  The SuMIRe Prime Focus Spectrograph (PFS), developed for the 8-m class SUBARU
telescope, will consist of four identical spectrographs, each receiving 600
fibers from a 2394 fiber robotic positioner at the telescope prime focus. Each
spectrograph includes three spectral channels to cover the wavelength range
0.38-1.26 um with a resolving power ranging between 2000 and 4000. A medium
resolution mode is also implemented to reach a resolving power of 5000 at 0.8
um. Each spectrograph is made of 4 optical units: the entrance unit which
produces three corrected collimated beams and three camera units (one per
spectral channel). The beam is split by using two large dichroics; and in each
arm, the light is dispersed by large VPH gratings. The proposed optical design
was optimized to achieve the requested image quality while simplifying the
manufacturing of the whole optical system. The camera design consists in an
innovative Schmidt camera observing a large field-of-view (10 degrees) with a
very fast beam. To achieve such a performance, the classical spherical mirror
is replaced by a catadioptric mirror (i.e meniscus lens with a reflective
surface on the rear side of the glass, like a Mangin mirror). This article
focuses on the optical architecture of the PFS spectrograph and the perfornance
achieved. We will first described the global optical design of the
spectrograph. Then, we will focus on the Mangin-Schmidt camera design. The
analysis of the optical performance and the results obtained are presented in
the last section.
