The Hydra Positioner

The Hydra fibers are positioned within a 40' field approximately 380mm in diameter. Each fiber moves radially into the field from the periphery. A tiny prism is cemented to the tip of each fiber. The tip/prism assmebly is in turn cemented to a small magnetic button. Each button can be picked up and set down anywhere within a pie-shaped area emanating from its "Home" position just outside of the edge of the field.

Fibers are positioned with a "Gripper" which moves above the field on a high precision, computer controlled X-Y stage. The gripper is capable of a small amount of vertical (Z) motion. To move a fiber, the gripper jaws are opened. It is positioned over the fiber/button assembly, moves down, picks up the fiber and moves it to the target position with an rms precision of less than 10µ (.06"). The cycle is completely automatic and takes approximately 4 seconds per fiber.

There are 288 fibers, alternating between two sets each of which has 138 fibers and 6 spares. The "large" fibers are 300µ (2") in diameter. The "small" fibers are 200µ (1.3") in diameter. The small fibers were brittle and are not usable. Thus the instrument can position 138 2" fibers. A few are broken or have low throughput so that roughly 130 independent targets can be simultaneously observed.

The fibers are positioned on a flat plate. After they have been positioned, the plate is warped by applying a partial vacuum behind it, pulling it against some hard stops which hold it into a radius of 8.6M which conforms to the focal plane of the telescope. The telescope is "telecentric" meaning that the pupil is at the center of curvature of the field so that the light enters the fibers parallel to their optical axes.

The gripper stage carries a television camera so that you can see the gripper in action. Very occasionally the gripper drops a fiber. Should this happen to you, don't despair! It is usually not difficult to use the TV camera to find and recover a lost fiber. The procedure for doing so is described in the User Manual.