Zwicky Transient Facility team members install the instrument's new camera. 

Caltech Optical Observatories

New California telescope aims to catch quickly moving celestial events

Astronomers in California have taken a telescope built before most of them were born and converted it into a new instrument dedicated to one of the newest and fastest-moving branches of astronomy: spotting objects in the sky that change from one day to the next.

The new Zwicky Transient Facility (ZTF), which today opened its eye to the sky, was created by retooling the 1.2-meter Samuel Oschin Telescope at the Palomar Observatory near San Diego, California, which, starting in 1948, took pictures of the night sky onto specially curved glass photographic plates. The ZTF, named in honor of Fritz Zwicky, the Bulgaria-born astronomer who worked for most of his career at the California Institute of Technology (Caltech) in Pasadena, has been fitted with a new camera made up of 16 charge-coupled device (CCD) detectors. That will enable it to snap single images covering an area more than 200 times the size of the full moon.

With such a wide field of view—the biggest of any telescope more than 0.5 meters wide—the ZTF can survey the whole northern sky visible from Palomar every night. By doing so, astronomers can spot anything that changes from the previous night’s images, enabling them to identify quickly changing celestial phenomena, including supernovae, variable and binary stars, the active cores of distant galaxies, potentially Earth-threatening asteroids, and the flash of merging neutron stars that could also emit gravitational waves. 

Although the scientific haul is expected to be high, the ZTF is also a testbed for a larger upcoming instrument, the Large Synoptic Survey Telescope (LSST), which will begin observing from Chile in 2022. The LSST is expected to be so prolific that researchers will have to automate the process of sifting through observed events to find ones worth following up, and then getting a more detailed spectrum. To build such automated systems, ZTF researchers are involved in efforts to create the necessary data processing systems and robotic follow-up telescopes. “The headline goal is to get [an automated system] working and implemented in a way that astronomers can interact with it and use it,” says Adam Bolton of the National Science Foundation’s National Optical Astronomy Observatory (NOAO) in Tucson, Arizona.

Scaling up

In the past, the discovery of sudden events like supernovae was largely a matter of chance; a fortunate accident while astronomers were looking at something steadier. But in recent years interest in quickly changing phenomena has grown and a range of instruments has sprung up that use small telescopes to scan the sky on a nightly basis. The ZTF continues this trend but with a larger telescope to see fainter and more distant objects.

A key target will be a particular kind of supernova, known as type Ia, that is an important celestial yardstick because it is thought to always explode with the same luminosity. It was by studying type Ia supernovae that astronomers discovered in the 1990s that the expansion of the universe is accelerating, an effect ascribed to the mysterious dark energy. But what causes these explosions is not known for sure, so lingering doubts hang over their reliability as a measure. With the ZTF expecting to find thousands of type Ias per year, astronomers hope to put those doubts to rest and improve the accuracy of this yardstick.

The team behind the ZTF—which includes Caltech and partners at other U.S. universities and institutions in Israel, Sweden, Germany, and Taiwan—also expects to see each year more than a hundred superluminous supernovae, extra-bright explosions thought to be linked to gamma ray bursts, and a dozen or so tidal disruption events, flashes that occur when a star wanders too close to a supermassive black hole and is torn apart by its extreme gravity. Only a handful of such events have so far been confirmed, but astronomers hope if they can see more they will illuminate the notoriously camera-shy gravitational giants lurking at the centers of galaxies.

To achieve this, the team had to create a compact camera with a very wide field of view—47 square degrees. The camera’s 16 CCDs and electronics are enclosed in a cryostat to keep the detectors cold, and all of the gear must be small enough to fit in the center of the telescope at its focal point. The telescope’s drive motors were also changed so that the instrument can slew quickly from one patch of sky to another during observations. “It was amazing to take an old telescope and turn it into a supertelescope,” says Shrinivas Kulkarni, director of Caltech Optical Observatories, which has led the project.

New era of automation

Next decade, the LSST is expected to conduct regular surveys that are similar to the ZTF’s, but its 8.4-meter mirror will be able to see much more distant and fainter objects. The LSST is expected to detect as many as 10 million “events”—objects that change or appear—every night. “The ZTF will be an excellent forerunner for the fire hose of data expected from the LSST,” says Jonathan Grindlay of Harvard University, founding chair of the American Astronomical Society’s working group on time-domain astronomy.

That’s why automating data analyses will be critical. Researchers say stargazing will inevitably join forces with big data approaches, machine learning, and so-called event brokers, programs that can pick out the interesting events and then instruct robotic telescopes to take a closer look and obtain a spectrum so that an astronomer can peruse it later. Researchers at NOAO are working on such a broker system, as are others at the University of Washington in Seattle—who are partnered with the ZTF team—and elsewhere. “This is the best way to get ready” for the LSST, Kulkarni says. “We’ll go live, take data, and start doing things. ZTF will be the laboratory.”

The third part of the process—automated follow-up telescopes that can obtain spectra so astronomers can characterize events—is perhaps the least well prepared. The ZTF has teamed up with the Las Cumbres Observatory, which runs a worldwide network of 18 robotic telescopes, and the Liverpool Telescope, a robotic scope in the Canary Islands run by Liverpool John Moores University in the United Kingdom. “The challenge is to put all the pieces together so that scientific intelligence can be applied at the right time,” Bolton says.

If the whole system works, it may herald a new era when astronomers no longer have to spend long nights tending to observations but can simply turn up to work in the morning and have a whole menu of celestial delights waiting for them. Less romantic perhaps but, Kulkarni says: “The most efficient way to do astronomy is to get astronomers out of the dome.”

*Correction 14 November, 4:15 p.m.: An earlier version of this article misstated where Fritz Zwicky was born.