Hydra (spectrograph)

Hydra (spectrograph). Hydra is a versatile, multi-object fiber-fed spectrograph permanently mounted at the WIYN Observatory on Kitt Peak. Commissioned in 1994, it was designed to efficiently obtain simultaneous spectra of up to 100 celestial targets within a wide field of view, dramatically increasing the survey power of the 3.5-meter WIYN Telescope. Its unique robotic fiber positioning system and dual spectral channel configuration made it a workhorse instrument for galactic archaeology, stellar population studies, and galaxy cluster dynamics for over two decades.

Overview

Hydra was developed as a cornerstone instrument for the then-newly built WIYN Observatory, a partnership between the University of Wisconsin–Madison, Indiana University, Yale University, and the National Optical Astronomy Observatory (NOAO). Its primary design goal was to exploit the excellent image quality of the WIYN telescope to conduct spectroscopic surveys of dense stellar fields and extended galactic structures. The instrument's name reflects its ability to observe many targets simultaneously, akin to the multi-headed creature of Greek mythology. For much of its operational life, Hydra provided a critical, wide-field spectroscopic capability that complemented other instruments on larger telescopes like those at the W. M. Keck Observatory and the Cerro Tololo Inter-American Observatory.

Design and capabilities

The core innovation of Hydra is its robotic fiber positioner, located at the telescope's focal plane, which uses small robotic grippers to place up to 100 optical fibers into pre-drilled holes in a magnetic aluminum plate. This plate is configured to match the positions of target objects in the sky, a process informed by imaging from telescopes like the Sloan Digital Sky Survey. The fibers then feed light to a bench-mounted spectrograph in a thermally stable room below the telescope floor. Hydra originally operated with a single blue-sensitive CCD but was later upgraded to a dual-channel system, incorporating a red-sensitive CCD to cover a broader wavelength range from the atmospheric ultraviolet cutoff to the near-infrared. This allowed for detailed studies of stellar metallicities, radial velocities, and chemical abundances.

Scientific contributions

Hydra has been instrumental in numerous landmark astronomical surveys and studies. It played a key role in the Galactic Globular Cluster survey, precisely measuring motions and compositions of stars within clusters like M15 and M92 to understand their formation. The instrument was heavily used for probing the dynamics and dark matter content of galaxy clusters such as the Coma Cluster and Virgo Cluster. In the field of galactic structure, Hydra contributed to mapping the Milky Way's halo and thick disk through surveys like the Spaghetti Survey, identifying stellar streams and accreted dwarf galaxies. Its data also supported investigations into the planetary nebula luminosity function and the stellar populations of nearby dwarf galaxies like Leo I.

Technical specifications

Hydra's field of view is approximately one degree in diameter, matching the wide-field corrector of the WIYN telescope. The positioner can configure fibers with a precision of better than 25 microns. The spectrograph itself offers multiple gratings, providing spectral resolutions (R = λ/Δλ) ranging from about 500 to 20,000. The dual-channel configuration typically used a 600 line/mm grating in the blue channel and a 600 or 1200 line/mm grating in the red channel. The fibers have a core diameter of 300 microns, corresponding to about 2 arcseconds on the sky, which is well-matched to the typical seeing conditions at Kitt Peak. The instrument's throughput and stability were enhanced by its location in a controlled environment away from the telescope dome.

Operational history

First light for Hydra was achieved in 1994. It quickly became one of the most requested instruments on the WIYN telescope. A major upgrade in 2002-2003 replaced the original single CCD camera with the dual-channel Bench Spectrograph, significantly expanding its scientific reach. Throughout the 2000s and 2010s, it remained a premier survey instrument, competing with and complementing newer multi-object spectrographs like AAOmega on the Anglo-Australian Telescope and later the Dark Energy Spectroscopic Instrument. Hydra was eventually retired from regular service in 2019, succeeded by the NEID spectrograph and other specialized instruments, concluding a 25-year legacy of foundational spectroscopic research.

Category:Astronomical instruments Category:Spectrographs Category:WIYN Observatory