## C

leading to the required form of the secondary as bs2 = -1+(r^) = -! + (! - Ra)3 , (3.331)

from (3.326) and (3.330). If Ra = 0.25, this gives bs2 = -0.578, a moderate elliptical form. For normal values of Ra, the value of r3 from (3.326) will require M3 to be placed behind the primary in order to provide a reasonable final image position. This implies a perforated primary, as proposed by Baker.

The "aspheric plate theory" of § 3.4 can be applied in exactly the same way as for the Paul system above to prove that field coma and astigmatism are zero, so that the Paul-Baker system gives

Schroeder [3.22(b)] comments on the first large practical realisation of such a Paul-Baker telescope as a transit instrument with a 1.8 m primary working at f/2.2 and an axial obstruction ratio Ra ~ 0.32. He states near-diffraction-limited imagery is given over a 1° field. As with all such three-

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