Scanning spectrometer of the gillieson type.

A new type of plane grating monochromator has been devised in which the grating is illuminated by convergent light. The main feature of this design is an arrangement by which the grating is simultaneously translated and rotated by means of a simple rotation about an axis displaced from the grating face. The motion is such that the spectrum remains in good focus on a fixed exit slit over a range of wavelengths from zero order to 5000 A in the third order for a grating with 600 lines/mm. This type of mount has the advantages of small aberrations and small astigmatism over the usable range. Because there are only two reflections, the instrument is best suited for studying problems where low light level might be of concern, such as for astronomical work or for use in the far ultraviolet.