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Galileo's Telescopes
Galileo's Telescopes

Florence, Italy, is the home of many museums, some of which contain the greatest works of art humanity has ever produced. It’s easy to overlook the small Institute and Museum of the History of Science, which looks not unlike a small warehouse, but this museum, which I visited in 2006 on my way home from viewing the March 29 eclipse in Libya, houses many treasures of the history of science.

As an astronomer, I was particularly excited by the contents of the display case dedicated to Galileo Galilei. This contains the only two surviving telescopes made by Galileo, plus a grisly relic: the mummified remains of Galileo’s hand.

In the eighth year of the 17th century, the idea of a new invention called a telescope was percolating throughout Europe. Spectacle makers in several locations, notably Hans Lipperhey in the Netherlands, discovered that by combining two lenses of different focal lengths they could make a device which magnified and brought distant objects closer. Though most saw this as a new tool for use in warfare, a few applied this device to science, using it to study the heavens.

Most notable of these early telescope users was Galileo Galilei in Padua, Italy. The son of an eminent lutenist, Vincenzo Galilei, he had long been interested in science. In 1609 he built his first telescope, and soon became one of the leading makers of telescopes, demonstrating and selling his telescopes in the various courts of Italy. He apparently made many telescopes, but the only two that survive are the ones in this museum.

His discoveries formed the basis of the “new astronomy,” which he popularized through his many books, unusual for the day being written in ordinary Italian, rather than scholarly Latin. He wanted everyone, not just scholars, to learn about his discoveries of the spots on the Sun, the craters and mountains on the Moon, Jupiter’s four bright moons, and Saturn’s mysterious shape.

Galileo’s telescopes are simple refractors with objective lenses only an inch or so in diameter. Lenses were hard to manufacture in those days and suffered from severe chromatic aberration, unless they were kept small in size. Chromatic aberration is the spreading of the light into a tiny rainbow called a spectrum, and this plagued all refractors until the achromatic lens was discovered nearly two centuries later. One way of reducing chromatic aberration was to increase the focal length of the objective: Galileo’s telescopes are typically long and skinny, looking more like a walking stick than a telescope. The tubes were made of various materials, including wood and cardboard, but usually not of metal. Because Galileo was selling his telescopes to the aristocracy, they are often beautifully decorated with brightly colored bindings and wrappings.

The eyepiece of Galileo’s telescopes is a very unusual one compared to most telescope eyepieces. Rather than being a positive lens, similar to a magnifying glass, it is a negative lens that intercepts the converging light rays coming from the objective and makes them parallel to enter the eye. This has one big advantage: the image is erect; but many disadvantages, especially its extremely narrow field of view. This design survives today only in simple “opera glasses” with only 2 or 3 times magnification. Any more magnification, and the field of view becomes impractically small. Most later telescopes use a design developed by Johannes Kepler which uses a positive lens for an eyepiece, inverting the image but allowing a much wider field of view.

The magnification of Galileo’s telescopes was very low by modern standards, with a maximum of about 20 power. It speaks well for Galileo’s talents as an observer that he was able to see so much with such a limited instrument. Even the least expensive beginner’s telescope of today vastly exceeds what Galileo had in both aperture and magnification, as well as freedom from aberrations.

March 2009

Geoff has been a life-long telescope addict, and is active in many areas of visual observation; he is a moderator of the Yahoo "Talking Telescopes" group.

Details
Date Taken: 06/08/2011
Author: Geoff Gaherty
Category: Archives

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