A method for correcting the longitudinal color of (thin) doublets at more than two colors, due to Mercado and Robb, is very successful in practice. However, although ostensibly based on the demand for correction at three wavelengths it, in fact, often leads to correction at four or more wavelengths. This at first sight strange situation is analyzed here. The true state of affairs is illustrated by simple generic examples which are in harmony with what is found in practice. Thin systems with more than two elements are also briefly considered.
© 1985 Optical Society of America
H. A. Buchdahl, "Many-color correction of thin doublets," Appl. Opt. 24, 1878-1882 (1985)