A slit pupil is a limiting case of a rectangular pupil whose one dimension is negligibly small. It is used in spectrographs. The power series aberrations of a rotationally symmetric imaging system with a slit pupil are the 1D analog of the corresponding aberration terms discussed in Chapter 1. In this chapter, we discuss the PSF of a slit pupil and the incoherent image of a slit parallel to the slit pupil. The Strehl ratio for and the balanced aberrations of a slit pupil are discussed. It is shown that the balanced aberrations are represented by the Legendre polynomials. We show further that the slit pupil is more sensitive to a primary aberration with or without balancing, except for spherical aberration, for which it is slightly less sensitive.
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