Refracting Telescopes: 400th Anniversary
Posted by Brian Ventrudo
Four hundred years ago, Dutchman Hans Lippershey looked through two lenses and noticed distant objects appeared larger. He had invented the telescope. Because his device used lenses to bend, or refract, light, this type of scope is called a refractor, one of the two main types of telescopes.
• Every refractor has two main parts, a large lens to collect light from a distant object and a second smaller lens to magnify the image of the first lens.
• To “non-astronomers”, a refractor is the stereotypical telescope, a long thin tube you look directly through, like a paper towel roll.
• Lippershey’s telescope magnified 3 times (or 3x). In 1609, Galileo built a telescope that magnified 30x, and became the first to turn such a device to the sky. Both telescopes had a ton of image distortion, or aberration, yet still yielded useful results.
A Deeper Look
• Some tech terms: the distance from the lens to the focal plane is the “focal length”; the ratio of the focal length to the objective lens diameter is the “focal ratio”; the ratio of the objective lens focal length to the eyepiece focal length is the magnification of the telescope.
• The first refractors had a single objective lens with spherical curvature. This caused much image distortion. And red, green, and blue light rays focused at different planes, a problem called “chromatic aberration”. To minimize these problems, these early telescopes needed large focal ratios, which meant even telescopes with a small objective lens were enormously long… ten to twenty feet or more.
• In the mid-1750s, a lawyer (!!) Chester Moore invented a refractor with a two-lens objective, each of which was made from a different kind of glass to partially correct for chromatic aberration. Today, all refractors have at least two lenses in the objective.
A Bit of History
Hans Lippershey called his new invention a “looker”. Galileo, the first to turn a refractor to the stars and planets, instead called it a perspicillum, then changed to telescopium in Latin and “telescopio” in Italian.
You can imagine, I trust, what your spouse would say when you headed out into the night with your “looker” to gaze at the stars.