Overview of the Instrumentation for the Dark Energy Spectroscopic Instrument

Abstract

The Dark Energy Spectroscopic Instrument (DESI) has embarked on an ambitious five-year survey to explore the nature of dark energy with spectroscopic measurements of 40 million galaxies and quasars. DESI will determine precise redshifts and employ the Baryon Acoustic Oscillation method to measure distances from the nearby universe to beyond redshift z > 3.5, as well as employ Redshift Space DESI CollaborationDistortions to measure the growth of structure and probe potential modifications to general relativity. In this paper we describe the significant instrumentation we developed to conduct the DESI survey. The new instrumentation includes a wide-field, 3.2 • diameter prime-focus corrector that focuses the light onto 5020 robotic fiber positioners on the 0.812 m diameter, aspheric focal surface. This high density is only possible because of the very compact positioner design, which allows a minimum separation of only 10.4 mm. The positioners and their fibers are evenly divided among ten wedge-shaped 'petals.' Each petal is connected to one of ten spectrographs via a contiguous, high-efficiency, nearly 50 m fiber cable bundle. Two fibers per petal direct light into a separate system to monitor the continuum sky brightness. The ten identical spectrographs each use a pair of dichroics to split the light into three wavelength channels, and each is optimized for a distinct wavelength and spectral resolution that together record the light from 360 − 980 nm with a spectral resolution that ranges from 2000 to 5000. We describe the science requirements, their connection to the technical requirements on the instrumentation, the management of the project, and interfaces between subsystems. DESI was installed at the 4 m Mayall telescope at Kitt Peak National Observatory, and we also describe the facility upgrades to prepare for DESI and the installation and functional verification process. DESI has achieved all of its performance goals, and the DESI survey began in May 2021. Some performance highlights include root-mean-squared positioner accuracy of better than 0.1 , signal-to-noise ratio (SNR) per √ Å > 0.5 for a z > 2 quasar with flux 0.28 × 10 −17 erg s −1 cm −2 Å−1 at 380 nm in 4000 s, and median SNR = 7 of the [O II] doublet at 8 × 10 −17 erg s −1 cm −2 in a 1000 s exposure for emission line galaxies at z = 1.4 − 1.6. We conclude with additional highlights from the on-sky validation and commissioning of the instrument, key successes, and lessons learned.

The DESI Survey Validation: Results from Visual Inspection of the Quasar Survey Spectra

“…DESI (DESI collaboration et al 2016b;Abareshi et al 2022) is a multi-object spectrograph on the NOAO 4 m Mayall telescope at Kitt Peak in Arizona. It uses 5000 robotically controlled positioners to place fibers across a 7.5 deg 2 field of view (Abareshi et al 2022;Miller et al 2022;Silber et al 2022). The optical signal from the fibers is fed to optical spectrographs with sensitivity over 360-980 nm.…”

Target Selection and Validation of DESI Emission Line Galaxies

“…The DESI instrument, described in details in DESI Collaboration et al (2016b) and Abareshi et al (2022), is a multi-spectroscopic instrument mounted at the prime focus of the 4m Mayall Telescope at Kitt Peak, Arizona. The focal plane covers a field of view of about 8 deg 2 (Miller 2022) and is equipped with 5,000 fiber positioners (Silber et al 2022) distributed in ten 'petals'.…”

The DESI Survey Validation: Results from Visual Inspection of Bright Galaxies, Luminous Red Galaxies, and Emission Line Galaxies

“…and 5020 robotic fiber positioners installed on the 4meter Mayall Telescope at Kitt Peak National Observatory (Abareshi et al 2022;Silber et al 2022;Miller et al 2022). The DESI spectrograph covers the optical wavelength region from 3600 Å to 9800 Å with a spectral resolution ranging from ∼ 2000 in the shortest wavelength channel to ∼ 5000 in the longest wavelength channel.…”