Hundreds of fibers, arranged by hand, capture light at the Sloan Digital Sky Survey’s New Mexico telescope.


It was one of the stranger and more monotonous jobs in astronomy: plugging optical fibers into hundreds of holes in aluminum plates. Every day, technicians with the Sloan Digital Sky Survey (SDSS) prepped up to 10 plates that would be placed that night at the focus of the survey’s telescopes in Chile and New Mexico. The holes matched the exact positions of stars, galaxies, or other bright objects in the telescopes’ view. Light from each object fell directly on a fiber and was whisked off to a spectrograph, which split the light into its component wavelengths, revealing key details such as what the object is made of and how it is moving.

Now, after 20 years, the SDSS is going robotic. For the project’s upcoming fifth set of surveys, known as the SDSS-V, plug plates are being replaced by 500 tiny robot arms, each holding fiber tips that patrol a small area of the telescope’s focal plane. They can be reconfigured for a new sky map in 2 minutes. Other sky surveys are also adopting the speedy robots. They will not only save valuable observation time, but also allow the surveys to keep up with Europe’s Gaia satellite, the upcoming Vera C. Rubin Observatory in Chile, and other efforts that produce huge catalogs of objects needing spectroscopic study. “It’s driven by the science of enormous imaging surveys,” says astronomer Richard Ellis of University College London.

COVID-19 has delayed the SDSS’s robotic makeover. The survey’s northern telescope at Apache Point Observatory in New Mexico began to take SDSS-V data in October 2020 using plug plates. It aims to switch over to the robots by mid-2021. The southern scope at Las Campanas Observatory in Chile will follow later in the year. “It’s bananas,” says SDSS-V Director Juna Kollmeier of the Carnegie Observatories, “but we’re seeing the end of the tunnel.”

The robots mark a new chapter for the SDSS. For 10 years much of its time went to the study of dark energy, the mysterious force that is accelerating the universe’s expansion. The SDSS prised apart the light of millions of galaxies to determine their distance, via a redshift—a Doppler shift in their light due to the expansion of the universe, like the wail of a receding siren. Results from the galaxy survey, released in July 2020, traced the universe’s expansion back through 80% of its history with 1% precision, confirming the effects of dark energy, perhaps the biggest mystery in cosmology. Cracking it will require looking further back in time to fainter galaxies, which is beyond the capabilities of the survey’s 2.5-meter telescopes.

Instead, the scopes will carry out three new surveys. Milky Way Mapper will gather spectra from 6 million stars, probing their composition to find out how long they’ve been burning and forging heavy elements. “Stars are all clocks,” Kollmeier explains. With age estimates, astronomers can work out when parts of the Milky Way formed. Subtle shifts in composition can also reveal whether a group of stars originated in another galaxy or star cluster that has been subsumed into ours—an unwinding of Milky Way history called galactic archaeology.

In a second survey, Black Hole Mapper, the optical fibers will gather light from bright galaxies to learn about the supermassive black holes they harbor. Doppler shifts in the spectra of glowing gases surrounding these black holes could reveal how fast they fling this material around—and thus how heavy they are. Shifts in the spectra could trace how they gobble up and spit out streams of this gas. By tracking the gases over time, Kollmeier says, astronomers may learn how the black holes grow, seemingly in concert with their galaxies.

The third survey, Local Volume Mapper, will bunch fibers together like a multi-pixel detector to get spectra from clouds of interstellar gas within nearby galaxies. “We’re mapping a whole galaxy in exquisite detail at one time,” Kollmeier says. By determining the motions and composition of the gas clouds, the SDSS team hopes to identify why some collapse into stars and others don’t.

Meanwhile, the dark energy quest pioneered by the SDSS will move to the Dark Energy Spectroscopic Instrument, a 5000-fiber robotic spectrograph on a 4-meter telescope in Arizona. It will soon begin to track the distances to tens of millions of galaxies in the remote universe.

Robot revolution

To speed up the ability to split light from thousands of stars at once, sky surveys are turning to robot-controlled optical fibers.

LAMOST China 4.9 meters 4000 2008
DESI Arizona 4 meters 5000 2019
SDSS-V New Mexico and Chile 2.5 meters 1600 2021
WEAVE Spain 4.2 meters 1000 2021
4MOST Chile 4.1 meters 2400 2023
MOONS Chile 8.2 meters 1000 2023
Prime Focus Spectrograph Hawaii 8.2 meters 2400 2022

In the coming months, the William Herschel Telescope, a 4.2-meter telescope in the Canary Islands, will join the robot revolution by sending light to a 1000-fiber spectrograph called the WHT Enhanced Area Velocity Explorer (WEAVE). Instead of using robots to hold fibers in place, WEAVE has two of them working offline, picking and placing magnetic fiber ends onto a metal plate—automating what the SDSS’s plate pluggers did. One of WEAVE’s goals is to gather Doppler shifts from the billion stars Gaia has mapped, nailing down their full 3D motions. Then, “We can run the clock backwards and see where they came from,” says project scientist Scott Trager of the University of Groningen. It’s another way to do galactic archeology.

Next year, the European Southern Observatory’s (ESO’s) 4-metre Multi-Object Spectroscopic Telescope in Chile will be fitted with yet another robotic technology. Its 2400 fibers will be fed through controllable “spines” that stick up into the telescope’s focal plane and can be made to move, like wheat stalks in a breeze. Like WEAVE, it will follow up on sources identified by European spacecraft, including Gaia and Euclid, an upcoming dark energy mission.

It and other fiber spectrographs will also help with studies of fast-moving cosmic events such as supernovae or the violent collisions that produce gravitational waves. The Rubin Observatory will spot many of them. From 2023, it’s expected to detect 10 million fast-changing objects every night. For the thousands that demand scrutiny, “spectra are really critical for understanding what a source is,” says Eric Bellm of the University of Washington, Seattle, who is the science lead for Rubin’s alert stream.

Even some of the world’s largest scopes, in the 8-meter range, are adding robotic spectrographs. Japan’s Subaru and ESO’s Very Large Telescope are both developing systems that will vacuum up spectra from faint, distant objects. Ellis says a fiber spectrograph combined with Subaru’s 8.2-meter mirror would be able to pick out spectra of individual stars in the Andromeda galaxy, the Milky Way’s nearby twin. “With a big telescope, we can do galactic archaeology in our nearest neighbor,” he says.

*Correction, 5 February, 3:10 p.m.: An earlier version of the table in this story misstated the total number of fibers for both of the SDSS-V telescopes.