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