The field lens is a large fused silica element located immediately behind the focal surface. The lens is mounted in a statically athermal cell. The lens/cell unit is removable for re-coating. The lens cell includes features to simplify removal and re-installation. The cell is kinematically attached to the instrument mainframe for stress isolation, and to simplify collimation.
The field lens design parameters are:
diameter 635 mm (25 inches)
thickness 85 mm (3.35 inches)
material fused silica