Telescope attachment allows ground-based observations of new worlds to rival those from space

Telescope attachment allows ground-based observations of new worlds to rival those from space

A team of astronomers led by researchers at Penn State
have developed beam-shaping diffusers – small pieces of etched
glass that can be mounted on telescopes – that increase the
quality of ground-based photometric observations. The research
team tested the diffusers on the ARC 3.5m Telescope at Apache
Point Observatory in New Mexico (left), the 5m Hale telescope
at Palomar Observatory (middle), and the 0.6m telescope at
Davey Lab Observatory at Penn State (right). Credit: Gudmundur
Stefansson; middle: Caltech/Palomar Observatory.

A new, low-cost attachment to telescopes allows previously
unachievable precision in ground-based observations of
exoplanets—planets beyond our solar system. With the new
attachment, ground-based telescopes can produce measurements
of light intensity that rival the highest quality photometric
observations from space. Penn State astronomers, in close
collaboration with the nanofabrication labs at RPC Photonics
in Rochester, New York, created custom “beam-shaping”
diffusers—carefully structured micro-optic devices that
spread incoming light across an image—that are capable of
minimizing distortions from the Earth’s atmosphere that can
reduce the precision of ground-based observations. A paper
describing the effectiveness of the diffusers appears online
on October 5, 2017, in the Astrophysical Journal.

“This inexpensive technology delivers high photometric
precision in observations of exoplanets as they transit—cross
in front of—the bright stars that they orbit,” said Gudmundur
Stefansson, graduate student at Penn State, NASA Earth and
Space Science Fellow, and lead author of the paper. “This
technology is especially relevant considering the impending
launch of NASA’s Transiting Exoplanet Survey Satellite (TESS)
early in 2018. It is up to ground-based facilities to rapidly
and reliably follow-up on candidate planets that are identified
by TESS.”

Diffusers are small pieces of glass that can be easily adapted
to mount onto a variety of telescopes. Because of their low
cost and adaptability, Stefansson believes that
diffuser-assisted photometry will allow astronomers to make the
most of the information from TESS, confirming new planet
candidates from the ground.

“Beam-shaping diffusers are made using a precise
nanofabrication process,” said Suvrath Mahadevan, associate
professor of astronomy and astrophysics at Penn State and an
author of the paper, “where a carefully designed surface
pattern is precisely written on a plastic polymer on a glass
surface or directly etched on the glass itself. The pattern
consists of precise micro-scale structures, engineered to mold
the varying light input from stars into a predefined broad and
stable output shape spread over many pixels on the camera.”

Telescope attachment allows ground-based observations of new worlds to rival those from space

Left: Light from a laser pointer is shaped into a wide and
stable output using a beam-shaping diffuser. A carefully designed
pattern is precisely molded into plastic polymers or directly
into a glass substrate, creating micro-structures on the surface
of the diffuser (inset). Right: The diffuser installed at the ARC
3.5m Telescope at Apache Point Observatory. Credit: RPC
Photonics; right: Gudmundur Stefansson

The research team tested the new diffuser technology “on-sky”
on the Hale telescope at Palomar Observatory in California, the
0.6m telescope at Davey Lab Observatory at Penn State, and the
ARC 3.5m Telescope at Apache Point Observatory in New Mexico.
In all cases, images produced with a diffuser were consistently
more stable than those using conventional methods—they
maintained a relatively consistent size, shape, and intensity,
which is integral in achieving highly precise measurements.
Using a focused telescope without a diffuser produced images
that fluctuate in size and intensity. A common method of
“defocusing” the telescope—deliberately taking the image out of
focus to spread out light—yielded higher photometric precision
than focused observations, but still created images that
fluctuated in size and intensity.

“Diffused observations are by far the most stable”, said Ming
Zhao, data scientist at The New York Times and former research
associate at Penn State who led the diffuser effort at the 5m
Hale telescope at Palomar.

By shaping the output of light, the diffuser allows astronomers
to overcome noise created by the Earth’s atmosphere. “The
stable and smooth images delivered by diffusers are essential
in minimizing the adverse effects of the turbulent atmosphere
on our measurements, and in maximizing our precision,” said
Zhao.

The stability of the stellar image on a telescope detector is
integral in achieving high precision photometric measurements.
This video compares diffuser-assisted observations (right) to the
two other most commonly used observing modes to observe
transiting exoplanets: (left) observations using a defocused
telescope, creating broad, but often highly unstable stellar
images, and (middle) in-focus observations, which often fluctuate
and “dance” around on the detector, degrading their precision.
The diffused observations consistently deliver a broad and stable
image of the star throughout the observations. The video images
were obtained with the wide-field Infrared camera (WIRC) on the
5m Hale Telescope at Palomar Observatory, with and without a
diffuser. Credit: Ming Zhao

“This technology works over a wide range of wavelengths, from
the optical—visible by humans—to the near infrared,” said Jason
Wright, associate professor of astronomy and astrophysics at
Penn State and an author of the paper. “As such, diffusers can
be used for a wide range of exoplanet science. We can use them
to precisely measure the times exoplanetary worlds transit
their stars, which will help us measure their masses and
compositions, and even find new planets in their systems; and
we can use them to study the temperature structures of giant
planets’ atmospheres.”

The research team is already establishing collaborations to
implement this technology on other telescopes around the world.
“Our goal is to equip the broader exoplanet community with
low-cost precision tools to deliver to aid future observations
in exoplanet science,” said Stefansson.

Explore further:

Astronomers discover two ‘warm Jupiter’ exoplanets orbiting
distant stars

More information: Gudmundur Stefansson et al. Toward
Space-like Photometric Precision from the Ground with
Beam-shaping Diffusers, The Astrophysical Journal
(2017). DOI: 10.3847/1538-4357/aa88aa

Journal reference: Astrophysical
Journal

Provided by: Pennsylvania
State University

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