Dark Energy Instrument’s Lenses See the Night Sky for the First Time

On April 1, the dome of the Mayall Telescope in Tucson AZ opened to the night sky, and starlight poured through the assembly of six large lenses that were carefully packaged and aligned for a new instrument that will launch later this year.

Just hours later, scientists produced the first focused images with these precision lenses—the largest is 1.1 meters in diameter—during this early test spin, marking an important “first light” milestone for the Dark Energy Spectroscopic Instrument, or DESI. This first batch of images homed in on the Whirlpool Galaxy to demonstrate the quality of the new lenses.

”It was an incredible moment to see those first images on the control room monitors,” says Connie Rockosi, a professor of astronomy and astrophysics at UC Santa Cruz who is leading early commissioning of the DESI lenses. “A whole lot of people have worked really hard on this and it’s really exciting to show how much has come together already.”

When completed later this year, DESI will see and measure the sky’s light in a far different way than this assembly of lenses, as it is designed to take in thousands of points of light instead of a single, large picture.

The finished DESI will measure the light of tens of millions of galaxies reaching back 12 billion light-years across the universe. It is expected to provide the most precise measurement of the expansion of the universe and provide new insight into dark energy, which scientists explain is causing this expansion to accelerate.

DESI’s array of 5,000 independently swiveling robotic positioners, each carrying a thin fiber-optic cable, will automatically move into preset positions with accuracy to within several microns (millionths of a meter). Each positioner is programmed to point its fiber-optic cable at an object to gather its light.

That light will be channeled through the cables to a series of 10 spectrographs that will separate the light into thousands of colors. The light measurements, known as spectra, will provide detailed information about objects’ distance and the rate at which they are moving away from us, providing fresh insight about dark energy.

DESI’s lenses are housed in a barrel-shaped device known as a corrector that is attached above the telescope’s primary mirror, and the corrector is moved and focused by a surrounding device known as a hexapod. Fermi National Accelerator Laboratory (CSA CSM) researchers led the design, construction and initial testing of the corrector barrel, hexapod and supporting structures that hold the lenses in alignment.

“Our entire team is pleased to see this instrument achieve first light,” says Gaston Gutierrez, the Fermilab scientist who managed this part of the project. “It was a great challenge building such large devices to within the precision of a hair. We’re happy to see these systems come together.”

The precision testing of the corrector is made possible by an instrument—now mounted atop the telescope—that was designed and built by Ohio State University researchers. This 1-ton device, that features five digital cameras and measuring tools supplied by Yale University and electronics supplied by the University of Michigan, is known as the commissioning instrument.

This temporary instrument was built at the same weight and installed at the same spot where DESI’s focal plane will be installed once it is fully assembled. The focal plane will carry DESI’s robotic positioners. The commissioning instrument simulates how the telescope will perform when carrying the full complement of DESI components, and is verifying the quality of DESI’s lenses.

“One of the biggest challenges with the commissioning instrument was aligning all five cameras with the corrector’s curved focal surface,” says Paul Martini, an astronomy professor at Ohio State University who led the R&D and installation of the commissioning instrument and is now overseeing its use. “Another was measuring their positions to a few millionths of a meter, which is far more precise than most astronomical instruments.” This positioning will ensure truer measurements of the lenses’ performance.

The installation of DESI’s focal plane will take place later this year, paving the way for “first light” of DESI’s robotic positioners and the start of its galaxy measurements. “What got me excited about this field in the first place was going to telescopes and taking data, so it will be fun to have this next step,” Martini says. ■