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Reference.org
Astigmatism (optical systems)
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<p>An <a href="/facts/Optical_system/p0vq4S1T">optical system</a> with astigmatism is one where <a href="/facts/Ray_(optics)/PgUEC7Sm">rays</a> that propagate in two perpendicular <a href="/facts/Plane_(mathematics)/tAUtRFcn">planes</a> have different <a href="/facts/Focus_(optics)/QuRKfhg6">foci</a>. If an optical system with astigmatism is used to form an image of a <a href="/facts/Cross/a3W7gkFH">cross</a>, the vertical and horizontal lines will be in sharp focus at two different distances. The term comes from the <a href="/facts/Greek_language/pzDd6eIz">Greek</a> α- (<i>a-</i>) meaning "without" and στίγμα (<i>stigma</i>), "a mark, spot, puncture". </p>