Telescopes from the Ground Up

The open-air telescope shows the need for a new design

Now that refracting telescopes had grown to more than 100 feet in an effort to reduce spherical aberration, the standard shape — lenses enclosed in one long tube — wasn’t working out.

In 1686, Christopher Huygens, a Dutch astronomer, decided to stop using long tubes altogether. He mounted his primary lens in a short iron tube and attached it to a high pole. He left the eyepiece in another small tube on the ground, and ran a length of cord between the two to help line them up. The arrangement left him with a 123-foot telescope that was open to the air.

Like Johannes Hevelius’ 150-foot telescope, the Huygens telescope didn’t work well. It was difficult to line up the lenses. On dark nights, the primary lens was hardly visible from the eyepiece end. Huygens would have to hold up a lantern and search for its reflection in the far-away primary lens in order to line the lenses up. Bringing the lenses into line could take most of the night, which left little time for observations. In addition, any stray light in the area washed out the view, making images difficult to see.

While the intentions behind the lengthiest telescopes were good, astronomers were finding that they were just too hard to use to be truly successful. Instead, most of the new discoveries were coming from refracting telescopes no larger than 40 feet long. Huygens, for instance, found a moon of Saturn — Titan — with his 12-foot, 50-power telescope, and realized Saturn was surrounded by rings with his 23-foot, 100-power telescope.

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Early Refractors
Map of The Hague, Netherlands, where Huygens built his refractors.
Illustration of one of Huygens' refractors.Enlarge picture
longest refractor
Year completed: 1686
Telescope type: Refractor
Light collector: Glass lens
Telescope length: 123 feet
(37 m) long
Light observed: Visible
Discovery Highlights:
  • Using his telescopes, Huygens discovered that Saturn's "bulges" were rings that were not attached to the planet at all.