What is Reflection?

The refracting telescope was one of humankind’s great inventions, rendered even greater by the presence of a genius like Galileo to use it. However, the limitations of the refracting telescope soon became apparent:

• Even the most exquisitely crafted lens produces distortion, which can be corrected only by the introduction of other lenses, which, in turn, introduce their own distortion and loss of brightness, since a little of the energy is absorbed in all that glass. The chief distortion is chromatic aberration.
• Excellent lenses are expensive to produce, and this was even more true in the days when all lenses were painstakingly ground by hand. Lenses are particularly difficult to produce because both sides have to be precision crafted and polished. For mirrored surfaces, like those found in reflecting telescopes, only a single side must be polished.
• Generally, the larger the lens, the greater the magnification and the brighter the image; however, large lenses get heavier faster than large mirrors. Lenses have volume, and the potential for imperfections (such as bubbles in the glass) is higher in a large lens. All of this means that large lenses are much more difficult and expensive to produce than small ones. Recognizing the deficiencies of the refracting telescope, Isaac Newton developed a new design, the reflecting telescope, in 1668.

Instead of the convex lens of a refractor, the reflector uses a concave mirror (shaped like a shallow bowl) to gather, reflect, and focus incoming light. The hollow side of your breakfast spoon is a concave mirror (the other side is a convex one). This curvature means that the focal point is in front of the mirror—between the mirror and the object being viewed. Newton recognized that this was at best inconvenient—your own head could block what you are looking at—so he introduced a secondary mirror to deflect the light path at a 90-degree angle to an eyepiece mounted on the side of the telescope.

Refracting telescope design continued to develop throughout the eighteenth and nineteenth centuries, culminating in the 40-inch (that’s the diameter of the principal lens) instrument at Yerkes Observatory in Williams Bay, Wisconsin, installed in 1897.
But due to the limitations just mentioned, the biggest, most powerful telescopes have all been reflectors. In the eighteenth century, the great British astronomer Sir William Herschel persuaded the king to finance an instrument with a 47-inch (1.2-meter) mirror.

With this telescope, Herschel had a big enough light bucket to explore galaxies beyond our own Milky Way (though he did not know that’s what they were). By the middle of the nineteenth century, William Parsons, third Earl of Rosse, explored new nebulae (fuzzy patches of light in the sky, some of which are galaxies) and star clusters with a 73-inch (1.85-meter) instrument constructed in 1845. It ranked as the largest telescope in the world well into the twentieth century, until the 100-inch reflector was installed at the Mount Wilson Observatory (near Pasadena, California) early in the century.