The quad-mirror optical telescope concept for the Maunakea Spectroscopic Explorer

A quad-mirror (QM) telescope ooptical concept has been designed for the Maunakea Spectroscopic Explorer project. The goal to repurpose the existing CFHT site without changing the footprint of the observatory imposes numerous constraints on the telescope including space constraints limiting the size of the telescope and the space available for the large number of spectrographs required, dimensional constraints (limits the size of any monolithic mirrors), mass limits along with other logistical constraints that wouldn't necessarily exist for a facility designed from first principals. The QM concept offers several scientific and versatility advantages over the original prime focus baseline, but it poses some challenges, such as the size of the tertiary mirror. This design serves as input into the effort to understand how best to implement the QM design and to determine if there are any arising hard show stoppers. We present an overview of the optical design with its benefits, areas of compromise to meet currently known constraints, and evaluation of preliminary optical sensitivities and metrology needs.