This simple design for a grating spectrometer uses a single lens, or mirror, as shown in figure 6.18. The

blazed grating is oriented so that the incoming light is incident at the blaze angle and then the reflected

light returns almost parallel to the incident beam. The mirror is an off-axis section of a paraboloid, and

by giving this a slight tilt the reflected beam is cast a little below the plane of the diagram so that the

detector does not overlap the source. Littrow mountings suffer from astigmatism and coma, but are

extremely compact. A grating in the Littrow mounting can be used as a monochromator to tune the

wavelength of a dye laser, as shown later in figure 14.10.