The optical design of a highly segmented, very wide-field spectrograph

In this paper we work out the optical design of, basically, a limited Field of View off-axis camera. This element is the ingredient of a much more complex very wide field of view spectrograph and it is intended to avoid technological difficulties related with huge optics by replicating such element (or family of such elements). The optical design has to deal with the large off-axis aberration at a point in the Field of View as far from the optical axis as about 0.75 degree. This requires special tools for treating the convergence of the optical design as, for instance, vignetting on the edges can be severe because of the strong aberrations at the field lens entrance. Constraints into the optical design are particularly interesting as well: in fact the overall cross section of the design have to lie within the footprint of the entrance Field of View in order to allow for an array of such a design to be assembled together and guarantee the space for the allocation of micro-mechanisms required for movable slits and grisms in each module.