Spherical aberration

A type of aberration that occurs when an optical system forms an image. As shown in the figure , light rays that are parallel to the axis of a concave mirror do not converge to a single point if the size of the mirror surface is large. Light rays that are incident on the edge of the mirror intersect with the mirror axis closer to the mirror after reflection than light rays that are incident on the center of the mirror. This phenomenon is called spherical aberration of a spherical mirror. With a mirror that is a paraboloid of revolution rather than a spherical one, light rays that are parallel to the mirror axis converge completely to a single point. For this reason, paraboloids of revolution are used for astronomical reflecting telescopes. The image point that paraxial light rays form on the mirror axis is called the paraxial image point. The distance from the paraxial image point to the intersection of the light rays that entered the edge of the mirror and the mirror axis is called the amount of vertical spherical aberration. Vertical spherical aberration increases in proportion to the square of the distance from the mirror axis to the point of incidence of the light rays. Therefore, spherical aberration can be reduced by narrowing the optical system. To eliminate the spherical aberration and coma of a concave mirror, a Schmidt corrector plate is placed at the center of the curvature of the sphere. A wide-angle, bright telephoto camera that uses this method is called a Schmidt camera.

[Kazuo Miyake]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

光学系の結像に際して生ずる収差の一種。図のように凹面鏡の鏡軸に平行に入射した光線は、鏡面の大きさが大きいときには1点に集まらない。鏡の縁のほうに入射した光線は、反射後、鏡の中心部に入射した光線よりも鏡に近いところで鏡軸と交わる。この現象を球面鏡の球面収差という。球面ではなく回転放物面の鏡では、鏡軸に平行な光線は完全に1点に集まる。そのため天体反射望遠鏡には回転放物面が用いられる。近軸光線が鏡軸上に生ずる像点を近軸像点という。近軸像点から、鏡の縁に入射した光線と鏡軸との交点までの距離を、縦(たて)方向球面収差量という。縦方向球面収差は、光線の入射点の鏡軸からの距離の2乗に比例して増大する。したがって、光学系を絞ると球面収差を小さくすることができる。凹面鏡の球面収差とコマ収差を除去する方法として、球面の曲率中心にシュミットの補正板を置く。この方法を応用した、広角で明るい天体望遠カメラのことをシュミット・カメラという。

［三宅和夫］

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