This is the fiber optic coupler port that connects the fiber cable coming from the Raman probe to the spectrometer. The port also has a coupler on the inner backing to  re-focus the incoming beam to it's proper divergence.

Measurements are: 51mm from re-focusing lens to reflecting mirror, 61.77mm from centerline on reflecting mirror to diffraction grating, 51mm from diffraction grating to centerline on DSLR camera lens.