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which is converted into angular coma in arcsec by multiplying by (206 265/f'). Now (Comfl()j,s gives the coma for the case of the small field angle upr1 which is not the strict case of lateral decentering in the normal sense, shown in Fig. 3.97. Here, the principal ray coincident with the axis of the primary (upr1 = 0) is reflected back on itself, is incident on the decentered secondary at a small angle and reflected off as shown.

The difference between the cases shown in Figs. 3.96 and 3.97 is as follows. The Schiefspiegler case of Fig. 3.96 has been calculated from the recursion formulae with the "field angle" upr1 strictly for an image point on this reflected "axis": no "field" for the system is considered and the pupil can be considered at both primary and secondary. The case of strict lateral decenter shown in Fig. 3.97 involves a rotation of the beam relative to the mirror system clockwise, in a negative cartesian sense, i.e. through the angle —upr1. This introduces an additional coma corresponding to the field coma of the system for a field angle —upr1. Thus, the field coma corresponding to the field angle upr1 must be subtracted from the result above of Eq. (3.353) and this field coma will depend on the form of the telescope. If the form is aplanatic (RC in the case shown with a convex secondary), the field coma is zero and the decentering coma for the Schiefspiegler case of Fig. 3.96 and Eq. (3.353) gives the correct result; otherwise the field coma corresponding to upr1 must be subtracted.

The general formula for the field coma for a 2-mirror telescope is, from Table 3.5

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