Corrector Systems for Cassegrain TelescopesC. G. Wynne

Imperial College of Science and Technology, Applied OpticsSection, Physics Department, London, S.W.7., United King-dom.Received 2 August 1968.

May I make some comments on, and corrections to, the paperby Wilson.'

At the cassegrain focus of a Ritchey-Chretien telescope, theuseful field may be expanded by the use of an optical system tocorrect the field curvature and astigmatism of the mirror pair.The designing of these correctors may be facilitated if the mirrorsare allowed to depart from the Ritchey-Chr6tien form, and suchsystems have been proposed, e.g., by Khler.2 With this formof solution, the cassegrain focus when the corrector is removed

will not be aplanatic; this is undesirable, since astronomers some-times wish to use the cassegrain focus without the corrector.

Wilson refers to this fact, and states that it does not appear tohave been discussed in the literature; it was discussed at the IAUSymposium No. 27 referred to above (pp. 21, 22, 27, 29, 36).Wilson states that all secondary focus corrector systems proposedso far require a departure from the Ritchey-Chretien mirrorforms; this is not the case for the doublet correctors that I de-scribed briefly (making this point explicitly) at this Symposium(p. 27), fuller details of which were published in 1965.2 Insteadof these references, Wilson cites a paper of mine at the Conferenceon Astronomical Optics, Imperial College, July 1967; the pro-ceedings of this conference have not been, and will not be, pub-lished. Possibly Wilson has procured an advance copy of a paperof mine, with the title he quotes, describing more recent work,which has just appeared.4

References1. R. N. Wilson, Appl. Opt. 7,253 (1968).2. H. Khler, "The Construction of Large Telescopes," IAU

Symposium No. 27, 1965 (Academic Press Inc., New York,1966). Substantially the same paper (with some corrections)has appeared in European Southern Observatory Bulletin No.2, pp. 13-35 (1967) and Appl. Opt. 7, 241 (1968).

3. C. G. Wynne, Appl. Opt. 4, 1185 (1965).4. C. G. Wynne, Astrophys. J. 152, 675 (1968).

2326 APPLIED OPTICS / Vol. 7, No. 11 / November 1968