HIRES

HIRES. The High Resolution Echelle Spectrometer is a powerful optical spectrograph located at the W. M. Keck Observatory on Mauna Kea in Hawaii. Commissioned in 1993, it is one of the premier instruments for high-precision radial velocity measurements and detailed spectroscopic analysis in astronomy. Its design and capabilities have made it a critical tool for discoveries in exoplanet research and stellar astrophysics.

● Overview

HIRES is permanently mounted on the Nasmyth platform of the Keck I telescope, one of the twin 10-meter telescopes of the W. M. Keck Observatory. The instrument was designed and built by a team led by Steven S. Vogt of the University of California, Santa Cruz in collaboration with the California Institute for Technology and the University of California. Its primary scientific role has been the detailed study of stellar spectra, enabling the measurement of minute Doppler shifts caused by orbiting planets. This capability positioned HIRES as a foundational instrument for the California Planet Search, a long-term survey that has discovered hundreds of exoplanet candidates. The spectrometer's high stability and resolution have also made it invaluable for quasar absorption line studies and investigations into the chemical composition of stars in the Milky Way and nearby galaxies like the Andromeda Galaxy.

● Instrumentation and design

The core of HIRES is a large, white-pupil echelle spectrograph housed in a thermally stabilized vacuum chamber. This design disperses light across multiple diffraction orders using a large echelle grating, with cross-dispersion provided by a second grating, producing a two-dimensional spectrum captured by a large-format CCD detector. A key feature is the instrument's ability to employ an iodine cell for precise radial velocity calibration; the cell is placed in the light path to impose a dense, stable set of absorption lines onto the stellar spectrum for reference. The light from the Keck I telescope is fed to HIRES via a complex optical train that includes an image rotator and a slit viewer. The spectrometer offers several slit widths and a variety of optical filters, allowing astronomers to optimize for resolution, wavelength coverage, or specific spectral features from the ultraviolet to the near-infrared regions.

● Scientific contributions

HIRES has been instrumental in numerous landmark discoveries in astronomy. It played a pivotal role in the early confirmation and characterization of exoplanets, including detailed studies of the 51 Pegasi b system and the detection of the multiple-planet system around the star Gliese 876. The instrument's precise radial velocity measurements were crucial for the discovery of the first Saturn-mass planet and for constraining the masses and orbits of planets found by the Kepler mission. Beyond planet hunting, HIRES spectra have been used to measure the primordial deuterium abundance, providing key data for Big Bang nucleosynthesis models. It has also analyzed the chemical fingerprints of some of the oldest stars in the Milky Way, such as those in the globular cluster Messier 13, and probed the intergalactic medium by studying absorption lines in the spectra of distant quasars like ULAS J1342+0928.

● Technical specifications

The spectrometer achieves a spectral resolution (R = λ/Δλ) ranging from approximately 30,000 to over 100,000, depending on the chosen slit width and configuration. Its wavelength coverage typically spans from about 3100 Ångströms in the ultraviolet to 11000 Ångströms in the near-infrared in a single exposure, though specific settings can target narrower ranges. The detector is a mosaic of three CCD chips, providing a large combined pixel array. The instrument's radial velocity precision is famously stable, capable of detecting stellar motions as small as 1 meter per second over long time baselines, a feat enabled by the iodine cell calibration technique. This precision is rivaled by only a few other instruments globally, such as HARPS on the ESO 3.6-meter telescope at La Silla Observatory.

● Operational history

Since its first light in 1993, HIRES has been in nearly continuous operation, undergoing several significant upgrades to maintain its cutting-edge performance. Major refurbishments included detector replacements, with the original TI CCDs being upgraded to more modern CCD mosaics, and improvements to its image rotator and control software. The instrument has been a workhorse for the astronomical community, with time allocated through the competitive proposal processes of the University of California, the California Institute of Technology, and NASA. Its long observational baseline has been essential for the California Planet Search, which began systematic observations in the 1990s. HIRES continues to support a wide range of astrophysical research, from studying the atmospheres of transiting exoplanets like those in the TRAPPIST-1 system to investigating the outflows from active galactic nuclei. Category:Astronomical instruments Category:Spectrographs