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Section 5.1

Paragraph Unity

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

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

The latest electronic innovation, still under development,is called "adaptive optics." Adaptive optics is an electronicfeedback mechanism capable of correcting for the distortingeffects of the earth's atmosphere and thus allowing much sharperimages of astronomical objects. The earth's atmosphere isconstantly shimmering, because of moving pockets of air andchanges in temperature, and such shimmering causes passing lightrays to bend one way then another. In effect, the shiftingatmosphere acts as a rapidly changing lens, smearing out anddefocusing images. In adaptive optics, motorized cushions areplaced behind the telescope's secondary mirror and constantlyreshape the mirror's surface to counteract the defocusing effectof the atmosphere. The cushions are given instructions by acomputer, which analyzes the image of a "guide star" in the samefield of view as whatever the telescope is looking at. With noatmospheric distortion, the image of a star should be a singlepoint of light. By analyzing how the actual image of the guidestar differs from a point, the computer can infer the distortionof the atmosphere and tell the cushions how to alter the mirrorto bring the guide star, and all the objects near it, back intosharp focus. Corrections must be made rapidly, because theatmosphere is rapidly shifting. In practice, the computer willanalyze the image of a guide star and give new instructions tothe reshaping cushions every 0.01 to 0.1 seconds.

--Alan Lightman, Ancient Light

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