Flawed Hubble Still Sees With Great Clarity
But it must be modified before it can record the faint objects it was designed to study
IN spite of its flawed mirror, the Hubble Space Telescope is providing incisive new views of the universe. The $1.5 billion instrument cannot image the very faint objects it was designed to study. But what it can see, it sees with unprecedented clarity.
``Where stars are fuzz balls as seen from the ground, they snap into sharp images,'' says Ray Villard of the Space Telescope Science Institute in Baltimore. He notes that Hubble images already have inspired over a dozen research papers. Peter Jacobsen, Hubble project scientist for the European Space Agency (ESA), says, ``We're seeing objects with 10 times the sharpness of the view we get from the ground.''
New photos of Saturn illustrate this. They show the growth of a giant storm. Hubble has taken hundreds of images of the storm with a high quality available from no other observing system.
The wide field/planetary camera supplied by the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. - the instrument most affected by Hubble's sensitivity loss - took the pictures.
The camera has also provided new details on the much-studied Orion nebula. Mr. Villard says astronomers now know ``there's a lot going on in there they have not seen before.''
The Faint Object Camera (FOC) that ESA contributed to the Hubble project also suffers from the sensitivity loss. Yet it, too, is returning a rich scientific harvest.
For the first time, it has clearly imaged Pluto's moon Charon. Its computer-enhanced images show crisp details in star clusters that appear as blurs to ground-based telescopes. In fact, Duccio Macchetto, ESA's representative at the Space Telescope Science Institute, has said, ``We have covered everything from one of the nearest planets to one of the most distant quasars.''
Quasars (quasi stellar objects) are highly energetic objects that outshine an entire normal galaxy. They are easy to see even though they may be billions of light years away. Thus Hubble can image them.
As Dr. Jacobsen has noted, ``[Hubble] images are not blurred. They are hazy.'' Villard likens this to viewing a street lamp through a window with a greasy film on it. You can still see the lamp and any other object that's relativity bright. But some of the object's light goes into a halo surrounding its image.
Hubble's mirror was supposed to focus at least 70 percent of a star's light into a spot only 0.1 arc-seconds across. Instead, something like 88 percent of the light smears into a halo. Because of this, Hubble cannot image objects dimmer than 25th magnitude. That's the sensitivity of a typical ground-based telescope. Astronomers had hoped to see down to magnitudes of 28 to 30.
The National Aeronautics and Space Administration is studying a Hubble repair shuttle mission in 1993. Possibilities include replacing the JPL camera with a unit with corrective optics, modifying ESA's camera, and putting corrective mirrors in the telescope's light path. Solar panels, whose flexing sometimes jiggles Hubble, may be replaced. A preliminary plan is expected in December.
Whatever is done will require great care. The final report that the Hubble investigating panel released Tuesday identified lax management as the fundamental cause of Hubble's mirror trouble. NASA, it said, ``did not have the necessary expertise to critically monitor ... the program.'' NASA has taken steps to strengthen its project management generally. Hubble program manager Dougles Broome has said that the repair mission will do nothing that risks the telescope's present effectiveness.