Aspheric Ebert Monochromator for Simultaneous Observation of Many Wavelengths*

The Ebert spectrograph has recently been used extensively as a monochromator. In view of the spherical mirror being used as a collimator-camera mirror and the plane grating as the dispersing element, the instrument suffers mainly from spherical aberration if the f-number of the system is smaller than about 9 for a 1-m focal length. Also, if many exit slits are needed over the entire spectrum of interest, the coma cannot be neglected. In order to obtain a fast spectrometer in which many exit slits could be used, the two surfaces in the system must be aspherized. The theory of such a system is presented in this paper. Specifically, a numerical example is worked out for an f/5 system with 40-in. focal length and 1200 lines/mm rulings on the grating. The spectrum of interest is 1000 to 4000 Å in first order.