Only as Good as
Their Telescopes


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Lathe

After its importance to science became well known, lens grinding became fashionable. Elaborate lathes, like this one made by Andrea Frati in the 18th century, graced the parlors of wealthy nobles.

At some point in any scientific endeavor it’s necessary to bring philosophy into contact with the real world. The tools for doing this are scientific instruments that perform critical experiments. The theoretician is thus, sooner or later, at the mercy of the instrument maker.

For astronomy, instrumentation has come to mean better and bigger telescopes. Today’s instrument makers include the thousands who contributed to the Hubble space telescope and the thousands who are working on the Webb telescope. The earliest astronomers depended on the craftsmen who fabricated their tools of magnification—without them scientists like Galileo Galilei could not measure and prove their observations and theories.  

Lathes

Top and middle: Huygens' universal joint: In a vertical lathe for grinding lenses, the glass blank rotates rapidly. It is fastened to the end of a rod and swung across a slower-rotating, concave lap. Bottom: A 17th-century lathe similar was used to produce the tools that ground Galileo's lenses.

In 1609 Galileo published the observations he had made using a primitive three-power telescope. His news immediately removed mankind from his previous place at the center of the universe and astronomy left the realm of superstition to become real science.


Accounts from the 1600s typically describe complete telescopes. Lenses are mentioned, but little attention is given to the methods that produced them. Shaping lenses involved increasingly sophisticated use of primitive tools, all of which were some variation on the basic lathe. The operator tried to form a lens contour by comparing it to a metal gauge that had an edge cut to match a compass-scribed arc. The approach was soon abandoned in favor of a more sophisticated process that used a series of turning operations in which the tools, rather than the lens itself, were lathe-turned. Lenses were produced by grinding them against a metal tool called a "lap," which had been produced using a lathe. Convex lenses required a lap having a portion of a hemispherical cavity. A male form was used for concave lenses.

Ippolito Francini of Florence, who furnished lenses to Galileo, produced laps using a lathe with a pivoted boring bar that could cut an accurate portion of a hemisphere. The same machine also polished lenses by substituting a buffer for the cutting tool. The cavity's radius of curvature was controlled by adjusting the length of the rod that held the cutting tool. The machine was sophisticated enough to incorporate a flywheel to smooth the hand-cranked rotation.  

A later contribution to lens making was the "baton," attributed to Christiaan Huygens, the 17th-century Dutch mathematician. Prior to this invention, the lens had been manipulated on the lap by a handle glued with pitch to the upper surface of the lens. Since the handle was some distance above the worked surface, it was inevitable the craftsman would apply a tipping moment to the lens blank while grinding, creating an imperfectly formed surface to the outer radius of the lens.  

The baton's length was set roughly equal to the radius that had been lathe-turned into the lap. It could hold either a lens blank or a lap and allowed a right circular cylinder of glass to be ground to the desired radius. The baton kept the line of action of the force always normal to the spherical lens surface at the point of intersection with the lap.

After grinding the lens was polished, sometimes using the same lap that had been used to shape it. That required preparing the lap surface to remove roughness. Another method still used by amateurs to polish telescope mirrors involved pouring pitch on the lens surface and removing it after it hardened, creating a polishing tool that exactly matched the curvature of the lens.

As the body of knowledge regarding telescope-making grew, it became obvious that the spherical aberration due to the shape of simple lenses was a factor that limited optical quality.

Johannes Kepler first recognized that a hyperbolic lens would eliminate spherical aberration. This led to a major effort by a host of scientists, philosophers, and mathematicians who were not above actually grinding glass, including Descartes, Galileo, Huygens, Kepler, Newton, Spinoza, and Torricelli.

Another aberration was observed and explained by Isaac Newton in 1672, when he discovered that white light was, in fact, composed of colors that were refracted to varying degrees. That caused an image to be surrounded by a colored rainbow known as chromatic aberration. Later on he invented the reflecting telescope, recognizing that in this kind of telescope the light is focused independent of wavelength. The vast majority of amateur-made telescopes today are reflectors.  

The early history of astronomy was greatly dependent upon the ability of artisans to craft good lenses for telescopes. Technological advances initially focused on increasing magnification, followed by sharpening resolution. Each refinement in lens quality led to another discovery—the lunar mountains, the Milky Way, Jovian satellites, Saturn's rings. The list continues to grow today.

[Adapted from “Clear as Glass” by Robert O. Woods, ASME Fellow, for Mechanical Engineering, October 2006.]

Accounts from the 1600s typically describe complete telescopes. Lenses are mentioned, but little attention is given to the methods that produced them.

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March 2011

by Robert O. Woods, ASME Fellow