The development of telescope optics is a fascinating story. Until this century,
the optical theory of reflecting telescopes was essentially limited to the
Cartesian treatment of axial imagery. In 1905, Karl Schwarzschild initiated
a revolution by applying third order (Seidel) theory to the field imagery of
2-mirror telescopes. Since then, the whole gamut of possible telescope systems
has been invented, analysed and, in many cases, tried out in practice.
Over all its history, the optical development of the telescope has also depended
on technical inventions, above all in mirror materials, glasses, support
systems and means of achieving high reflectivity. Over the last 30 years, developments
have been particularly spectacular, above all in manufacture and
test techniques and generally in enhancing the image quality available.