Nineteen years ago today, on April 24, 1990, NASA launched its greatest Great Observatory — the Hubble Space Telescope — and ushered in a revolutionary era of astronomical discovery.

Hubble over Earth

The Hubble Space Telesocpe, as seen by a Space Shuttle repair crew.

The passage of time has been kind to Hubble and its handlers. We're largely forgotten that, almost immediately after opening its mirrored eye to space, astronomers realized that all was not right. An error in calibrating the focus point during optical fabrication left HST's 94-inch-wide (2.4-m) primary mirror with a bad case of spherical aberration.

It was ultimately due to human error, the kind of too-much-reliance-on-high-tech-wizardry mistake that any decent amateur telescope maker would have caught by using simple traditional testing methods. But it wasn't discovered until Hubble was in orbit. Fortunately, clever opticians designed a set of mirrors to fix the problem (dubbed "Corrective Optics Space Telescope Axial Replacement," or COSTAR). Astronauts installed COSTAR, a new camera, and other tune-up parts during their first in-orbit house call in December 1993. Hubble has been a cosmic discovery machine ever since.

This year I've been teaching astronomy to high-school seniors, and it occurred to me that the Hubble Space Telescope has been churning out observations throughout their young lifetimes. In a way, that's too bad. My schools' observatory boasts an impressive 25-inch Ritchey-Chrétien telescope with beautiful optics — it's essentially a ¼-scale model of that Big Eye in the Sky. But the subtly detailed views of nebulae and galaxies seen through its eyepiece can't hope to compete with two decades of fabulous, oversaturated Hubble results that permeate every astronomy textbook, website, and PBS documentary.

Still, I really can't complain. After all, we're living in a Golden Age of Astronomy. Hubble has been called the greatest telescope of all time. Methinks even Galileo would have agreed with that claim.

In a few weeks, the world's eyes will again be on Hubble, as the final team of spacewalking astronauts hover around it for one last round of repairs and upgrades. They plan to rocket into orbit aboard the Space Shuttle Atlantis on May 11th at 2:02 p.m. EDT, and you can bet that I'll put the textbook and PowerPoints aside that week to let my class watch — if not with wide-eyed wonder, then at least with nodding appreciation.


Image of Stephen


April 24, 2009 at 10:12 am

The HST mirror's spherical aberration was indeed tested with traditional methods, used by amateurs before launch. The guy who did the test argued that the mirror was clearly wrong and well within this test's abilities. The test results were dismissed as "an inferior test" to the "null corrector" results. The Hartford Current newspaper had an excellent writeup about this some years ago.

The fact that there was no integrated ground test was, IMO, reprehensible. And of course, once on orbit, it became clear who was right.

The software that was used in the early days to deconvolve and improve images took quite a bit of time on then-super computers. This code runs quite quickly on modern laptops, and is available, and can clean up ground based images taken by amateurs. That is, it isn't specific to the HST's spherical aberration. And as predicted in the early days, these early days are difficult to recall. It has turned out to be a small fraction of the life of the HST.

The HST was used for the Hubble Key project, contributed to the discovery of dark energy, detection of dark matter, and a zillion other things. My own favorite instrument are the Fine Guidance Sensors, which have been used to detect the proper motion - side to side wobble of a star under the influence of an orbiting planet. The FGS sensors participate in the mirror's spherical aberration uncorrected, but are not affected adversely very much.

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Image of Bob


April 25, 2009 at 3:40 pm

Stephen, you talk about some software used to clean up the early Hubble pictures being available. Can you give a pointer to where it might be living? Thanks

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