The camera system is an f/4.1 off-axis Maksutov design comprising a two-element corrector, a spherical mirror and a field flattening element adjacent to the focal plane. A 45flat mirror before the field flattener directs the convergent radiation to a focus outside of the spectrograph. Each element is interchangeable and ':identified as before by red or blue painted discs on the cells. The red and blue flatteners are of different optical design. Both the, camera casing and field flattener unit are accessed from outside of the casing.
Focusing of the spectrograph is achieved by movement of the camera mirror. The collimator position is not adjustable except by a small distance during alignment operations which are not the concern of the normal user. The camera mirror position is determined by a micrometer which is read from outside of the instrument. The mirror slide is additionally clamped by a nut which must be released before adjusting the micrometer. A Hartmann mask, located close to the slit dekker and thermometer allow Hartmann focus tests to be conducted. The camera position for best focus can be defined to within 50,m routinely.
In normal operations the Hartmann mask is withdrawn from the optical path and held by a nylon clamp. In use the two operational positions of the mask are determined by being fully ``in'' or at a detented position. Adjacent halves of the collimator aperture are irradiated in the two positions.