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01/02/2020 - Behold the Whole Sky

By Adam Hadhazy

a photograph of the under construction telescopeWhen construction is complete, the Large Synoptic Survey Telescope will be "the widest, fastest, deepest eye of the new digital age." Image credit: LSST.

There's about to be a new telescope in town—in the figurative sense, that is, unless you happen to literally live more than a mile-and-a-half up on the summit of a mountain named Cerro Pachón in the foothills of the Chilean Andes. 

There, construction is humming along for the Large Synoptic Survey Telescope, or LSST. Slated to start science operations early next decade, LSST in all likelihood will be a gamechanger for astronomy and astrophysics.  

What makes LSST so special is how big and fast it will be compared to other telescopes. "Big" in this case refers to the telescope's field of view, which captures a chunk of sky 40 times the size of the full Moon. "Big" also refers to LSST's mirror size, a very respectable 8.4 meters in diameter, which means it can collect ample amounts of cosmic light. Thirdly, "big" applies to LSST's 3.2 billion-pixel camera, the biggest digital camera ever built. Put all those bits together, and LSST will be able to record images of significantly fainter and farther-away objects than other ground-based optical telescopes. 

And finally, as for "fast," LSST will soak up more than 800 panoramas each night, cumulatively scanning the entire sky twice per week. That means the telescope will catch sight of fleeting astrophysical events, known as transients, that are often missed because telescopes—even today's state-of-the-art, automated networks of 'scopes—are not gobbling up so much of the sky so quickly. Transients that last days, weeks, and months—for instance, cataclysmic stellar explosions called supernovae—are routinely spotted. But the shortest events, lasting mere hours or even minutes, are another, untold story. 

"Unfortunately, we still know relatively little about the transient optical sky because we have never before had a survey that can make observations of a very large fraction of the sky repeatedly every few nights," says Steven Kahn, Director of the LSST project. "LSST will meet this need."

Kahn, the Cassius Lamb Kirk Professor in the Natural Sciences and Professor of Particle Physics and Astrophysics at Stanford University, is also a member of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC). He stepped into the director role back in 2013 when LSST was on the drawing board. Now the huge instrument is nearing the completion of its construction. Kahn and his colleagues are dearly looking forward to all that LSST will bring to the table, building on the pioneering work into gauging the transient sky underway with other, precursor projects worldwide.

"LSST will go significantly deeper and cover the sky more rapidly," says Kahn. "By covering more sky per unit time, we are more sensitive to very rare events, which are often the most interesting."

In this way, LSST is going to open up a major discovery space, for phenomena both (poorly) known and (entirely) unknown. 

"The Universe is far from static," says Kahn. "There are stellar explosions of many different kinds that allow stars to brighten dramatically and then fade away on different timescales." Some of these transient flashes of light are expected from the vicinities of neutron stars and black holes as they interact with matter that strays too close. Researchers hope to gain new insights into these dense objects' properties, whose extreme physics challenge our best-supported theories. 

Another primary goal for LSST is to advance our understanding of the "dark universe" of dark matter and dark energy. Together, these entities compose 95 percent of the cosmos, with the "normal" matter that makes up stars, planets, and people registering as the remaining rounding error. Yet scientists have only stabs in the dark, as it were, on what exactly dark matter and dark energy really are. LSST will help by acquiring images of billions of galaxies, stretching back to some of the earliest epochs in the universe. Analyzing the shapes and distributions of these galaxies in space as well as time (recall that the farther away you see something in the universe, the farther you're seeing back in time) will better show dark matter's role in building up cosmic structure. The signature of dark energy, a force that is seemingly accelerating the universe's expansion, will also be writ across the observed eons of galactic loci. 

Closer to home, LSST will vastly expand our knowledge of our own Solar System. It will take a census of small bodies, such as asteroids and comets, that fly by overhead, too faint for us humans to notice but there all the same—and in rare instances, potentially dangerously so; just ask the dinosaurs. 

"LSST will measure everything that moves in the sky," says Kahn. "Of particular interest, we will provide the most complete catalogue of potentially hazardous asteroids, those objects whose orbits might allow them to impact the Earth."

Not done yet, LSST will also extend our catalogue of stars in the galaxy, aiding in charting the history and evolution of our own Milky Way galaxy. Furthermore, LSST will be a premier instrument for discovering the sources of gravitational waves, the ripples in spacetime first predicted by Albert Einstein in 1915 and finally directly detected in 2015 by the LIGO experiment. It can be a tough business today, even with the rich array of telescopes in operation, to rapidly pinpoint the visible light that gravitational wave-spawning neutron star collisions give off. LSST should aid in that regard admirably. 

The wait is nearly over. The LSST building is nearly complete, the large mirrors are on site, and the camera is being integrated at the at SLAC National Accelerator Laboratory in California, which co-hosts KIPAC along with Stanford. 

"Basically, everything that needed to be fabricated for the LSST telescope and camera has been fabricated," says Kahn. "The remaining work largely involves putting the system together and getting it working."

Kahn has been to the telescope site recently, in both September and October. He likes what he sees. 

"Visiting the site in Chile is a remarkable experience," Kahn says. "It is a beautiful site, and the LSST facility sits prominently atop the edge of a cliff on Cerro Pachón. The sheer size of the building and its complexity is striking." 

Before long, the impressiveness of the building will recede into the background as the profundity of the science LSST generates takes center stage.

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