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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 1-μm in the diameter and 1-μm 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.
This concept has several advantages relative to a design based on Vgrooves, which is the classical option. It is
much easier and quicker to assemble, much cheaper, more accurate, easier to adjust; and it also offers the possibility of
making a device much more strong, robust and completely miniaturized.
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