  The Fiber Optical Cable and Connector System, FOCCoS, subsystem of the Prime
Focus Spectrograph, PFS, for Subaru telescope, is responsible to feed four
spectrographs with a set of optical fibers cables. The light injection for each
spectrograph is assured by a convex curved slit with a linear array of 616
optical fibers. In this paper we present a design of a slit that ensures the
right direction of the fibers by using masks of micro holes. This kind of mask
is made by a technique called electroforming, which is able to produce a nickel
plate with holes in a linear sequence. The precision error is around 1micron in
the diameter and 1 micron in the positions of the holes. This nickel plate may
be produced with a thickness between 50 and 200 microns, so it may be very
flexible. This flexibility allows the mask to be bent into the shape necessary
for a curved slit. The concept requires two masks, which we call Front Mask,
and Rear Mask, separated by a gap that defines the thickness of the slit. The
pitch and the diameter of the holes define the linear geometry of the slit; the
curvature of each mask defines the angular geometry of the slit. Obviously,
this assembly must be mounted inside a structure rigid and strong enough to be
supported inside the spectrograph. This structure must have a CTE optimized to
avoid displacement of the fibers or increased FRD of the fibers when the device
is submitted to temperatures around 3 degrees Celsius, the temperature of
operation of the spectrograph. We have produced two models. Both are mounted
inside a very compact Invar case, and both have their front surfaces covered by
a dark composite, to reduce stray light. Furthermore, we have conducted
experiments with two different internal structures to minimize effects caused
by temperature gradients.
