ATM Plus for Minus

[Anthony Stillman <atmer@flash.net>]

I have found a typo in my copy of "Telescope Optics Evaluation and Design"
by Rutten and van Venrooij (third printing.)  Specifically, in the section
entitled "Designing a Maksutov-Cassegrain", in equation 21.6.8, the minus
sign should be a plus sign.

The previous numbered equation (21.6.7) is correct.  I originally believed
that Y had been redefined to help with the simplification.  However, a
numerical model and a re-examination of the referenced paper "Compound
Catadioptric Telescopes with All Spherical Surfaces" by Sigler (AO vol. 17
pp. 1519-1526)  proved to me the presents of the typo.

Anthony