A geometrical theory of aberrations for the vicinity of the focus of arbitrary off-axis sections of conic mirrors is derived. It is shown that an off-axis conic mirror introduces linear astigmatism in the image. However, in classical two-mirror telescopes this aberration can be eliminated by tilting the secondary parent mirror axis. It is also shown that the practical geometrical-optics performance of a classical off-axis two-mirror telescope with no linear astigmatism is equivalent to the performance of an on-axis system, proving that both systems have identical third-order coma. To demonstrate the applicability of the theory developed in a practical system, a fast (i.e., f∕2), compact, obstruction-free classical off-axis Cassegrain telescope is designed.
© 2005 Optical Society of America
Atmospheric and Oceanic Optics
Seunghyuk Chang and Aluizio Prata, Jr., "Geometrical theory of aberrations near the axis in classical off-axis reflecting telescopes," J. Opt. Soc. Am. A 22, 2454-2464 (2005)