## Y

it follows that the factor (H3E)V is simply dent of the stop position. The stop-shift formulae are given by:

y because yv is indepen-

= Ev Sv + Ev (H3E)(Sjv + 3Sm) + £v 3(H3E)2Sii + £v(H3E)3Sj

Thus, to third order accuracy, the coma coefficient £ S+ is independent of the stop position if the spherical aberration coefficient £ Si is zero; and the astigmatism coefficient £ S+-j is independent of the stop position if both £ Si and £ Su are zero. These principles are of great significance in understanding the effect of different stop positions in various telescope forms treated below.

In telescope optics, the first order aberrations of Table 3.1 are also of great practical significance for the reasons given in § 3.2.1 above. It can easily be shown [3.3] [3.6] that these can be expressed as wavefront aberrations in terms of the lateral and longitudinal focus shifts ¿n' and ¿z of the centre of curvature of the image forming wavefront by

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