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The Mayfield Handbook of Technical & Scientific Writing
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Section 5.1

Paragraph Unity

Unify paragraphs by making every sentence contribute to a controlling idea, which is usually stated in a topic sentence.

In the following paragraph, which is part of a discussion of recent advances in optical astronomy, the topic sentence is the second sentence. It introduces the controlling idea of the paragraph--"adaptive optics." Note how each sentence of the paragraph supports that idea.

The latest electronic innovation, still under development, is called "adaptive optics." Adaptive optics is an electronic feedback mechanism capable of correcting for the distorting effects of the earth's atmosphere and thus allowing much sharper images of astronomical objects. The earth's atmosphere is constantly shimmering, because of moving pockets of air and changes in temperature, and such shimmering causes passing light rays to bend one way then another. In effect, the shifting atmosphere acts as a rapidly changing lens, smearing out and defocusing images. In adaptive optics, motorized cushions are placed behind the telescope's secondary mirror and constantly reshape the mirror's surface to counteract the defocusing effect of the atmosphere. The cushions are given instructions by a computer, which analyzes the image of a "guide star" in the same field of view as whatever the telescope is looking at. With no atmospheric distortion, the image of a star should be a single point of light. By analyzing how the actual image of the guide star differs from a point, the computer can infer the distortion of the atmosphere and tell the cushions how to alter the mirror to bring the guide star, and all the objects near it, back into sharp focus. Corrections must be made rapidly, because the atmosphere is rapidly shifting. In practice, the computer will analyze the image of a guide star and give new instructions to the reshaping cushions every 0.01 to 0.1 seconds.

--Alan Lightman, Ancient Light

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