The Refracting Telescope

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In the beginning, there was the simple refractor, the lens-type scope that was probably first turned on the heavens by Galileo Galilei on a mythic Italian evening in 1609. Galileo did not invent the telescope and may not even have been the first person to use it for viewing the night sky. He was the first real astronomer to wield a telescope, however, recording his observations and trying to understand what they meant. The puzzling thing is not that Galileo turned his scope to the Moon, planets, and stars or that he did it in 1609. What is mystifying is that it took so long for someone to stumble onto the idea of the telescope itself since it is such a laughably simple thing.

The secret of Galileo's telescope or any refracting telescope is at the end of its tube, where a large lens is found (Figure 1), the refractor's objective. This objective may be, as it was in Galileo's telescope, a single lens or element, or, as in today's refractors, it may be composed of two or more elements. The purpose of the objective

R. Mollise, Choosing and Using a New CAT,

DOI: 10.1007/978-0-387-09772-5_2, © Springer Science + Business Media, LLC 2009

Drin Rack Beams
Figure 1. (Telescope Types) The three most common telescope types: the refractor, the (Newtonian) reflector, and the catadioptric (CAT). Credit: Image courtesy of Meade Instruments Corporation.

is easy to understand. Its job is to collect light, lots of light, much more than the tiny lens of the human eye can gather.

The objective not only gathers light, it also brings it to a focus at the opposite end of the telescope's tube. The image formed at this focus is bright but small. In order for the human eye to make out details in this telescopic image, a magnifying glass is placed just past the focus point. This magnifying glass, like the telescope's objective, may be made from one lens element or many and is commonly referred to as an "eyepiece" or "ocular." In modern telescopes the eyepiece can be removed and replaced by one with differently shaped lenses that delivers a different magnification ("power"). A refracting telescope's images are focused, brought to best sharpness, by moving the eyepiece in and out, placing it closer or farther away from the objective. That's all there was to Galileo's telescope and all there was to any astronomer's telescope for many years: a lens to collect and focus light and a lens to magnify this image for human inspection.

Simple as these first telescopes were, astronomers in the seventeenth and eighteenth centuries used them to take humankind's first steps towards unlocking the mysteries of the cosmos. It soon became clear, however, that Galileo's version of the telescope, with its single element objective lens, had some debilitating defects. The most severe of these was chromatic aberration. The Galilean telescope's simple lens could not bring all rays of light to the same focus. Red rays, for example, focus at a slightly different position than blue rays. No matter how the focus of the telescope was adjusted by moving the eyepiece, the image remained slightly blurry and deviled by (usually) purple-colored halos around bright objects. Eventually, a means of making refractors "color free" would be found, but lens-type telescopes completely free of this "spurious" color would not be possible for a long time, not until the twentieth century.

Fortunately, it wasn't too long after Galileo's time that a genius turned his attention to the telescope problem. Isaac Newton, perhaps the greatest scientific mind the human race has yet produced, came up with an elegant solution for chromatic aberration. It was obvious the spurious color was due to the basic properties of the telescope's objective. The lens brought images to a focus by bending, by refracting, light; that's where the color came from. Why not use something other than a lens, then? A mirror can collect light as well as a lens, and a concave mirror can bring this light to a focus.

In Newton's reflecting telescope (Figure 1), a large concave primary mirror does just that. It gathers light from the sky like a lens. The "Newtonian's" primary mirror then reflects this light back up the tube, where it is intercepted by a small, flat, secondary mirror tilted 45°. This secondary diverts light rays out the side of the tube to an eyepiece for viewing. Since there is no refraction going on, there is no chromatic aberration. Reflecting telescopes have optical problems of their own, but colored halos around bright stars is not one of them.

The refractor and the reflector sound like very different animals, but in some ways they are quite similar. Their basic characteristics are measured and stated in the same ways. The diameter of a telescope's lens or mirror is its aperture and is expressed in inches or millimeters. The point at which the lens or mirror brings the light to a focus is the focal point. The distance from lens or mirror to this focal point is the telescope's focal length. The ratio of the telescope's aperture to its focal length is its focal ratio ("speed"). For example, a 6-inch (150-mm) diameter mirror with a focal length of 48-inches (1,200-mm) has a focal ratio of "f/8" (48/6). Telescopes with low ("fast") focal ratios deliver smaller, brighter images and wider fields, eyepiece for eyepiece, than telescopes with high, slow focal ratios. An f/4 telescope with a 12-inch (300-mm) aperture mirror produces a magnification of 48x with a 25-mm eyepiece (300 x 4/25 mm = 48x). A 12-inch mirror with a focal ratio of f/6 gives 72x (300 x 6/25 = 72x).

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