A new far infrared grating monochromator of Czerny-Turner type has been devised and constructed which has an output optical system comprising two small spherical mirrors to compensate astigmatism of the Czerny-turner system. Anastigmatism of the system has been confirmed by ray tracing. The far infrared monochromator covers 100–240
m wavelengths and is useful to measure spectral response of stressed Ge
Ga photoconductors having response up to 200
m, which have been studied for applications to far infrared astronomical and atmospheric observations by balloons.