The twin FPS units. Credit: The SDSS collaboration.

Pasadena, CA—After 21 years of observers loading heavy aluminum plates night after night, the Sloan Digital Sky Survey (SDSS) is now seeing the cosmos through robotic eyes. Following more than five years of design, development, and construction, survey members worked over the final months of 2021 to install the new robotic Focal Plane System (FPS) on the Sloan Foundation 2.5m Telescope at Apache Point Observatory (APO).

“We are thrilled to have reached this technological milestone despite being in the midst of a global pandemic and are excited to witness how this shift will enhance the work of the project,” said Juna Kollmeier. She is the Founding Director of Carnegie’s Theoretical Astrophysics Center, the Director of the Director of SDSS’s fifth iteration, and current Director of the Canadian Institute for Theoretical Astrophysics (CITA). “This project has been truly collaborative, involving the contributions of scientists at more than 50 institutions from around the world.”

Previously, optical fibers were plugged into plates by hand. That system has been replaced by hundreds of high-precision robots that can put fibers anywhere in the focal plane. The system now installed at APO is the first of two units; its twin is currently under construction and will soon head to Las Campanas Observatory in Chile to survey the southern sky.

“This new system is a huge accomplishment for the fifth phase of SDSS,” said Gail Zasowski, a professor at the University of Utah and the SDSS-V project spokesperson. “It’s been an exciting road to this point, sometimes fraught with challenges, but it’s wonderful to have arrived here at last!”

The development of the new robotic FPS is a global effort built by an international team, including Carnegie, the Ohio State University’s Imaging Sciences Laboratory, the University of Washington, and the École Polytechnique Fédérale de Lausanne (EPFL). These design teams overcame numerous challenges posed by the global pandemic by developing and constructing components wherever they were—some in their own garages and backyards—and shipping them elsewhere for further assembly. The robots were built in Switzerland and integrated into the main mechanical units in Columbus, Ohio. From there, they traveled to their final home in New Mexico—and soon, they will head to Carnegie’s Las Campanas Observatory in Chile as well.

 “This is such a major milestone for SDSS-V, its success had required a major technical and coordination effort,” said Carnegie’s Solange Ramirez, the Project Manager of SDSS-V.

The FPS will enable two of the three core science programs in SDSS-V: the Milky Way Mapper and the Black Hole Mapper. Together, these projects will collect data from millions of objects spread across the sky, from stars in our own galactic backyard to unimaginably distant supermassive black holes.

The Milky Way Mapper will study our home galaxy in unprecedented detail. The ecosystem of stars, gas, dust, and dark matter in large galaxies like our own has been shaped over billions of years by numerous physical processes that dominate on different scales in space and time. It will take advantage of our unique perspective within the Milky Way to create a uniquely high-resolution map of the galaxy’s stars and how they are moving.

The Milky Way Mapper will also measure masses, ages, chemical compositions, presence of companions, and a slew of other properties for vast samples of stars of all types—including hot massive stars, stars that are just forming, and the white dwarfs that are dead remnants of stars like our Sun. It will also target tens of thousands of multi-star and planetary systems in order to understand how often multi-companion systems form and what determines how they are structured.

“Without the observing speed and flexibility that the FPS gives us, it would be simply impossible to target so many objects across the entire sky,” said Jennifer Johnson, a professor at Ohio State and head of the Milky Way Mapper team.

Looking farther afield, the Black Hole Mapper will study quasars, which are among the most luminous objects in the universe. Powered by accretion onto supermassive black holes at the centers of galaxies, quasars can be used as beacons to trace the growth of these titans over cosmic time. SDSS-V will collect data on more than 300,000 quasars to measure the masses of their black holes, understand the physics of how they gobble up matter, and trace their growth over many billions of years.

By observing these objects many times, and combining the data with earlier SDSS observations, SDSS-V will be able to see how these systems evolve on timescales ranging from days to decades. The survey will also provide follow-up observations of hundreds of thousands of sources identified at X-ray wavelengths by the SRG/eROSITA satellite, to determine their nature and their distance. This vast sample of sources will include not only new quasars, but also distant clusters of galaxies—the largest gravitationally bound structures in the universe—as well as nearby X-ray-emitting stars.

These vast samples of strikingly different types of targets—millions of Milky Way stars, hundreds of thousands of distant quasars—are among the key aspects that set SDSS-V apart from other surveys and are enabled by the new FPS system.

Added Kollmeier, “The SDSS team is an inspiration. While the world was shutting down, they were showing up. Everyone from the undergraduate students to the project leadership to our industry partners kept at it as best they could and supported each other to get here. I am so proud of this team’s perseverance, and I look forward to the mysteries we both solve and uncover as the survey gets underway in full force.”

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