- Press Release
- Mar 27, 2023
Jupiter’s Moon Io Caught Erupting in Keck Telescope Image
MAUNA KEA, Hawaii — The successful installation and operation of a 21-st century
adaptive-optics system at the W.M. Keck Observatory has ushered in a new era of ultra
high-resolution astronomical imaging capability.
Recent observations with the Keck adaptive-optics (AO) system have produced the highest
spatial-resolution images ever obtained from ground or space by an optical-infrared telescope.
Funded by the W.M. Keck Foundation, the $7.4 million AO system was installed on the 10-meter
Keck II telescope in February 1999. A clone of the instrument will be installed on the twin Keck I
telescope this year.
While adaptive optics previously have been demonstrated on 4-meter class telescopes, the Keck
AO system is the first to be installed on the new generation of 8-10 meter telescopes — thus
taking advantage of the larger apertures’ diffraction limit.
The Keck AO system senses and adjusts for distortions in starlight introduced by the Earth’s
atmosphere. The system splits off the optical portion of the light to sense the distortion while
transmitting the corrected infrared light to the science instrument. Corrections are sent to a
flexible mirror at up to 670 times per second.
The resultant images have an improved resolution more than tenfold — from about half an
arc-second to tens of milli-arc seconds.
“The Keck AO system has opened up new astronomical frontiers by realizing the full angular
resolution capability of a 10-meter telescope combined with its light-gathering power,” said
Keck optics manager Dr. Peter Wizinowich. “We can now observe objects at a level of detail 10
to 20 times higher than without an AO system.”
Since AO first light, astronomers at Keck have used the system to image Neptune, Saturn’s
moon Titan, the asteroid Vesta, Jupiter’s volcanic moon Io; explore binary-star systems
previously beyond resolution, the heart of our Milky Way galaxy, and distant galaxies. The
recent images of Io caught an apparent limb eruption and the surface albedo variations are
clearly visible. A series of images of Vesta have been made into a movie that clearly shows the
asteroid’s rotation. (See images at:
The first phase of the Keck AO system currently uses natural guide stars to sense atmospheric
distortion. An artificial guide-star system using a sodium-wavelength laser designed by a team
from Lawrence Livermore National Lab will open the full sky to AO observations when it comes
online in the next year. Currently less than 10 percent of the sky has a guide-star bright enough
for AO correction.
The Keck AO system now uses an engineering-grade camera (K-Cam) with a 256×256
indium-antimonide detector for science imaging. The addition of the Near Infrared Spectrometer
(NIRSPEC) and the next-generation Near Infrared Camera (NIRC 2) this year with their 1024 x
1024 InSb arrays and improved sensitivities will add powerful new tools to the Keck II
The installation of the Keck I AO system later this year will be an important step toward
combining the light of the two Keck telescopes to achieve even higher resolution.
“We are currently working with JPL (Jet Propulsion Laboratory) to build an interferometer that
will have 10 times the angular resolution of an individual Keck telescope,” said Wizinowich.
The Keck Adaptive Optics Team members included Scott Acton, John Gathright, Olivier Lai,
William Lupton, Chris Shelton, Kevin Tsubota and Peter Wizinowich at Keck Observatory; and
Jong An, Ken Avicola, Herb Friedman, Don Gavel, Erik Johansson, Bruce Macintosh, Scot Olivier
and Claire Max at Lawrence Livermore National Laboratory.
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CAPTION: At top, the Galileo satellite image of Io from the proximity of a Jovian orbit shows
clearly the dark spots on the moon, which are clearly identifiable in the Keck II
telescope/Adaptive Optics image of Io at top right. Note the bright flare on the upper left limb
of the moon, an apparent eruption from the volcanically active moon. The three images at the
bottom are the various wavelengths in the infrared that were combined and computer
processed to make the final image. Wavelengths: J is 1.25 microns, K’ is 2.12 microns, H is 1.6
microns. (Visible wavelengths of light are around a half micron). The resolution of Io is on the
order of 0.040 arc-seconds, the most detailed image ever made of Io from the Earth. Credit:
W.M. Keck Observatory/Adaptive Optics team
The images were captured in late November, 1999.