PFS fiber system
The PFS fiber system consists of 2400 fibers
which relay light incident at the prime focus focal plane to 4
spectrographs located remotely in a room adjacent to the
telescope. A key component of the fiber system is fiber
connectors, which provide flexibility in instrument exchange as
well as testing, and retain the possibility of using this fiber
system to feed other spectrographs or photometers. In this system,
each of the 2400 fiber entrances is retained by the Cobra
displacement robot with a microlens attached. The microlens is
attached to each fiber entrance edge on the focal plane in order
to transform the F-ratio to a fainter/slower one, which ensures an
efficient acceptance of the input beam from the wide field
corrector (WFC) instead of overfilling the fiber, and also eases
the spectrograph design.
1. Fiber Optical Cable and Connector System (FOCCoS)
The design of the PFS fiber optical cable and
connector system (FOCCoS) is lead by LNA/USP team. A possible
route for the fiber cable in the Subaru telescope is illustrated
in Fig. 1. The cable will be segmented in 3 parts along the way in
the telescope structure; the called cable A, cable B, and cable C.
These parts will be connected by a set of multi-fiber connectors.
Cable B will be permanently attached to the Subaru telescope. The
first set of multi-fiber connectors will connect the cable B to
the cable A from the spectrograph system at the IR-side tertiary
mirror (M3) floor. The second set of multi-fiber connectors will
connect the other extremity of cable B to the cable C, which is
part of the structure of the Prime focus instrument (PFI). The
whole system of the FOCCoS is thus defined by four interfaces as:
1. Telescope (external physical interface). 2. Spectrograph
(external optical interface). 3. Cobra fiber positioner (external
optical interface). 4. Connectors (internal optical interface).
Fig. 1. illustration of the PFS
fiber cable route.
At the spectrograph room side, the cable A is
allocated with 4 branches each containing 600 fibers into each
spectrograph. Inside each spectrograph, the cable A will be an
extension of a slit device obtained with the linear disposition of
the extremities of the optical fibers and fixed by epoxy at a base
of composite substrate (upper part in Fig. 2). Cable A is a set of
parallel cables starting in the slit device, crossing a
distribution box, and finishing at the connectors system (Fig. 2).
By such slit device, the 600 fibers are linearly arrayed in a
circular arrangement for better orientation of each fiber to the
spectrograph collimator mirror. A meniscus lens whose inner radius
R1 is the same as the radius of the fiber slit and whose outer
radius R2, longer to make the lens negative, with AR coating on
both sides may be glued against the slit block to protect the
fibers extremities and avoid scratches or damages. The physical
size of the slit device is 140 mm long in which 600 fibers are
held with the center-to-center spacing of 230 microns.
Fig. 2. Cable A and Fiber Device
termination, as the entrance unit of the spectrograph.
On the other hand, at the telescope prime focus
side, the cable C will be installed onto the PFI as part of the
PFI structure (Fig. 3) and will be ended with
hexagonal-close-packed disposition of 2400 optical fibers each
controlled by a Cobra displacement robot on the focal plane. The
cable C which is inside the PFI chamber will be composed by a set
of segmented tubes articulated and secured through several plates
(Fig. 3). The whole cable C includes a set of parallel cables that
start in the fiber arm devices, crossing the Cobra plate, fiber
plates, strain relief boxes, and finishing at the connector
systems (Fig. 3).
Fig. 3. illustration of the
fiber cable allocation near the top end position of the
telescope. Concept for the segmented tubes encasing the fibers
and the plates system inside the PFI is highlighted on the right
A micro-negative lens will be attached/glued to each
optical fiber entrance to slow down the fast light beam from the
WFC (F/2.178). The articulation condition between Cobra positioner
robot, fiber arm, optical fiber, and microlens is shown in Fig. 4.
Each Cobra positioner unit contains a fiber arm that holds one
fiber whose positions are articulated to be positioned over a
small region of sky. This fiber needs to be polished and secured
to the Cobra unit in a robust but stress-free manner, which
retains the intrinsic focal ratio degradation of the fibers
themselves. The fiber extremity will be glued with a microlens,
which is probably of the meniscus type, to accept the fast beam
input from the WFC (Fig. 4). The F-ratio transformation of the
microlens is designed to have a factor of 1.28, which can slow
down the input beam entering the fiber from F/2.183 to F/2.79. The
physical size of the microlens is about 1 mm in diameter and about
0.6 mm in length.
Fig. 4. Articulation condition
between Cobra positioner robot, fiber arm, optical fiber, and