HUBBLE REVISITED: IT'S ALL IN HOW YOU LOOK AT IT - The Explorer: Import

HUBBLE REVISITED: IT'S ALL IN HOW YOU LOOK AT IT

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Posted: Tuesday, April 30, 2002 11:00 pm | Updated: 7:46 am, Thu Mar 24, 2011.

When I was a kid, my older brother received a telescope for Christmas. It was a huge (or so it seemed to me then) white cardboard tube with a smaller one within that would slide in and out by twirling a cheap black plastic dial to adjust its focus. It had a large lens in the front, and in the back, a removable eyepiece that was used more often than not for observing and frying insects on long summer days when attention spans were short. It stood on three legs of stainless steel that were not so stainless after a few nights of stargazing on dew-dampened grass.

Its optics were probably no better than that of a pair of binoculars. But it didn't matter. It was our connection with NASA, the space program with its Apollo missions, and Star Trek back in the days before Star Trek was cool. Before the days of personal computers and educational astronomy software, it was our only way to touch the cosmos. Good or bad, it was a telescope, and that was all that mattered in our eyes.

If you ask anyone at random to name a telescope, it is likely that "Hubble" will be the name you will hear most often--and for good reason. No other telescope has ever been so celebrated and so ridiculed as the telescope that soared like an eagle, but soon after was depicted as being as blind as a bat.

The Hubble Space Telescope was named after Edwin Powell Hubble, a Rhodes scholar who became a lawyer only to return to graduate school to pursue a degree in astronomy.

It was Edwin Hubble who later became known as the "Father of the Cosmos" for two discoveries he made: The first, that galaxies containing millions of stars exist far outside of the Milky Way. The second, that these galaxies are receding with velocities that increased in proportion to their distances from us - in other words, that the Universe is expanding. Owing as much to his ego as to his and others sense of the importance of his discoveries, Edwin Hubble died in 1953 bitter, having never been awarded a Nobel Prize on the technicality that there was no Nobel Prize category for Astronomy.

Therefore, honoring Edwin Hubble by christening the space telescope as his namesake was apt since the main objectives of the Hubble Space Telescope were cosmological in purpose.

In 1977, when the Hubble Space Telescope was little more than a twinkle in the eyes of the scientists and engineers who would create it, four scientific objectives justified its birth: (1) To determine the constitution, physical characteristics, and dynamics of celestial bodies. (2) To determine the nature of processes which occur during the extreme physical conditions existing in and between astronomical objects. (3) To discover the history and evolution of the universe.

(4) To determine whether the laws of nature are universal in the space-time continuum. Built from 1978-1990, the Hubble Space Telescope had a length of 43.5 feet, a diameter of 14 feet, and weighed approximately 25,500 pounds at a cost of $1.5 billion.

Launched within the shuttle bay of space shuttle Discovery on April 24, 1990, Hubble was released into an orbit 375 miles above the surface of the earth traveling at a speed of 17,000 miles per hour, circling the earth every 100 minutes.

Months later, the news gradually leaked out to the public that the Hubble Space Telescope was flawed, and rather than a boon may have become more of a boondoggle.

The trouble with Hubble lay in its primary mirror. The primary mirror is the part of a reflecting telescope that directly captures light rays from a distant object and focuses it for imaging by a series of other mirrors, lenses, and electronic instrumentation.

Hubble's primary mirror was flawed with an optical defect called "spherical aberration." How a spherical aberration was a problem for Hubble can be explained through analogy.

Imagine a hollow ball the size of a house with its interior coated with a highly polished metal. Now cut the ball in half and attach one of the halves to a support making it look like a large satellite dish antenna aimed horizontally. Next, place a small raised target that is a short distance from, and that is inline to the principal axis (the center) of the inner concave surface of the dish. Stand a distance behind the target and shoot a rifle past the target at the inner concave surface of the dish. What will happen is that due to the shape of the concave surface the bullets will ricochet back toward the target. By adjusting the distance of the target from the face of the dish, you will find that there is a position at which the target will be struck by the majority of the ricocheting bullets as long as the bullets fly near and are parallel to the principal axis before striking the dish's surface. Let's call this position of the target "the focal point."

Here is where spherical aberration comes in. Because the dish was made from a ball, the concave dish half is spherical in shape. A spherical concave shape cannot ricochet a bullet back to the same area every time when the bullets striking the dish are too far away from the principal axis. Instead, some bullets will ricochet back creating different focal points. Some of these focal points will be either around, behind or in front of the target focal point.

However, a parabolic shaped concave surfacethink of a parabolic surface as a special slightly flattened spherical shapecan ricochet a bullet to the same focal point every time. No matter what distance from the principal axis the bullet strikes the inner concave surface, as long as the bullet flies parallel to the principal axis before striking the dish, the bullets will ricochet back to the same spot.

This analogy explains how a reflecting telescope works. Light from a star is at such a great distance, that relative to a concave shaped parabolic mirror within the body of a telescope, it is parallel to the principal axis. The light is gathered by the mirror and bounced back to a tight focal point where a clear magnified image can be seen through an eyepiece. If the parabolic surface is not perfect, then some of the gathered light will be reflected back in multiple focal points causing

the image to appear blurry. This blurring is what is referred to as spherical aberration because it is the same type of optical phenomenon you will see with a spherical mirror. With respect to the Hubble Space telescope, the optics is a little more complex, but the aberration is the same. Because the primary mirror used by Hubble to capture starlight is so large, its focal point lies at a distance three times longer than the length of the space shuttle that ferried the space telescope. To solve this problem, Hubble was designed with a second smaller mirror that reflected light back to the primary mirror. The reflected light from the secondary mirror passes through a hole in the center of the primary mirror to form a focal point behind the primary mirror. Think of it as a mirror reflecting a mirror in order to fold light.

In order to fold light like this, Hubble's primary mirror was not a parabolic shape but a hyperboloid shape, which is basically a flatter parabolic curve. Hubble is referred to as a Ritchey-Chreien telescope named after a pair of early 20th century opticians who proved that optical distortions from folding light with multiple mirrors are minimized if the mirrors are shaped as hyperboloids rather than as the usual paraboloids used in reflecting telescopes.

The spherical aberration Hubble suffered from occurred during the making of the hyperboloid concave surface of the primary mirror. Too much glass from the mirror had unknowingly been removed from the edges during polishing. This deviation from the true shape needed was approximately 1/25th of the diameter of a strand of hair!

In essence, this defect caused multiple focal points to appear as noisy halos around the correct focal point causing the image to blur. Afterwards, computer enhancement was successfully used to remove the noisy halos to produce a clean image, but in doing so meant that 85% of the light gathered by the mirror was lost leaving only 15% behind as useful. This in effect meant that Hubble suffered from a lack of sensitivity, not a lack of resolution as many news sources have erroneously reported. In other words, with Hubble we could not see objects that were faint because its sensitivity to light lost was lost by spherical aberration; however, what we could see was still a much sharper image than we had ever seen before through any earth-bound telescope.

In 1993, a repair mission was made to Space Telescope Hubble that corrected the spherical aberration through corrective optics. The repair mission was a success improving Hubble's optics to its original design goals. The eagle still soars, albeit wearing contact lenses.

A child's toy telescope and flawed space telescopes have a lot in common. They may not have had the best of optics, they may have been flawed, but in truth, it probably did not matter. Perhaps the real value of either scope was rather than looking "out there," we were really looking within and asking ourselves, "What do I see?"

Tim Boyer is a molecular biologist.

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