The Adler has a number of large seventeenth-century telescopes (not shown here, but see the image from the Mensing catalog). Early objective lenses had very long focal lengths, due to the as-yet unsolved problems of spherical and chromatic aberration.

This closely resembles —it may actually be— the telescope with which Herschel discovered the planet Uranus in 1781. It appears here as it was mounted in a display case some years ago. The tube and primary mirror (displayed at the bottom left of the image) are original; the mount is a modern reproduction of Herschel's design.

For many years the Dearborn Telescope, the largest telescope in the world in its time, has been one of the most popular exhibits at the Adler Planetarium. The telescope has had an unusually varied career, and has recently returned from a careful, professional facelift, resuming its place in the New Adler.