and

A far infrared monochromator of Czerny- Turner type has been devised which has an output optical system comprising two small spherical mirrors to obtain non-astigmatic image. The far infrared monochromator covers both 50-120 um and 100-240 um wavelengths ranges and can be used to measure spectral response of stressed Ge:Ga photoconductors having response up to 200 um, which we have fabricated to apply them to far infrared astronomical observations by balloons and by sattelites. Experimental performance of the stressed Ge:Ga photoconductors under low photon fluxes conditions also will be given