Knowledge Bank

W. G. Fastie has described the basic theory of the Ebert type of grating $spectrometer1,2$ and has shown the wavelength invariance along circularly curved $slits.3$ Fastie and Sinton described a method using four skewed mirrors to obtain a triple-pass Ebert $spectrometer,4$ This paper will describe a method for multipassing in this type of spectrometer using two pairs of mutually perpendicular mirrors. Any number of passes greater than two may be obtained consistent with the length of the source at the entrance slit.