## Bf

n'yi

4n'yi rad n'yi

(206 265) arcsec

The second term of Eq. (3.21) gives for the coma term at the Gaussian focus: (w/)gf = 1 (S//cos/ (3.191)

The linear field dependence (n'/nm) in Eq. (3.21) can be ignored as we are only concerned with the aperture effects governing the size of the image for any given field position giving the value S . For the wavefront aberration, the aperture law for coma is, from (3.191), a cube law giving the S-shaped aberration shown in Fig. 3.16. The vertical y-axis shows the wavefront aberration relative to the Gaussian focus, which means in this case relative to its principal ray. If an oblique line is drawn through the origin, then this is equivalent to a tilt of the reference sphere, or simply that the aberration is referred to a height in the image plane different from that of the principal ray. If the tilted line is drawn at an angle such that it cuts the wavefront at ym, then it can be shown that the resultant wavefront error is minimized. This is the "best focus" equivalent of the spherical aberration case, although here the "focus shift" is a lateral one. The remarks made above concerning wave-front and ray aberrations for spherical aberration also apply here. Reference

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