Schematic drawing of the FISS-SP assembly using the existing FISS spectrograph (centre), the new FISS-SP slit unit (left), and the new spectral camera with polarizing beam splitter (PBS) (right). Rays with different colours correspond to different points in the field of view. In the interest of enhanced clarity, the projection of the detailed drawing of the slit unit was rotated by 90 degrees about the vertical and is viewed from the right. The spectral camera with PBS unit was rotated about the optical axis of the incoming beam and is viewed from the top left. See the main text for more details.