SEP. 9, 2015
MANTISSA Collaboration Uses Statistical Inference to Revolutionize Sky Surveys.
The roots of tradition run deep in astronomy. From Galileo and Copernicus to Hubble and Hawking, scientists and philosophers have been pondering the mysteries of the universe for centuries, scanning the sky with methods and models that, for the most part, haven’t changed much until the last two decades.
Now a Berkeley Lab-based research collaboration of astrophysicists, statisticians and computer scientists is looking to shake things up with Celeste, a new statistical analysis model designed to enhance one of modern astronomy’s most time-tested tools: sky surveys.
A central component of an astronomer’s daily activities, surveys are used to map and catalog regions of the sky, fuel statistical studies of large numbers of objects and enable interesting or rare objects to be studied in greater detail. But the ways in which image datasets from these surveys are analyzed today remains stuck in, well, the Dark Ages.
“There are very traditional approaches to doing astronomical surveys that date back to the photographic plate,” said David Schlegel, an astrophysicist at Lawrence Berkeley National Laboratory and principal investigator on the Baryon Oscillation Spectroscopic Survey (BOSS, part of SDSS) and co-PI on the DECam Legacy Survey (DECaLS). “A lot of the terminology dates back to that as well. For example, we still talk about having a plate and comparing plates, when obviously we’ve moved way beyond that.”
Surprisingly, the first electronic survey—the Sloan Digital Sky Survey (SDSS)—only began capturing data in 1998. And while today there are multiple surveys and high-resolution instrumentation operating 24/7 worldwide and collecting hundreds of terabytes of image data annually, the ability of scientists from multiple facilities to easily access and share this data remains elusive. In addition, practices originating a hundred years ago or more continue to proliferate in astronomy—from the habit of approaching each survey image analysis as though it were the first time they’ve looked at the sky to antiquated terminology such as “magnitude system” and “sexagesimal” that can leave potential collaborators outside of astronomy scratching their heads.
It’s conventions like these in a field he loves that frustrate Schlegel.
“There’s a history of how the data are used in astronomy, and the language and terminology reflect a lot of the problems,” he said. “For example, the magnitude system—it is not some linear system of how bright objects are, it is an arbitrary label dating back thousands of years. But you can still pick up any astronomy paper and they all use the magnitude system.”
When it comes to analyzing image data from sky surveys, Schlegel is certain existing methods can be improved upon as well—especially in light of the more complex computational challenges expected to emerge from next-generation surveys like DECaLS and higher-resolution instruments like the Large Synoptic Survey Telescope (LSST).
“The way we deal with data analysis in astronomy is through ‘data reduction,’” he said. “You take an image, apply a detection algorithm to it, take some measurements and then make a catalog of the objects in that image. Then you take another image of the same part of the sky and you say, ‘Oh, let me pretend I don’t know what’s going on here, so I’ll start by identifying objects, taking measurements of those objects and then make a catalog of those objects. ‘ And this is done independently for each image. So you keep stepping further and further down into these data reduction catalogs and never going back to the original image.”
Image:
(Left) The Víctor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile, where the Dark Energy Camera is being used to collect image data for the DECam Legacy Survey. The glint off the dome is moonlight; the small and large Magellanic clouds can be seen in the background.
(Right) A DeCAM/DeCALs image of galaxies observed by the Blanco Telescope. The Legacy Survey is producing an inference model catalog of the sky from a set of optical and infrared imaging data, comprising 14,000 deg² of extragalactic sky visible from the northern hemisphere in three optical bands and four infrared bands.