Coma-free double monochromator without intermediate slit

A conventional Czerny-Turner monochromator suffers from the aberration coma especially at longer wavelengths. Some earlier workers have suggested some simple means of correcting this aberration at a specific wavelength within the range of use of the instrument. Such an instrument will have zero coma at, say, 5000 A and slightly reduced coma at the shortest and longest wavelengths. In the present paper, we propose a modified Czerny-Turner monochromator in which the coma is eliminated at every wavelength setting of the instrument and, in addition, which has the desirable properties of double dispersion and low scattered light of a double monochromator.