Modifying Hydra for the WIYN telescope: an optimum telescope, fiber MOS combination

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Abstract
The KPNO fiber-fed, multi-object spectroscopic instrument, Hydra, is being moved from the Mayall to the WIYN telescope. The WIYN telescope has good concentricity between the focal surface radius and the exit pupil. The primary modification to the instrument will allow the fibers to align with the telescope exit pupil while lying along the curved focal surface. Maximum pupil alignment losses will be reduced from 15% at the Mayall to 5% at the WIYN. The new orientation of the instrument requires a different mechanism for accessing the fiber and focal plate for maintenance. We are also upgrading the manner in which the fibers are held in place around the focal plane in order to reduce neighboring fiber interactions beyond the pivot circle. The wavelength calibration assembly is being modified to take advantage of extra room in the instrument. For guiding, instead of using offset guide probes, we are developing an algorithm which will utilize the field orientation probes. The Bench Spectrograph associated with Hydra is also being moved over to the WIYN, however, we do not discuss the spectrograph in this paper.

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