HYDRA and Hale-Bopp

The Hydra fiber positioner is mounted on one of the two Nysmith foci of the WIYN Telescope. Hydra can position up to 96 individual fiber optic pickup "buttons" somewhat radomly about the one degree field of view of WIYN. To accomplish this it uses a two axis, computer driven stage with a "gripper" which can pick up buttons and move them to predetermined locations. The buttons hold small right angle prisms attached to the ends of the fibers. This allows the fibers, which come in from the sides, to look horizontally into the telescope focal plane. The buttons are held onto the thin steel focal plane by small magnets incorporated into the buttons. After positioning the fibers, the focal plane is then warped to the rather strong field curvature of WIYN. Hydra contains two independent fiber cables, a blue optimized cable with 300 micron, 3 arc second fibers and a red optimized cable with 200 micron, 2 arc second fibers. The cables snake down through the azimuth cone of the WIYN mount and into a climate controlled "Bench Spectrograph" room at ground level.

For the Hale-Bopp campaign of 1996 and 1997 we devised a fiber asignment file which placed about half of the blue fibers in as close proximity to one another as possible. The magnetic buttons are roughly 4mm in diameter so the minimum separation is 40 arc seconds. The stainless steel tubes which protect the glass fibers are not allowed to cross or touch other buttons so packing is limited. The patteren which was used is illustrated below:

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There is a single button at the center of the field and five buttons almost touching it in a hexagonal pattern at a radius of 40 arc seconds. One button of the hex-pattern has to be left out to allow access for the field center fiber. Six more fibers almost touch the 40 arc sec ring at a radius of 69 arc seconds. Then there are 12 fibers at 2.4 arc min and 24 at 6 arc minutes. The proximity of these fibers is limited by the restriction on fiber crossing/ button touching which is built into the fiber assignment code. Finally there are rings of 24 fibers at 15 and 22.5 arc minutes. Some of these could have been closer, but it was deemed desireable the have a substantial number of fibers at large distnces to provide good sky brightness measures.

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