The DESI spectrograph system and production

Abstract

The Dark Energy Spectroscopic Instrument (DESI) is a project in construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14,000 square degrees will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs covering a 360-980 nm passband with a spectral resolution (λ/Δλ) between 1500 and 4000. The spectrograph uses two dichroic beam splitters to separate the flux among three spectral cameras, each with a volume phase holographic grating and lens system that focuses onto a charge coupled device detector. We describe the spectrograph, its system requirements, design and construction.

Integration and testing of the DESI multi-object spectrograph: performance tests and results for the first unit out of ten

“…The DESI spectrographs are fully described in [3]. The optical layout and global implementation is shown in Figure 1.…”

Comprehensive Measurements of the Volume-phase Holographic Gratings for the Dark Energy Spectroscopic Instrument

“…An integrating sphere and photodiode a Incidence angles are to be within an allowed installation range of ±1°. 0 from the specified α 0 (Edelstein et al 2018).…”

“…It is also possible to tune the rocking curves into a more symmetric shape about the blaze wavelength. This tuning feature would certainly be advantageous for the blue gratings, especially since these were measured at the non-optimal α 0 ; however, this would mean neglecting certain DESI spectrograph design constraints (Edelstein et al 2018). Despite the apparent advantages of α-tuning, it is important to note that deviating from the optimal α 0 would result in a lower overall wavelength-averaged efficiency.…”

“…A suite of 5000 robotic fiber-positioners at the focal plane will take simultaneous spectra. The fibers are bundled into arrays of 500 that feed a suite of 10 three-channel spectrographs with resolution R=λ/Δλ between 1500 and 4000, depending on wavelength (Edelstein et al 2018;Poppett et al 2016). Light from the fiber arrays is collimated by a spherical mirror, and then split into three spectral bandpasses using two dichroics.…”

“…The wavelength bands are: blue (360-593 nm), red (566-772 nm), and near-infrared (NIR) (747-980 nm). The diffracted light is finally focused onto detectors by channel-specific cameras (Edelstein et al 2018). Detailed descriptions of the DESI instrument designs and the science objectives are described in the DESI Final Design Reports and related literature (DESI Collaboration et al 2016a, 2016bEdelstein et al 2018;Martini et al 2018).…”