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 side.


2. microlens

   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 microlens.