The Robotic Multi-Object Focal Plane System of the Dark Energy Spectroscopic Instrument (DESI)

TL;DR

The DESI Focal Plane System showcases a scalable, petal-based assembly of 5,020 robotic fiber positioners that reconfigure rapidly (<2 minutes) with micron-level precision, enabling DESI to map tens of millions of extragalactic targets. The system integrates a dense hardware stack (fibers, fiducials, GFAs, and metrology) with sophisticated software (PlateMaker, move scheduling, FVC feedback) to close the fiber-positioning loop in real time. On-sky performance demonstrates ~6 μm RMS positioner accuracy after turbulence correction and robust GFA guiding, yielding high target throughput and precise sky calibration. The work provides actionable lessons on large-scale, modular fiber-fed spectrographs, emphasizing calibration, fault management, and large-scale pre-production testing for future instruments.

Abstract

A system of 5,020 robotic fiber positioners was installed in 2019 on the Mayall Telescope, at Kitt Peak National Observatory. The robots automatically re-target their optical fibers every 10 - 20 minutes, each to a precision of several microns, with a reconfiguration time less than 2 minutes. Over the next five years, they will enable the newly-constructed Dark Energy Spectroscopic Instrument (DESI) to measure the spectra of 35 million galaxies and quasars. DESI will produce the largest 3D map of the universe to date and measure the expansion history of the cosmos. In addition to the 5,020 robotic positioners and optical fibers, DESI's Focal Plane System includes 6 guide cameras, 4 wavefront cameras, 123 fiducial point sources, and a metrology camera mounted at the primary mirror. The system also includes associated structural, thermal, and electrical systems. In all, it contains over 675,000 individual parts. We discuss the design, construction, quality control, and integration of all these components. We include a summary of the key requirements, the review and acceptance process, on-sky validations of requirements, and lessons learned for future multi-object, fiber-fed spectrographs.

Figures (41)

• Figure 1: Each of the 10 petals on DESI is a complete instrument with 502 robots, 12 -- 14 fiducial point sources, a guide or wavefront camera, a protective enclosure for fiber splices, power supplies, and control electronics. At left, a fully-assembled petal is viewed from the front, looking toward the robotically-controlled fiber tips. Fiducial sources are mounted in cylindrical housings, interspersed throughout the robot array. A guide camera (covered by a protective plate in this image) is mounted at the upper right corner, and the whole assembly here is surrounded by a protective metal enclosure (which is removed just prior to installation in the telescope). At right, a side view shows the layout of fibers, electronics, and support structure.
• Figure 2: This diagram shows the location of the focal plane system components as installed on the Mayall telescope. The focal plate assembly, which contains all robotic fiber positioners, guide and focus cameras, and fiducial sources, is mounted to the prime focus corrector. The Fiber View Camera (FVC) is mounted behind the primary mirror and measures the positions of the backlit fibers, as well as the fixed fiducials interspersed throughout the array.
• Figure 3: The DESI fiber positioner robot is designed for a minimum 10.4 mm pitch between neighboring units. It has 2 rotational axes, driven by independent ø 4 mm 337:1 gear motors. Drive electronics are integrated in a board mounted to the aft end. The assembly consists of 22 parts and 10 fasteners. The design was developed at LBNL and SSL, and mass-produced by UM and EPFL.
• Figure 4: Targeting accuracy for DESI fiber robots over a three-month period of survey operations in autumn 2021, during which the Focal Plane System acquired more than 7.8 million targets. The shallower red histogram shows 'blind' move accuracy, i.e. the first open-loop move made toward each target. The sharper blue histogram shows positioning error after a single correction move, based on feedback from the Fiber View Camera (FVC, § \ref{['sec:fvc']}). Blind move RMS error is 54 $\mu$m, with 0.35% of attempted targets having error $>$ 200 $\mu$m. After the correction move, the RMS error is 9 $\mu$m, with 0.03% outliers $>$ 200 $\mu$m. Errors here are with respect to the commanded target positions, and include significant contributions of measurement noise (§ \ref{['sec:fvc_performance']}) due to centroiding precision limits ($\sim$ 3 $\mu$m) and air turbulence in the dome ($\sim$ 3 -- 8 $\mu$m). Some turbulence subtraction code introduced in late 2021 further reduced the error to 6 $\mu$m RMS. (see fig. \ref{['fig:positioner_accuracy_with_turbcorr']})
• Figure 5: Fiber positioner robots are shown packed together on the focal plate. In addition to the fiber ferrule, each $\phi$ arm has a light-trap hole with a dark dye. The original concept for these light-traps was to be able to selectively extinguish bright stars in the field; in practice they have been most useful for identifying $\phi$ angles during hardware debugging. A projecting tab feature on each ferrule holder provides a simple but effective hard limit against clockwise over-extension.