The Prime Focus Imaging Spectrograph (PFIS) is a first light instrument for the Southern African Large Telescope
(SALT). PFIS is a versatile instrument designed to operate in a number of scientific modes by utilizing volume phase
holographic gratings, Fabry-Perot etalons, and polarimetric optics, which are manipulated in and out of the beam using
various placement mechanisms. The instrument is mounted at the prime focus 15m above the primary mirror and tilted at
37°. This remote placement and the need for 240° of rotation about the optical axis raises important design issues with
mass, flexure and access. The instrument structure provides the interface to the telescope Prime Focus Instrument
Platform (PFIP) as well as support points for all the optics, mechanisms and electrical equipment. The structure is a
welded open truss of hollow, square-section Invar beams. The open truss provides the highest stiffness to weight ratio
and minimizes the effect of wind loading, while the use of Invar negates the effects of thermal expansion. It has been
designed using finite element analysis in conjunction with an optical tolerance analysis of the optics nodes to minimize
effective image motion under the varying gravity load. The fundamentals of the design of the structure to minimize the
flexure and its effect on image motion, the motivation for using the open Invar truss structure, and the design of the
remotely operated mechanisms are discussed. In 2005 PFIS was installed and commissioned on SALT in South Africa.
Included in this text are some of the results and experiences of taking PFIS into operation.