The 40-inch telescope from the inside.

In 1890, this telescope was already partially under construction, and it was planned to be the world's largest telescope. It was, in fact, the world's largest telescope from 1897 (when it was commissioned) to 1909, and it is still the world's largest refractor. The lenses started as glass disks cast by Mantois of Paris and were polished into 40-inch lenses by Alvan Clark and Sons in Cambridgeport, Massachusetts. Warner and Swasey of Cleveland built the mounting for the telescope and a 90-foot diameter dome to house it. They also constructed a 75-foot diameter movable floor that raises astronomers to the telescope eyepiece. This is necessary because the telescope pivots about the middle of the telescope tube: if one end points to a star low on the horizon, the other end (through which you want to look) will be far off the ground. So, the floor rises so that you can be hoisted up to the eyepiece.

In all of the pictures below, note the scale of things. The lens itself is 40 inches across, and the tube is 63 feet long. This is a big telescope!

This view of the 40-inch shows tourists seated around the edge of the floor, listening to the tour guide lecture.

Note the size of the gears in this picture. Although you can't see them in this picture, the grease blobs on them are about the size of your fist.

You can barely see the operator's desk far below, between the pier and the tube. Note the spiral staircase going up to the telescope.

Another view of the telescope and pier.

The 75-foot diameter floor, all 37.5 tons of it, is suspended by 4 pairs of cables, like the ones our fearless tour guide (Rich Dreiser) is threatening to grab in this picture. The floor has only fallen once -- at 6:43 am on 29 May 1897, about 1 hour 20 minutes after sunrise, so (fortunately) there were no astronomers there to be injured. (The previous evening there had been work going on "adjusting the telescope" until 9-10 pm.) Half the floor was in the basement, and the other half was hanging at a terrible angle. It sheared off half the spiral staircase on the way down, so it was very fortunate that there was nobody in the dome or on the telescope. You can read more about this in the Virtual Museum.


Various views of Professor Kyle Cudworth near the telescope; the third one has him standing near where the detectors (photographic plates or eyes) are placed, and the last one is standing near the lens. Professor Cudworth uses the 40-inch refractor nearly every night that weather permits to study the motions of stars. The 40-inch is an ideal instrument for this kind of research because there are lots of pictures taken with this same instrument over many (more than 100!) years. There is more description of his research on the page on science going on at Yerkes today.

Yes, people who work here do have a sense of humor. (yes, that is Spiderman up there!)

This is a close-up of the Moon taken with the 40-inch. This is the Southern Highlands and (in center) the crater Clavius.

Looking along the 40inch tube at the Moon.

Lots of famous research has been done at Yerkes, much of it with the 40-inch. The 40-inch is used as often as the weather will permit. The current emphasis is stellar proper motions, where the long history of excellent photos from this telescope enables significantly higher precision than anyone else gets anywhere else in the world: combining photos by Ritchey and Barnard before 1910 with those we take now.

Go back to the page on the telescopes at Yerkes

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Content originally generated in March 1995.
This file was last modified on 20 April 1999.