A color image of the Yerkes refracting telescope within its dome
The Yerkes Telescope. Image courtesy of Wikimedia Commons.

The construction of the 40-inch refracting telescope at Yerkes Observatory in Williams Bay, Wisconsin, was directed by George Hale, an astrophysicist at the University of Chicago, and funded by Charles Yerkes, a Chicago businessman. The telescope is made of 40-inch glass lenses manufactured by the Alvan Clark and Sons Company in Massachusetts. The lenses had to be free of defects and impurities in order to function correctly. So, Clark had to use clay rods to continuously stir the glass in a furnace to prevent bubbles and produce perfect lenses. A 62-inch long steel tube supports the lenses at the front and the eyepiece at the back. The entire telescope is mounted on an elevating floor in a 90-foot diameter dome in Yerkes Observatory.

Two months before the opening of the observatory, the movable floor collapsed because of faulty cable fastenings. Luckily, construction workers were able to rebuild the floor before opening day for a price of $3,000. All together, the construction of the telescope cost $350,000, which is approximately equivalent to 10 million dollars today.

The telescope at Yerkes Observatory was significant because it surpassed the 36-inch refracting telescope at the Lick Observatory in California, making the Yerkes telescope the largest refracting telescope in the world. The 40-inch telescope has a light-gathering power 25% more powerful than the telescope at the Lick Observatory.

A color image showing the exterior of the Yerkes Observatory with cars in the foreground
The exterior of the Yerkes Observatory. Image courtesy of Wikimedia Commons.

Refracting telescopes consist of two convex lenses, called the objective and the eyepiece. When light passes through the objective lens, the light is refracted and converged to a focal point to produce an image. The image is then magnified through the eyepiece.

The main issue with refracting telescopes is the dependency of the image’s focus on the light’s wavelength, called chromatic aberration. Because different light wavelengths refract at different degrees, refracting telescopes produce images that are blurred. Also, glass lens manufacturing is expensive and complex. So, in the early 20th century, astronomers began to use reflecting telescopes over refracting telescopes to produce better-quality images with a less-expensive telescope. As a result, the 40-inch refracting telescope at Yerkes Observatory was the last and biggest refractor ever made, even though much larger reflecting telescopes exist today.

Written by Kelsey Corrigan, December 2014.


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