Hundreds of fibers, arranged by hand, capture light at the Sloan Digital Sky Survey’s New Mexico telescope.
DAN LONG / APACHE POINT OBSERVATORY
By Daniel Clery
It was one of the strangest and more monotonous jobs in astronomy: stuffing optical fibers into hundreds of holes in aluminum plates. Each day, technicians with the Sloan Digital Sky Survey (SDSS) prepared up to 10 plates that would be placed in the focus of the survey telescopes in Chile and New Mexico that evening. The holes corresponded to the exact positions of stars, galaxies or other bright objects in the view of the telescopes. The light of each object fell directly on a fiber and flew to a spectrograph, which divided the light into its wavelengths, revealing important details such as what the object was made of and how it moves.
Now, after 20 years, the SDSS robot is going. For the upcoming fifth set of recordings of the project, known as the SDSS-V, plug plates will be replaced by 500 small robotic arms, each with fiber tips that patrol a small portion of the focus plane of the telescope. It can be reconfigured for a new aerial map within two minutes. Other aerial recordings also adopt the fast robots. This will not only save valuable observation time, but also that the recordings keep pace with the Gaia satellite of Europe, the emerging Vera C. Rubin observatory in Chile, and other efforts yielding large catalogs of objects that require spectroscopic study. “It’s driven by the science of enormous imagery,” says astronomer Richard Ellis of University College London.
COVID-19 delayed the robust makeup of the SDSS. The northern telescope of the survey at Apache Point Observatory in New Mexico began taking SDSS-V data in October 2020 using plug plates. It aims to switch to robots by mid-2021. The southern range at the Las Campanas Observatory in Chile follows later in the year. “These are bananas,” says SDS-V director Juna Kollmeier of the Carnegie Observatories, “but we see the end of the tunnel.”
The robots mark a new chapter for the SDSS. For ten years much of his time was spent studying dark energy, the mysterious force that accelerates the expansion of the universe. The SDSS appreciated the light of millions of galaxies to determine their distance via a red shift – a Doppler shift in their light due to the expansion of the universe, such as the weeping of a waning siren. Results from the galaxy survey, launched in July 2020, traced the universe’s expansion through 80% of its history with 1% accuracy, confirming the effects of dark energy, perhaps the biggest mystery in cosmology. To crack it, you have to look further back in time at fainter galaxies, which fall outside the capabilities of the 2.5-meter telescopes.
Instead, the scope will perform three new surveys. Milky Way Mapper will collect spectra of 6 million stars and examine their composition to find out how long they have been burning and forging heavy elements. “Stars are all watches,” Kollmeier explains. With age estimates, astronomers can practice when parts of the Milky Way have formed. Subtle compositional shifts can also reveal whether a group of stars originated in another galaxy or galaxy infested in ours – a sequel to Milky Way history called galactic archeology.
In a second survey, Black Hole Mapper, the optical fibers collect light from bright galaxies to learn about the supermassive black holes they contain. Doppler shifts in the spectra of glowing gases around these black holes can reveal how fast they throw this material around – and therefore how heavy it is. Shift changes in the spectra can detect how they are engulfed and spit out currents of this gas. By detecting the gases over time, says Kollmeier, astronomers can learn how the black holes grow, apparently in line with their galaxies.
In the third survey, Local Volume Mapper, fibers are joined together like a multipixel detector to obtain spectra of clouds of interstellar gas in nearby galaxies. “We map an entire galaxy in fine detail at once,” says Kollmeier. By determining the motions and composition of the gas clouds, the SDSS team hopes to determine why some crash into stars and others do not.
Meanwhile, the dark energy search, which the SDSS did pioneering work, will move to the Dark Energy Spectroscopic Instrument, a 5000-fiber robot spectrograph on a 4-meter telescope in Arizona. It will soon begin tracking the distances to tens of millions of galaxies in the remote universe.
Robot revolution
To accelerate the ability to distribute light from thousands of stars simultaneously, aerial photography turns to robot-controlled optical fibers.
| INSTRUMENT | LOCATION | TELESCOPE DIAMETER | NO. OF FIBERS | FIRST LIGHT |
|---|---|---|---|---|
| LAMOST | China | 4.9 meters | 4000 | 2008 |
| DESI | Arizona | 4 meters | 5000 | 2019 |
| SDSS-V | New Mexico and Chile | 2.5 meters | 800 | 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 participate in 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 to work offline, and to pick and place magnetic fiber ends on a metal plate, which automatically does what the SDSS’s plugs did. One of WEAVE’s goals is to collect Doppler shifts from the billion stars that mapped Gaia, and nail down their full 3D movements. Then, “We can turn the clock back and see where it’s coming from,” says project scientist Scott Trager from the University of Groningen. This is another way of doing galactic archeology.
Next year, the European Southern Observatory (ESO)’s four-meter spectroscopic telescope with many objects in Chile will have another robotic technology. Its 2400 fibers are fed by controllable ‘spines’ that are stuck in the focal plane of the telescope and can be moved, like wheat stalks in a breeze. Like WEAVE, it will follow up on the sources identified by European spacecraft, including Gaia and Euclid, an upcoming mission for dark energy.
This and other fiber spectrographs will also help with studies of fast-moving cosmic events such as supernovae or the violent collisions caused by gravitational waves. Many of them will be spotted by the Rubin Observatory. As of 2023, it is expected to detect 10 million rapidly changing objects every night. For the thousands who need research, “spectra are very important to understand what a source is,” says Eric Bellm of the University of Washington, Seattle, who is the scientific leader for Rubin’s warning stream.
Even some of the world’s largest scopes, in the range of 8 meters, add robust spectrographs. The Japanese Subaru and ESO’s Very Large Telescope are both developing systems that will vacuum the spectra of vague, distant objects. Ellis says a fiber spectrograph in combination with Subaru’s 8.2-meter mirror will be able to select the spectra of individual stars in the Andromeda galaxy, the nearby twins of the Milky Way. “With a large telescope, we can do galactic archeology in our nearest neighbor,” he says.