There are two problems with Optical Design 2 which lead to Optical Design 3. First, the reduction of focal length from 180 to 120 mm caused an increase in the size of the secondary mirror. This blocks a significant amount of light to the primary mirror. Secondly, there were difficulties in mounting the camera to the primary mirror due to the size of the camera and lack of space on the back of the mirror. Both of these problems are avoided by moving the PSD in front of the primary mirror and mounting the camera on the back of the secondary mirror, facing out.
It was felt that the FOV in Optical Design 1, 3.18 mm, was too small for the purposes of the demonstration, so the new design has a focal length of 120 mm which leads to a FOV of 4.76 degrees. In this design the PSD was also positioned 25 mm behind the primary mirror.
The initial design called for the placement of the PSD behind the primary mirror and the use of a beam splitter to divert part of the signal to a camera mounted on the back of the primary mirror.