由位于主轴外的某一轴外物点，向光学系统发出的单色圆锥形光束，经该光学系统折射后，若在理想平面处不能结成清晰点，而是结成拖着明亮尾巴的彗星形光斑，则此光学系统的成像误差称为彗差。
对光学成像产生影响的主要成分是散光像差、彗差和三叶草像差。
Meanwhile, astigmatism aberration, coma aberration and trefoil aberration are three main effects that influence the quality of the optics.
结果：总高阶相差、水平彗差、垂直彗差和球差均随瞳孔直径的增大而增大；
Results: Total higher order aberrations, spherical aberration, horizontal primary coma, and vertical primary coma, were increased significantly with increasing pupil size.
结果表明，与基于硅片曝光的彗差测量方法相比，基于空间像的彗差测量技术速度上的优势十分明显。
Compared with the resist-based coma measurement techniques, the technique based on the aerial image possess the advantage of time-saving.
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