Reflecting telescopes do not use a lens to gather the light from the heavens. Instead, they use a mirror to do so, and reflect the image back to the eyepiece.
In 1616, just seven years after Galileo built his first refractor, Niccolo Zucchi attempted to build a reflecting telescope using a metal reflector. He found it impractical. In 1663 James Gregory described the design of a reflector, but it was ten years later that Robert Hooke actually built it. It was small and the astronomer’s head blocked much of the incoming light.
Then in 1668, Sir Issac Newton had one of his many flashes of genius. If he put another mirror in the path of the light reflected from the mirror, he could redirect the image to a comfortable viewing position. He did, and the Newtonian Reflector, the basis of all astronomical telescopes since, was born.
Newton’s reflector was however primitive. The mirror, made of metal, was only two inches in diameter. And it was spherical, which meant the light rays did not come to a single focal point.
Newton continued to work to improve his design, and eventually built a six inch reflector. But he never found a way to make anything but a spherical mirror. That was left to Scotsman James Short. He found a way to make a parabolic mirror. This focused all the light to a single focal point. The shortcomings of Newton’s reflector were overcome. Telescope makers made some large mirrors. Sir William Parsons built a 72-inch mirror, but it, like all metal mirrors of the period, tended to tarnish quickly. The huge instrument was also so unwieldy it was all but impossible to use. Most telescopes of the period were 19 inch to four feet in diameter.
Then, about 1850 German chemist Justus von Liebig developed a method for coating glass with a thin coating of silver. German astronomer Carl von Steinheil and French doctor Leon Foucault applied this technique to telescope mirrors. The glass mirrors were half the weight of metal mirrors, did not tarnish as quickly, and were easily resilvered when they did.
Still, telescope makers could not make satisfactory mirrors large enough to be completely useful until Andrew Common, an English amateur in Yorkshire, England persevered and produced a 36-inch silvered glass mirror towards the end of the 19th century. Common’s instrument was eventually donated to the Lick Observatory, where it began the discovery of galaxies.
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 will be described in upcoming articles.