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Refracting Telescopes

         Telescopes function by focusing light and then magnifying the focused image with an eyepiece or charge-coupled device called a CCD. Refracting telescopes (or "refractors") use lenses to focus light. In a refractor, the light comes in through the front, or objective lens (above diagram, left), and is focused at various eyepiece lenses (above diagram, right). The eyepiece's job is magnify the focused image it receives. Focusing light by bending it through a lens is called "refracting," hence the name refractor for telescopes that utilize this process.

         The lens of a refracting telescopes is physically limited to about 40" in diameter. Above that diameter an objective lens, necessarily supported only at its circumference (so that the support will not block incoming light) will sag or deform due to the force of gravity.  Though microscopic, this deformation is unacceptably significant given the precision demanded of the optics. This limit in diameter is currently held by the world's largest refracting telescope, the famous 40" Yerkes refractor built for the University of Chicago in Williams Bay, Wisconsin by George Ellery Hale.


         In 1892 Warner and Swasey received the order for this behemoth of telescopes, and the next year the mount was on display in Chicago at the Columbian Exposition. (When fire ravaged the Manufactures Building where the exposition was being held, horses dragged the giant tube clear of the flames and it was found to be unharmed.) Made of 1/8" to 7/32" sheet steel to support the great weight of the 40" objective lens, the 62' tube varies in diameter from 52" at the center to 42" at the objective end and 38" at the eyepieces. It weighs approximately 6 tons, was the first ever to use an electric motor to move it, and in turn is supported by polar and declination axes weighing 3.5 and 1.5 tons respectively (shown below).

         The upper end of the 13.5' x 12"-13.5" polar axis carries the 20-ton, 8'-diameter main driving gear with 330 teeth. The declination axis is 12" in diameter and 11.5' long. All this is supported by a 50-ton column which rises 43' above the lowest part of a movable floor that rises to meet the eyepiece at any elevation. The column itself consists of four bolted-together sections tapering from 11' x 5' to 10' x 5'.  This in turn was placed on an 18' x 14' footing, that itself in turn is anchored to a 32' x 28' x 5' brick pier on a concrete foundation.

         Alvan G. Clark, the optician who figured the great 40" lens, although in poor health set them into the telescope on May 20, 1897. He died within weeks of the installation. As the 60' tube can be oriented in almost any position, it was inverted, and the lens cell frame was bolted to the end of the telescope.  Then the two glass disks comprising the lens were put in place, one at the extreme end of the tube, the other about 11" within.

         The movable floor had it's drama as well. At 6:43 a.m. on May 29, 1987 a support cable broke and the movable floor crashed to the ground. Fortunately no one was in the building at the time. Looking through the telescope to see if the lens were damaged, Hale and his staff found a network of fine lines crossing their view.  A workman sent up into the instrument for further inspection discovered that the lines were just a spider web in the tube!  The lens was unharmed, but it took until September to fix the rising floor and the movable dome.


go to . . .

telescopes in general
reflecting telescopes
compound-catadioptric telescopes
charge-coupled devices (CCDs)
adaptive optics





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Carl Woebcke, Refracting Telescopes, 1991-2017. All rights reserved.