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Up: THE HYDRA FIBER POSITIONER
Fibers selected for use are positioned onto a thin, flat, steel plate during target configuration. After all fibers are in place, a vacuum is applied on the back side of the plate to warp it against a spherical backstop. This both draws the fibers onto the true focal surface and aligns them axially with the telescope exit pupil.
Unused fibers are left positioned in either stow or park positions, located around the outer circumference of the assembly. These positions are at a depth lower than the surface of the warpable plate so that fibers in use can cross over unused neighbors.
There are 288 total fiber positions evenly spaced around the focal plate numbered from 0 through 287. The red fibers occupy every third position beginning at location 1 and include an additional two fibers at positions 276 and 279 (spares which can be used with the nominal complement). The blue fibers are also located at a spacing of every third position starting at location 2. The Field Orientation Probes are set at spacing intervals of every 24th with the first occupying position 0. The remaining 82 empty slots are available for future fiber cable implementations.