First Light For Most Powerful Sky-mapping Machine Ever Created
April Flowers for redOrbit.com – Your Universe Online
Approximately 8 billion years ago, the light from distant galaxies began streaming towards Earth. Now, at a mountaintop observatory in Chile, the newly constructed Dark Energy Camera (DECam), the most powerful sky-mapping machine ever created, has captured that ancient starlight and recorded it for the first time.
Early on September 12, 2012, the DECam, mounted atop the Victor Blanco Telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile, recorded its first images of a southern sky filled with galaxies. DECam’s focal plane consist of 62 charge-coupled devices (CCDs) invented and developed by engineers and physicists at the U.S. Department of Energy’s Berkeley National Laboratory (Berkeley Lab).
The Victor Blanco Telescope is a 4-meter telescope at CTIO, a complex of astronomical telescopes and instruments. CTIO is part of the U.S. National Optical Astronomy Observation operated under an agreement with the University of Chile.
Built by the Dark Energy Survey (DES) collaboration based at the Fermi National Accelerator Laboratory, DECam is a photometric imaging camera. The device, roughly the size of a phone booth, measures the amount of light in various colors from astronomical objects rather than details of their spectra. The first pictures of the southern sky were taken by the 570-megapixel camera on September 12.
“Early in the planning of DECam, Fermilab realized that the high-redshift galaxies they sought would require longer exposures to get secure photometric results, and that the Berkeley Lab CCD’s higher quantum efficiency in the near infrared would make the survey much faster and more efficient,” says Natalie Roe, Director of Berkeley Lab’s Physics Division.
Most astronomical CCDs at the time were fragile affairs because, to be sensitive to faint light, they had to be thinned to about 20 micrometers (millionths of a meter), a fraction of the roughly 100-micrometer width of a human hair.
The DECam CCDs measure a robust 250 micrometers thick – yet they maintain high resolution across the spectrum, including in blue light. When photons hit the back surface of the chip, the holes they create (the positively charged equivalents of electrons) are pulled through to the circuitry on the front by an electric field generated by a bias voltage, which permeates the entire thickness of the CCD.
Berkeley Labs CCDs are far superior, in red light, with more material to capture long-wavelength red photons and enough thickness to suppress surface reflections that cause interference fringes, to the typical astronomical CCD. Fabrication methods for ultrapure silicon originally developed for high-energy physics insure that the Berkeley Lab CCD’s “dark current” – charges originating inside the chip, a source of false signals – is also low.
“Fermilab was attracted to our CCDs because of their improved red response,” says Holland, “but considering that there were as yet no big cameras using them when DECam was planned; they had to decide to take a risk.”
Teledyne DALSA Semiconductor, along with the Physics Division’s Microsystems Laboratory, built the DECam chips. Partially finished wafers holding four CCDs, each with eight of eleven masking steps completed, were commercially thinned, then sent to the Microsystems Laboratory for completion. They were “cold-probe” tested at minus 45 degrees Celsius to check for shorts, defects and excessive dark current.
“In the first months, manufacturing went slowly,” Roe said, “but we used data from each lot of wafers to feed back processing improvements, and the yield steadily improved. We used conservative estimates and overshot the requirements – at the end, we produced twice as many science-grade CCDs as needed.”
The collaboration between Fermilab and Berkeley Lab started early in the large-scale development of the Berkeley Lab CCD so the experience benefited both parties quite a bit. DECam will produce the largest-ever 3D map of the universe. This will replace the record currently held by the third Sloan Digital Sky Survey and its largest component, the Baryon Oscillation Spectroscopic Survey (BOSS), led by Berkeley Lab astrophysicists. The red channel of the SDSS-III spectrograph, whose development was led by Roe, also uses Berkeley Lab CCDs.
The Sloan/BOSS map of the Universe was released in early August. The map covers one-third of the night sky, and is a fully functional 3D map.
Says Holland, “We could not have made the BOSS CCDs, which are twice the size of the DECam CCDs, without learning what we did in ramping up for DECam.”
Roe adds, “Delivery of CCDs has often ended up being the bottleneck for astronomical instrumentation, but in this case we delivered on time and things came together as planned. It was an example of great teamwork between Berkeley Lab and Fermilab.”
The DECam is the most powerful survey instrument of its kind. It is able to see light from over 100,000 galaxies up to 8 billion light years away in each snapshot. In conjunction with the Blanco telescope’s large light-gathering mirror (which spans 13 feet across), DECam will allow scientists from around the world to pursue investigations ranging from studies of asteroids in our own Solar System to the understanding of the origins and the fate of the universe.
“The achievement of first light through the Dark Energy Camera begins a significant new era in our exploration of the cosmic frontier,” said James Siegrist, associate director of science for high energy physics with the U.S. Department of Energy. “The results of this survey will bring us closer to understanding the mystery of dark energy, and what it means for the universe.”
“We’re very excited to bring the Dark Energy Camera online and make it available for the astronomical community through NOAO’s open access telescope allocation,” said Chris Smith, director of the CTIO. “With it, we provide astronomers from all over the world a powerful new tool to explore the outstanding questions of our time, perhaps the most pressing of which is the nature of dark energy.”
DES scientists will use the new camera to carry out the largest survey ever undertaken, and then use the data from that survey to carry out four probes of dark energy, studying galaxy clusters, supernovae, the large-scale clumping of galaxies and weak gravitational lensing. This is the first time all four of these methods will have been possible in a single experiment.
The DES is expected to begin in December, after the camera is fully tested, and will take advantage of the excellent atmospheric conditions in the Chilean Andes to deliver pictures with the sharpest resolution seen in such a wide-field astronomy survey. In just its first few nights of testing, the camera has already delivered images with excellent and nearly uniform spatial resolution.
Over five years, the survey will create detailed color images of one-eighth of the sky, or 5,000 square degrees, to discover and measure 300 million galaxies, 100,000 galaxy clusters and 4,000 supernovae.
The Dark Energy Survey is supported by funding from the U.S. Department of Energy; the National Science Foundation; funding agencies in the United Kingdom, Spain, Brazil, Germany, and Switzerland; and the participating DES institutions.