LSST: mining the sky in 4D

The Large Synoptic Survey Telescope (LSST) is going to be a unique astronomical instrument.
“Unlike a lot of other telescopes around the world, LSST is actually aptly named,” joked Dr. David Reiss of the University of Washington at a recent gathering of Astronomy on Tap Seattle at Peddler Brewing Company in Ballard. Reiss and Dr. John Parejko, two UW astronomers involved in the project, gave an overview of the telescope, which is under construction in Chile.

John Parejko (left) and David Reiss, research scientists at the
University of Washington, discussed the Large Synoptic Survey
Telescope at an Astronomy on Tap Seattle event October 28 at
Peddler Brewing Company in Ballard. Photo: Greg Scheiderer.

As for the name, Parejko noted the scope will be truly large. It will have an 8.4-meter mirror, a 3.2-gigapixel camera, and will take an image of the night sky every 30 seconds.

“We’re going to generate 15 terabytes of data every single night,” Parejko noted. “That means by the end of the survey we’ll have 30 trillion database entries, and over half an exabyte of data and images being catalogued.”

“That’s a lot of data even for those of you who work at Amazon,” he quipped.

Synoptic is the word even the scientists say they have to look up every time. Essentially it means that the instrument will look at everything as a whole and provide a synopsis.

“Unlike a lot of other telescopes, the LSST has been designed to serve thousands of astronomers with interests ranging from supernovae or exploding stars, to planets and asteroids, to the universe as a whole,” Reiss explained.

It’s a survey because LSST will not look at just one object.

“Not only is it covering all kids of different science, it’s actually covering the whole sky,” Parejko said. They hope to start observing in 2022, and the 10-year survey will photograph the entire sky every three nights. They expect to discover 37 billion stars and galaxies.

Lastly, it’s a telescope, but it’s much more.

“The main thing that LSST is going to produce is lots and lots of data,” Reiss said, “images and catalogs and databases of all of the objects in the sky that are going to be shared with everybody in real time.” With new information coming in constantly, they’ll be effectively creating a 10-year, multi-color, ultra high-resolution movie of the night sky.

The building

Parejko described the facility, which is being built on the Cerro Pachón ridge at 8,700 feet elevation, not far from town of La Serena in the mountain desert of Chile. It’s a good site for an observatory, with high elevation and low humidity. The building has been designed with a lab for working on the mirror and other parts of the telescope so that they don’t have to send things off the mountain for repairs.

“That means we minimize our down time; we can spend as much time as possible taking data,” Parejko said. You can watch progress of the construction on the LSST webcam.

An artists’ concept of the Large Synoptic Survey Telescope.
Image: LSST.

The telescope itself will be short, squat, and compact, with the secondary mirror and camera located out at the end. They’re building it short to reduce wobble when it moves—another measure for minimizing down time. They were able to keep it short by using a different shape on the outside of the primary mirror than on the inside. Light will come into the scope, reflect off the outside of the primary to the secondary mirror, back down to the inside of the primary, which will beam it up to the camera.

“That’s how we can keep the telescope so short and compact, by folding the light like that,” Parejko explained.

The camera, about the size of a Smart Car, will have three lenses and space for five filters. The detector will feature 21 “rafts” each with nine CCDs. If one raft breaks, they’ll just pull it out, plug in another, and keep imaging.

The building will also include a major computer lab. That’s still under design.

LSST software

Reiss explained that, with so much data being collected, computing will be important. Essentially, they’re building, “sort of a Google index of the entire night sky over the course of ten years.” To do that, they’re creating a high-speed network to connect the telescope in Chile to a supercomputing center in Illinois. There, they’ll look for things that move or blow up, and expect to spot some 10 million events every night. Information about these discoveries will go out in nightly alerts to interested users.

“We’re basically providing the equivalent of astronomical Twitter, Google, and Amazon Web Services to the community,” Reiss said.

“We’re going to be sending out nearly 600 gigabytes worth of simply just these alerts every night,” he added. “If one of you were going to subscribe to these you’re going to max out your Comcast monthly allocation in one night.”

Researchers will be able to upload their software or algorithms into the LSST computing cluster and do calculations in the cloud, rather than having to download all of that data. Many institutions will receive the alerts and write algorithms that will help users pick and choose data. There will likely be smartphone apps that will allow users to, say, track their favorite asteroid, and people will be able to use the data to learn about the universe or do citizen science. Reiss noted that, by keeping a constant eye on the sky, we will be able to spot lots of the sorts of things that we only find today through the luck of looking in the right place at the right time.

LSST goals

The main science goals of the LSST are to learn about dark matter and dark energy, catalog the solar system, watch how things change, and learn about the structure and formation of the Milky Way.

The LSST team includes 39 institutional members, among them 21 colleges and universities. The UW is a founding member. The project employs 200 astronomers and engineers from 19 different countries. The total cost of getting LSST up and running by 2022 will be about $400 million. That sounds like a lot of money, but Reiss and Parejko pointed out, given the season, that it’s about what Americans spend on Halloween costumes for their pets in a typical year. Funding for the project has come from the National Science Foundation, the U.S. Department of Energy, and through fundraising by the nonprofit LSST Corporation.

Astronomy on Tap Seattle is organized by graduate students in astronomy at the University of Washington. The events are free, but you can help them cover the costs of creating them by donating online to the Friends of Astronomy Fund at the UW.

Learning about LIGO at Astronomy on Tap

The most recent gathering of Astronomy on Tap Seattle brought to town two scientists working in one of the most groundbreaking areas of astronomy: detection of gravitational waves.

Nature was kind to us

Jeff Kissel, a control systems engineer at the LIGO Hanford Observatory, talked about how exciting it was when they switched on advanced LIGO back in September 2015.

“Boom! Right out of the gate we saw this whopper of an event,” Kissel said, detecting gravitational waves from the merger of a pair of stellar-mass black holes. “Nature was very kind to us.”
What they spotted at Hanford and at LIGO in Livingston, Louisiana was a match.

“Inside our data, which is almost always noise, we saw this very characteristic wave form that was predicted by general relativity,” Kissel recalled. They found gravitational waves from a couple of other black-hole mergers in the following months.

“This is the beginning of gravitational wave astronomy,” Kissel said.

Gravitational waves oscillate through spacetime in a way
demonstrated by this animation. Credit: ESA–C.Carreau

Kissel pointed out that LIGO only detects a small part of the gravitational wave spectrum. As with light, gravitational waves can come in a wide range of wavelengths with periods ranging from milliseconds to billions of years. Longer-length waves might come from the mergers of galactic nuclei, or even from quantum fluctuations from the early universe.

“There’s a whole zoo of things to find out there,” Kissel said. He anticipates more ground-based observatories as well as some space LIGOs that could have detector arms millions of kilometers long.

How LIGO works

LIGO sounds awfully complicated, but, broken down, the idea is pretty simple. Jenne Driggers
is a Caltech postdoctoral scholar stationed at the LIGO Hanford Observatory, where her gig is improving the sensitivity of the interferometers. Driggers explained that, essentially, they shoot a laser beam into a splitter that sends beams down two equal arms four kilometers long. The beams reflect from mirrors and return to be put back together.

A simplified look at how LIGO works. A laser beam is split and sent down two equal
arms four kilometers long, then reflected back by mirrors. When they return to be
recombined, they will usually cancel each other out and no light will get to the detector.
But if a gravitational wave distorts the system, the light will be spotted by the detector.Credit: T. Pyle, Caltech/MIT/LIGO Lab

“When they recombine they can be exactly out of phase, and then there’s no laser light (at the detector),” Driggers said. “They cancel each other out totally. Or the lengths will change and these two electromagnetic waves can add up, and so we do get some light.”

When that happens it means that a gravitational wave has distorted the LIGO arms ever so slightly. They measure the light received at the detector to learn more about the wave.

In practice it’s a lot more complicated. It all happens in a total vacuum to avoid any distortion from air. The mirrors are suspended from a system of four pendulums, which helps to eliminate vibration. The mirrors are highly reflective pieces that each weigh around 100 pounds and cost half a million dollars. The laser is about the best there is.

“The laser wavelength itself is our ruler that we’re using to measure the distance between those two mirrors,” Driggers said, “and we need to be able to measure that distance to 10^-19 meters.”

“This is one of the highest-power, frequency stable, power-stable lasers on the planet,” she added.
Driggers invited people to tour LIGO Hanford. Public tours are held twice each month, and groups of 15 or more can arrange for a private tour.

Up next: LSST

Astronomy on Tap Seattle is presented and organized by astronomy graduates students at the University of Washington. Their next event is planned for Friday, October 28 at Peddler Brewing Company in Ballard and will feature UW scientists Dr. John Parejko and Dr. David Reiss, who are working on the Large Synoptic Survey Telescope project. The events are free. Enjoy beer and astronomy!