Innovation that is Making an Impact, Making Careers
A student working on a precision photometry project that got its start at Scialog: Time Domain Astrophysics in 2015 has won the prestigious Robert J. Trumpler award, given each year to a Ph.D. thesis in North America of unusual importance to astronomy. Gudmundur Stefánsson, whose thesis work helped develop a novel Engineered Diffuser — a nanofabricated optical element — for achieving high-precision ground-based differential photometry, is currently a Henry Norris Russell postdoctoral fellow at Princeton University.
“This high-risk effort really took off thanks to Scialog funding,” said Stefánsson’s Ph.D. advisor, Suvrath Mahadevan of Penn State. “It is making award-winning careers and a major scientific impact.”
Mahadevan was part of a team with Leslie Hebb of Hobart and William Smith Colleges and John Wisniewski of the University of Oklahoma who developed the concept, proposed and won seed funding for their project at the 2015 Scialog: Time Domain Astrophysics meeting. Today, Engineered Diffuser technology is being used by an increasing number of telescopes around the world to better study extrasolar planets from the ground.
The idea began with Hebb’s serendipitous discovery on a foggy night of observing the transit of an exoplanet at the 0.94m James Gregory Telescope in Scotland. During an exoplanet transit the planet blocks out a small portion of the light from the host star making it slightly dimmer, which can be measured with high precision photometric brightness measurements of stars. With water vapor thick in the air, the images she took that night were the best she had seen at that telescope, showing large, spread-out stars of high photometric precision. “Surprisingly, the photometry that I took on this one night had the best precision that I ever got on this small telescope,” Hebb said. “It was amazing!”
Hebb knew that other astronomers were putting optical elements full of iodine gas in the light path of telescopes to achieve high-precision measurements of planet-hosting stars. She wondered whether a similar device filled with water vapor might help her reproduce the effect of her foggy night of observing.
Over the years, Hebb would mention the idea to other astronomers, but nothing came of it until the Scialog: Time Domain Astronomy meeting. During one of the brainstorming sessions, she mentioned this idea. Mahadevan said he knew what kind of device she was talking about: an Engineered Diffuser.
“My idea about the cell full of water vapor was naive, but Suvrath explained to me about these specialized optical elements that could be designed and fabricated to shape the point spread function of a beam of light into any shape,” Hebb said.
Mahadevan was already investigating their use for time-series photometry applications in the infrared, so he and Hebb teamed up to write a Scialog proposal with Wisniewski, whose institution was a partner at the Apache Point Observatory in the Sacramento Mountains of south-central New Mexico. Their project was to develop, optimize, and install an Engineered Diffuser for use on Apache Point Observatory’s 3.5m Telescope as an inexpensive way to improve observation of exoplanet transits from the ground, instead of from space.
Mahadevan said the original Scialog funding was a “vote of confidence” in undertaking the project.
Stefánsson was a third-year Ph.D. student at Penn State when the Scialog funding began in early 2016, and he quickly started work developing and implementing the diffuser. The year before, he had worked on installing a new 24” telescope on the roof of Davey Lab Observatory at Penn State. That telescope provided a valuable and rapid-feedback test bed for gauging the performance of different diffusers in observations, directly complementing the team’s work in the lab.
On the basis of this work, Stefánsson applied for a NASA Earth and Space Science Fellowship for his work, which he received later that year.
“It was an exciting project from the start,” Stefánsson said. “And as an early graduate student, the stakes were high: if successful, this could offer independent research funding for the rest of my Ph.D.”
With guidance, support, and insights from Mahadevan, Hebb, Wisniewski, and many others, Stefansson tested a number of different diffusers of varying designs, raising and answering a myriad of questions about the optimal diffuser design to use. Their diffuser was commissioned in September 2016 at Apache Point.
With their initial observations, the team knew they were on to something. They stayed up well into the morning reducing their data and interpreting the results.
“We were in disbelief,” Stefánsson said. “We had obtained some of the highest precision photometry achieved from the ground.”
The group’s results describing the effectiveness of the diffuser were published on October 5, 2017, in the Astrophysical Journal. In their paper, Towards Space Quality Photometry from the Ground with Beam-Shaping Diffusers, the group showed that from the ground they were achieving a precision within a factor of two to that of the Kepler spacecraft, the de-facto photometric precision standard from space.
Since their original observations, their diffuser team has grown to include more than 25 members including postdocs, graduate students and undergraduates, many of whom are getting direct exposure and training observing and reducing publication-quality exoplanet transit observations with diffuser-assisted data. The team, along with a fourth team member, Paul Robertson, University of California, Irvine, was recently awarded a three-year collaborative grant from the National Science Foundation to support their research efforts in using diffuser-assisted photometry to characterize different transiting exoplanets and their host stars.
As Engineered Diffusers are relatively inexpensive devices, they are now being used at large and small telescopes around the world to better detect and characterize transiting exoplanetary systems. Since the installation on the 3.5m Telescope at Apache Point, the team has collaborated with other teams eager to install them on other telescopes, including collaborations involving diffuser tests on the current largest optical telescope in the world, the 10m Gran Telescopio Canarias in the Canary Islands. According to Stefánsson, diffuser-assisted photometry is being used at over 12 telescopes around the world, with 16 papers using or discussing diffuser-assisted photometry, with more papers in prep.
As a postdoc at Princeton, Stefánsson is now actively collaborating with the team to use the high precision capabilities of diffuser-assisted photometry to detect and characterize the properties of different transiting planets, with a particular goal to detect and characterize small rocky planets around nearby stars that could potentially harbor life.
