larger telescope. Ritchey gave him one—a 100-inch mirror that at the time was the largest telescope in the world.
Ritchey began his design of the instrument before he and Chretien developed their Cassegrain design, so the giant has its focal point high up in its cage. Fortunately, few astronomers have a fear of heights.
But even as Edwin Hubble was turning astronomical thought on its ear with the 100-inch, George Hale was dreaming of a mirror twice as large—a 200-inch telescope that would dwarf anything ever made. He set Ritchey to its design.
Ritchey wanted to use his and Chretien’s new design, but Hale balked. The two came to a parting of the ways, so the big telescope would be a Newtonian/Cassegrain with a Coude focus point for spectrographic analysis. Coude is French for ‘at an angle.’ At the prime focus, the instrument would have a fl of 3.3, giving it a very wide angle view. At the Cassegrain focus its fl would be 30. It would be a highly flexible astronomical instrument.
A 200-inch mirror turned out to be a problem. Glass was unstable with temperature changes, so Hale’s team tried quartz. That did not work as the quartz formed bubbles too readily.
But a new type of glass had just been introduced by Corning Glass Works. It was called Pyrex. It was perfect for the task. And to save weight, the team developed a new approach to casting the blank. Rather than casting a thick cylinder of Pyrex, they created a mold that formed a thin blank with a ribbed structure on its back. It was very light but very strong.
Later, after WWII, they would make one more advance in mirror construction. They would use aluminum for the reflective coating, not silver. Aluminum does not tarnish.
Unfortunately, George Hale never saw the completion of his giant mirror. He died in 1938. Upon completion in 1948, it was named for him, and today is known as the Hale Telescope.
And in a twist of fate, Edwin Hubble, who had been slated to use the giant to continue his churning of astronomical theory, suffered a heart attack just before the telescope was completed. He had to retire.
The Hale remained the world’s largest telescope until 1975 when the Soviet Union built its BTA-6 236 inch reflector. But the Soviet giant never functioned properly, so the Hale remained the world’s largest viable instrument until the millennium neared. Then the entire concept of telescope design changed. A single giant mirror would be replaced by myriad smaller mirrors, each movable, in instruments such as the Keck, the Large Binocular Telescope and the Magellan.
These 21st century telescopes are described in related articles.