Cassegrainian-type reflecting objectives are useful as in-line imaging devices for fast spectrometer systems. They are inherently free from chromatic aberrations and may be corrected for spherical aberration. A particularly simple design, consisting of two mirrors with oppositely equal curvatures, is possible when unit magnification is desired. This system has zero third-order spherical aberration, and all other aberrations are small also. Its main disadvantage is the loss due to the central obstruction of the beam, amounting to about 28% exclusive of reflection losses. The author is with the Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234.
Klaus D. Mielenz, "Spherically Corrected Reflecting Objective for Unit Magnification," Appl. Opt. 13, 2580-2584 (1974)