Overview of the Instrumentation for the Dark Energy Spectroscopic Instrument

TL;DR

The DESI paper outlines a groundbreaking, multi-faceted instrument designed to perform a 5-year spectroscopic survey of ~40 million galaxies and quasars to probe dark energy via BAO and Redshift-Space Distortions. It details a 3.2-degree prime-focus corrector with six large lenses, a 0.812 m curved focal surface housing 5020 robotic fiber positioners in ten petals, and ten bench-mounted spectrographs covering 360-980 nm with resolutions up to ~5000. A sophisticated Instrument Control System, data systems, and extensive Mayall upgrades enable near real-time survey planning, fiber assignment, calibration, and data processing, delivering high throughput (~30-40%), precise fiber placement (~0.1

Abstract

The Dark Energy Spectroscopic Instrument (DESI) has embarked on an ambitious five-year survey to explore the nature of dark energy with spectroscopy 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 z > 3.5, as well as measure the growth of structure and probe potential modifications to general relativity. In this paper we describe the significant instrumentation we developed for the DESI survey. The new instrumentation includes a wide-field, 3.2-deg diameter prime-focus corrector that focuses the light onto 5020 robotic fiber positioners on the 0.812 m diameter, aspheric focal surface. The positioners and their fibers are 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. The ten spectrographs each use a pair of dichroics to split the light into three channels that together record the light from 360 - 980 nm with a resolution of 2000 to 5000. We describe the science requirements, technical requirements on the instrumentation, and management of the project. DESI was installed at the 4-m Mayall telescope at Kitt Peak, 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 RMS positioner accuracy better than 0.1", SNR per \sqrtÅ > 0.5 for a z > 2 quasar with flux 0.28e-17 erg/s/cm^2/A at 380 nm in 4000s, and median SNR = 7 of the [OII] doublet at 8e-17 erg/s/cm^2 in a 1000s exposure for emission line galaxies at z = 1.4 - 1.6. We conclude with highlights from the on-sky validation and commissioning of the instrument, key successes, and lessons learned. (abridged)

Figures (32)

• Figure 1: Model of the 4 m Mayall telescope with the new instrumentation built for the DESI project. All of the main components are labeled and described in this paper. The major subsystems are the new, 8 deg$^2$ corrector (see §\ref{['sec:corr']}), the new top ring, vanes and cage that support the corrector (see §\ref{['sec:corrsupp']}), the focal plate assembly (see §\ref{['sec:fps']}), the fiber system (see §\ref{['sec:fibers']}), and the spectrograph system (see §\ref{['sec:spec']}). Also labeled are the Fiber View Camera (see §\ref{['sec:fvc']}) and the Calibration Lamp System (see §\ref{['sec:cals']}). The ten spectrographs are located in a thermally-controlled environment called the "Shack" (see §\ref{['sec:shack']}) that was custom built in the Large Coudé Room of the Mayall building.
• Figure 2: DESI corrector barrel. ( Left) Model of the corrector that emphasize the lenses ( top left) and the barrel design with the hexapod ( bottom left). ( Right) Photo of the reassembled corrector barrel on the ground floor of the Mayall telescope in August 2018.
• Figure 3: False color image of the nearby galaxy M51 obtained with the corrector on the first night of observations (1 April 2019). This $r-$band image was obtained with the Commissioning Instrument (see § \ref{['sec:ci']}) and shows M51 at the center of the corrector field of view. The image quality is approximately $0.65"$ FWHM. The size of the image is approximately $6.5' \times 4.5'$. North is up and East is to the left.
• Figure 4: Best delivered image quality obtained with the corrector during commissioning. On 3 April 2019 we obtained somewhat better than $0.6"$ FWHM images on axis. ( Left Panel) This image is the average of six bright stars observed with the Commissioning Instrument, which has a plate scale of approximately $0.13"$ per pixel. ( Right Panel) Average radial profile of the six stars. The vertical arrow marks the size of a nominal $1.5"$ diameter fiber.
• Figure 5: Properties of the corrector optical design as a function of radius. ( Top Panel) Focal ratio of the meridional and sagittal rays. The anamorphic distortion causes the focal ratio to vary from approximately $f/3.68$ at the center of the field to between $f/3.85$ (sagittal) and $f/4.23$ (meridional) at the edge. ( Middle Panel) Plate scale of the meridional and sagittal rays. While the $107\,\mu$m diameter fibers are $1.585"$ diameter circles on axis, they project to $1.39" \times 1.51"$ (meridional $\times$ sagittal) ellipses at the edge of the field. ( Bottom Panel) Chief Ray deviation at 500 nm as a function of radial position. This shows the difference between the absolute angle of the chief ray and the local surface normal, and minimizing this difference is important to achieve excellent coupling into the fibers (see §\ref{['sec:fibers']}).