A case of Hubble envy?
Posted: Wednesday, September 05, 2007 6:09 PM by Alan Boyle
Cambridge / Caltech / STScI / NASA / ESA |
The left view shows a "before" image of the Cat's Eye Nebula from the Hale
Telescope. The middle view shows the nebula after Lucky Imaging correction. The
right view shows the same nebula as seen by Hubble. The coloring and orientation
of the Hubble image are different due to the method used to create the picture.
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Do we really need the Hubble Space Telescope anymore? You could put that spin on the news that astronomers have developed a ground-based telescope system that can produce pictures twice as sharp as Hubble’s. But that would be wrong.
"Just line up the pictures, and you be the judge," said Ray Villard, a spokesman for the Space Telescope Science Institute. A leader of the Lucky Imaging team, the University of Cambridge's Craig Mackay, agrees that the sharper-than-Hubble claim depends on what you’re looking at - and that anyone with a case of telescope envy is missing the point.
"Lucky Imaging" is what the team members are calling their new twist on adaptive optics, a method that has been used for years to sharpen up pictures of celestial objects as seen through the blur of Earth's atmosphere. It's not usually possible to get super-high-resolution astronomical images from the ground because heat currents and atmospheric water vapor make the picture all fuzzy and fluttery. You don't even need a telescope to see the effect: Our turbulent atmosphere is the reason why stars twinkle at night.
Astronomers from Cambridge and the California Institute of Technology got around that problem by installing a high-speed camera on the 200-inch (5.1-meter) Hale Telescope at the Palomar Observatory, then snapping multiple pictures of the same targets at the rate of 20 frames per second. The Lucky team used computer software to sift through the pictures and select the sharpest elements from each one - capitalizing on their lucky shots (hence the team's name). The team explains the differences between their technique and traditional adaptive optics on this Web page.
"The system performed even better than we were expecting," Caltech researcher Nicholas Law, the principal investigator for the instrument, said in a news release. "It was fantastic to watch the first images come in and see that we were easily doing better than Hubble."
Better than the $2 billion-plus Hubble? A Cambridge news release makes that claim as well, saying that the Lucky pictures were "sharper than anything produced by the Hubble telescope, at 50 thousandths of the cost."
That sounds pretty impressive. But if you compare the sharpened Lucky image of the Cat's Eye Nebula (NGC 6543) with the latest Hubble image, as Villard suggests, Hubble wins hand down.
When I asked Mackay about that, he admitted that offering the Cat's Eye pictures for comparison was a "distinctly unfortunate" move. That picture serves as a perfect illustration - not to show whether ground-based or space-based telescopes are better, but to show why both are needed.
It takes a relatively long time for an imager to collect the pixels that make up the classic wide-field view of the Cat's Eye - and that doesn't play to the system's strengths, Mackay said. "We need to be able to take pictures rapidly," he explained. "We need to freeze the motion of the image [as it twinkles in the atmosphere]. If we use a big detector, then that ends up by reading out more slowly."
Cambridge / Caltech / STScI |
The upper photo shows stars in the
globular star cluster M13, as seen by a
ground-based telescope with the Lucky
Imaging system. The lower photo is the
same field of stars, as seen by Hubble.
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The Lucky team's technique works much better with a smaller field of view - for instance, a zoom view of stars in the globular star cluster M13. That's the kind of picture Mackay and Law had in mind for their comparison with Hubble. Mackay told me that the Lucky system produced pictures of those stars with a resolution of 50 milliarcseconds, compared with 100 milliarcseconds for the Hubble view.
"The star cluster picture is twice the resolution of Hubble," Mackay said from Cambridge.
Back in Baltimore, where Hubble science operations are based, Villard took the debate over whose telescope is better in good humor.
"It would be great if ground-based telescopes could do as well as Hubble routinely," he told me. "Then we'd have less demand for Hubble. We're oversubscribed [by a factor of] 9 to 1."
For some types of observations, such as pictures of sunspots and our solar system's outer planets, adaptive-optics systems have come a long way in the past few years. But in other fields, ranging from wide-field views of nebulas to deep-field views of the universe's frontiers, Villard said Hubble still reigns supreme.
"Sometimes the debate reminds me of audiophiles debating over whose speaker is better," Villard said. "The question is, what can you do with it?"
In the years ahead, both space-based and ground-based telescopes are due for upgrades: Hubble will be getting its final major overhaul during a shuttle mission scheduled next year. Meanwhile, astronomers are hoping to improve the adaptive-optics systems on the world's monster telescopes in Chile and Hawaii. That will give ground-based observers many of the benefits of Hubble at a bargain-basement price.
"Clearly there will always be a symbiosis because of the incredible cost of putting telescopes in space," Villard said. "The future of astronomy is going to be this symbiosis."
But Villard said even the best ground-based telescope won't replace the need for Hubble's heir, the $3.5 billion James Webb Space Telescope, which is due for launch in 2013.
"Nothing I have ever heard about can do what Webb does, and frankly that's true for Hubble. ... None of these techniques will ever replace all the advantages of being above the atmosphere," he said. "More power to 'em - but to paraphrase Mark Twain, reports of leapfrogging Hubble are greatly premature."
