Visible Light Display Optics.
The optical imager (Figure 2-17) focuses IR energy from the scan
mirror onto the IR detector array in the detector/Dewar. The optical
imager has three IR imaging lenses and a folding mirror. IR energy
passes through lens No. 1 and lens No. 2 to the folding mirror. The
folding mirror reflects the IR energy 90 degrees and transmits it
through temperature compensating focus lens No. 3 to the IR detector
Lens No. 3 is mounted on thermal compensating rods that
maintain a constant focal distance with changes in temperature.