Telecentric scanning lenses are a special configuration in which the arrangement of optics is designed to focus down the beam such that it is always perpendicular to the flat field.
This is accomplished by ensuring that the system 'stop' is located at the front focal point of the lens system. The 'stop' is located at the position where the beam is deflected from the axis. In a single-axis scanning system, this location is at the scanning mirror. For two-axis scanning, the stop is mid-way between the mirrors.

• Thread: M85X1
  
• X - Y mirror separation: 18mm
  
• Y mirror to the center of 1sr lens: 19.5mm
  
• Material: G-Ge (only transmit 10.6um)
  
• Material: Z-ZnSe (transmit both 10.6um and visible)