Astronomical telescopes are of two general types. The refractor is a telescope that passes light through an objective lens, then through an eyepiece. Galileo used a refractor to study the Moon and Jupiter. The diameter of the objective lens determines the light gathering ability. Double the diameter and you get four times the amount of light.
The world’s largest diameter refractor objective lens is 40 inches in the Yerkes Observatory telescope in Williams Bay, Wisconsin, northwest of Chicago. Their excessive weight and length make them unsuitable any larger. They can’t be supported well enough to avoid warping of the long tube. Some notable people who have used the Yerkes telescope include Edwin Hubble, Subrahmanyan Chandrasekhar, Russian-American astronomer Otto Struve, and Carl Sagan.
A. Einstein – Courtesy Yerkes Obs.
The second general type is the reflector. Isaac Newton developed a useful design. For this reason, the telescope is often called the Newtonian. Light is reflected off of a concave mirrored surface, parabolic for the highest quality telescopes. From there it reflects off of a small diagonal flat mirror at 45˚ and then through an eyepiece. Again, the diameter determines the light gathering ability. These telescopes don’t suffer from the aberrations caused by light passing through an objective lens as in the refractor.
This large and ungainly 40 foot long 48 inch diameter Newtonian was built in 1785 by William Herschel. He and his sister Caroline used it to look for faint objects. Nebula clouds and spiral objects were discovered and thought to be comets. Galaxies were not known then.
U. of Chicago Library
Another style of reflector is called the Cassegrain. It reflects the light from the objective mirror off of a smaller secondary curved mirror and back through a hole in the objective to an eyepiece. These telescopes can be made much larger than a refractor. Their size and weight can be supported better. This design effectively shortens the telescope tube length. It is much more compact relative to its diameter.
The largest telescopes in the world are of the Cassegrain type. Some single mirrors of 8.4 meters (27.6 feet) diameter are being made for them today. One such telescope, the Giant Magellan Telescope, is scheduled for completion in 2019. It will use 7 of these large mirrors together for an overall effective diameter of 24.5 meters (80 feet). Currently, the third of the seven mirrors is in production. The finished assembly will look like this. Note the human figure in the scene. Quick facts are here.
Only one mirror has a hole in the center as in the Cassegrain design. All seven will reflect their light to the smaller curved mirrors on the end of the supports. There, the light will reflect down and through the hole in the center mirror to the observing instruments. How do you build such large mirrors?
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The mirrors need to be as light as possible. They cannot be made of a thick piece of solid glass. Instead, they are built in a honeycomb hollowed out structure. It is a remarkable story of engineering and technology. The story of how these enormous mirrors are made is the next part of this post. Watch for it next time.