For high dispersion spectroscopy needing large instrumentation. Use two flat mirrors in light path. Requires a field rotator. Nasmyth Equivalent of Coude for Alt-Az telescopes. 2 large platforms available for instrumentation.
Refractive index is a function of wavelength, so blue light brought to focus nearer than red light.
Chromatic aberration, occurring for lenses only. The refractive index of glass varies with wavelength, so violet rays are brought to a focus nearer the lens than are red rays. The distance between two foci is greatly exaggerated in the diagram, but it is several percent of the mean focal length. Removed by parabolic mirror, but these are harder to make than spherical mirrors. For the positive coma illustrated here, the smaller images are nearer the axis.