Lens correction of astigmatism in a czerny-turner spectrograph

This paper presents measurements of astigmatism for a typical Czerny-Turner spectrograph and shows two ways of correcting astigmatism in this instrument by using lenses. For an f/6.3 instrument having a 0.75-m focal length collimating and camera mirror, the meridional and sagittal focal planes were separated approximately 15 mm at 60 mm left of the center of the film plane and approximately 43 mm at 70 mm to the right of the center of the plane, when the grating was set to put 5000 A in the center of the film plane. Measurements of astigmatism for different lines of the Hg spectrum with 5000 A in the center of the film plane showed the astigmatism to be determined primarily, if not entirely, by the angle at which the radiation left the grating and collimating mirror. The experimental results compared favorably with the theory given by Rosendahl. Two methods of correcting astigmatism by lenses were investigated: one experimental, the other theoretical. Both appear to be practical.