The Fiber Optical Cable and Connector System, ”FOCCoS”, for the Prime Focus Spectrograph, “PFS”, is responsible
for transporting light from the Subaru Telescope focal plane to a set of four spectrographs. Each spectrograph will be fed
by a convex curved slit with 616 optical fibers organized in a linear arrangement. The slit frontal surface is covered with
a special dark composite, made with refractory oxide, which is able to sustain its properties with minimum quantities of
abrasives during the polishing process; this stability is obtained This stability is obtained by the detachment of the
refractory oxide nanoparticles, which then gently reinforce gently the polishing process and increase its the efficiency.
The surface roughness measured in several samples after high performance polishing was about 0.01 microns.
Furthermore, the time for obtaining a polished surface with this quality is about 10 times less than the time required for
polishing a brass, glass or ceramic surface of the same size. In this paper, we describe the procedure developed for high
quality polishing of this type of slit. The cylindrical polishing described here, uses cylindrical concave metal bases on
which glass paper is based. The polishing process consists to use grid sequences of 30μm, 12μm, 9μm, 5μm, 3μm, 1μm
and, finally, a colloidal silica on a chemical cloth. To obtain the maximum throughput, the surface of the fibers should
be polished in such a way that they are optically flat and free from scratches. The optical fibers are inspected with a
microscope at all stages of the polishing process to ensure high quality. The efficiency of the process may be improved
by using a cylindrical concave composite base as a substrate suitable for diamond liquid solutions. Despite this process
being completely by hand, the final result shows a very high quality.