Bulletin of the Eastbay Astronomical
Society
Founded in 1924 at Chabot Observatory, Oakland, California
Volume 77, Number 1, September 2000
The twin domes of the Keck telescopes
at Mauna Kea dominate the Hawaiian nightscape in this new postage
stamp. More on this Space Achievement and
Exploration Commemorative Series.
Startling Images Using the Adaptive
Optics System at Keck
Dr. Imke de Pater
Department of Astronomy,
University of California, Berkeley
Saturday, 9 September,
2000
General Meeting 7:31 p.m.
Lecture 8:20 p.m.
Chabot Observatory, 4917 Mountain Boulevard, Oakland, California
Astronomers have always dreamed of escaping Earth’s atmospheric
turbulence, a limiting factor in their endeavors. And now, there
are two ways to do this. One is to launch a telescope above the
Earth’s atmosphere. At such a launch several things can go
wrong. The rocket can go astray or blow up, causing much emotional
distress to the apprehensive and anxious astronomers (who have
probably worked on the telescope for ten years). The telescope
can malfunction. The telescope can even have incorrectly ground
optics requiring a very expensive and dangerous repair mission
(to choose a random possibility). Furthermore, space-based telescopes
remain relatively small. To compare the Hubble and the Keck Telescopes,
HST cost roughly 20 times more to build and launch, yet Keck has
20 times the light gathering area and, potentially, 4-5 times
better resolution.
An alternative, though less glamorous but more economical, is
to observe from the Earth using adaptive optic techniques (AO)
to counter the effects of turbulence in the earth’s atmosphere.
The adaptive optics system at Keck uses a deformable mirror developed
at Lawrence Livermore National Laboratory to adjust the incoming
wavefront of light to match light from a reference beacon. A beam
splitter diverts the reference light, analyzes it relative to
what it should look like and adjusts the deformable mirror. The
mirror’s shape is adjusted several hundred times a second!
The results are images with dramatically improved resolution.
Our speaker, Dr. Imke de Pater, will show us some beautiful infrared
images of various objects, especially such solar system objects
as Neptune, the moons of various planets, Io, Titan, and even
the asteroid Vesta! The images are very sharp, and are directly
comparable with images obtained at visible wavelengths with the
Hubble Space Telescope. Keck images are four times better than
HST images at infrared wavelengths.
Questions that will be addressed include: What is adaptive optics
(in a few minutes)? What can we learn from such images, and how
do they compare with Hubble Space Telescope and spacecraft data?
Under ideal circumstances, the resolution of an optical system
is limited by the diffraction of light waves. This so-called “diffraction
limit” (the smallest angle that can be resolved) is generally
described as proportional to the wavelength and inversely proportional
to the aperture. Since the Keck has the largest aperture of any
single optical instrument in the world it should be able to resolve
the smallest angle. But note that the resolution is also a function
of the wavelength! The longer the wavelength, the higher the resolution.
Since Keck is located at a site that is particularly transparent
at infrared wavelengths, it gets a double bonus from adaptive
optics in the infrared.
Dr. de Pater received her Ph.D. from the University of Leiden
in the Netherlands. She is famous for her Keck images of the Shoemaker-Levy
impact on Jupiter. She claims that as a radio astronomer, Keck
is “the smallest telescope she has ever used!”
DINNER WITH THE SPEAKER
5:27 pm, Saturday, 9 September 2000
PEARL OF SIAM
5498 College Avenue, Oakland (510) 420-8600
Please call Betty Neall at 510/533-2394 by Friday, 8 September
to confirm your place. Note the time has been advanced to allow
everyone to be able to get to the meeting promptly at 7:31 pm.
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