This article appeared in Fermilab Today on May 27, 2015.
Dark energy makes up about 70 percent of the universe and is causing its accelerating expansion. But what it is or how it works remains a mystery.
The Dark Energy Spectroscopic Instrument (DESI) will study the origins and effects of dark energy by creating the largest 3-D map of the universe to date. It will produce a map of the northern sky that will span 11 billion light-years and measure around 25 million galaxies and quasars, extending back to when the universe was a mere 3 billion years old.
Once construction is complete, DESI will sit atop the Mayall 4-Meter Telescope in Arizona and take data for five years.
DESI will work by collecting light using optical fibers that look through the instrument’s lenses and can be wiggled around to point precisely at galaxies. With 5,000 fibers, it can collect light from 5,000 galaxies at a time. These fibers will pass the galaxy light to a spectrograph, and researchers will use this information to precisely determine each galaxy’s three-dimensional position in the universe.
Lawrence Berkeley National Laboratory is managing the DESI experiment, and Fermilab is making four main contributions: building the instrument’s barrel, packaging and testing charge-coupled devices, or CCDs, developing an online database and building the software that will tell the fibers exactly where to point.
The barrel is a structure that will hold DESI’s six lenses. Once complete, it will be around 2.5 meters tall and a meter wide, about the size of a telephone booth. Fermilab is assembling both the barrel and the structures that will hold it on the telescope.
“It’s a big object that needs to be built very precisely,” said Gaston Gutierrez, a Fermilab scientist managing the barrel construction. “It’s very important to position the lenses very accurately, otherwise the image will be blurred.”
DESI’s spectrograph will use CCDs, sensors that work by converting light collected from distant galaxies into electrons, then to digital values for analysis. Fermilab is responsible for packaging and testing these CCDs before they can be assembled into the spectrograph.
Fermilab is also creating a database that will store information required to operate DESI’s online systems, which direct the position of the telescope, control and read the CCDs, and ensure proper functioning of the spectrograph.
Lastly, Fermilab is developing the software that will convert the known positions of interesting galaxies and quasars to coordinates for the fiber positioning system.
Fermilab completed these same tasks when it built the Dark Energy Camera (DECam), an instrument that currently sits on the Victor Blanco Telescope in Chile, imaging the universe. Many of these scientists and engineers are bringing this expertise to DESI.
“DESI is the next step. DECam is going to precisely measure the sky in 2-D, and getting to the third dimension is a natural progression,” said Fermilab’s Brenna Flaugher, project manager for DECam and one of the leading scientists on DESI.
These four contributions are set to be completed by 2018, and DESI is expected to see first light in 2019.
“This is a great opportunity for students to learn the technology and participate in a nice instrumentation project,” said Juan Estrada, a Fermilab scientist leading the DESI CCD effort.
DESI is funded largely by the Department of Energy with significant contributions from non-U.S. and private funding sources. It is currently undergoing the DOE CD-2 review and approval process.
“We’re really appreciative of the strong technical and scientific support from Fermilab,” said Berkeley Lab’s Michael Levi, DESI project director.