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
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: "blue, "red", and “NIR”). The beam is split by using two large dichroics;
and in each arm, the light is dispersed by large VPH gratings (about 280x280mm).
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 (F/1.09). 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.