EAS LogoThe Refractor


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.

Front Page.

Roberts Rules. President Carter Roberts’ message.
Constellation of the Month. Aquarius

Globular Clusters By Jim Scala
Reynolds Wrap. News from Chabot Space & Science Center
Other Stuff
Bay Area Star Parties

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!”

5:27 pm, Saturday, 9 September 2000
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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