Hubble at 30: A Detailed Look at How It Snapshots the Stars

The Hubble Space Telescope has been expanding our view of the universe since it first launched in 1990 aboard the Space Shuttle Discovery. And while the telescope is nearing the end of its service life, it is still giving people on Earth an unprecedented look at the universe we call home.

At 43 feet long and 14 feet wide, and weighing 24,500 pounds, it’s not the biggest telescope ever built—but it’s been able to see farther than any other instrument ever created. 

Hubble’s main feature is its Optical Telescope Assembly (OTA). The OTA is configured in a modified Cassegrain telescope design using two mirrors to collect and focus light. When light enters the telescope, it strikes the primary mirror and is then bounced and focused onto the secondary mirror. 

The primary mirror is a massive concave surface measuring 7.8 feet in diameter and weighing 1,800 pounds. The light reflects off this mirror onto the secondary convex mirror, which is only 12 inches wide. That light is shot through an aperture in the primary mirror to a set of instruments for analysis. The mirrors are also curved hyperbolically, giving them a deeper curve than a standard Cassegrain parabolic setup, in a variation called a Ritchey–Chrétien design. This allows for sharper images over a wider field of view.

Both mirrors are coated with microscopically thin layers of aluminum and magnesium fluoride. The aluminum, which makes the mirror reflective, is only three millionths of an inch thick. The magnesium fluoride layer is coated on top, protecting the aluminum from oxidation and boosting Hubble’s ability to reflect ultraviolet light.

Once the light is collected, Hubble uses three kinds of instruments to analyze it: cameras, spectrographs and interferometers.

Hubble also deploys two spectrographs, which break light down into its component parts to determine the temperature, density, chemical composition and velocity of the object captured in the image. The Cosmic Origins Spectrograph (COS) and the Space Telescope Imaging Spectrograph (STIS) are complementary instruments: the COS breaks down faint traces of ultraviolet light into more easily studied components, while the STIS is a multipurpose instrument adept at analyzing bright objects.

The Fine Guidance Sensors are Hubble’s interferometers, which fulfill two functions: serving as targeting cameras to help the telescope keep a steady aim as it peers into distant galaxies, and measuring the relative positions and brightness of stars to each other.

From “Techno Turkey” to Unparalleled Success

Hubble was designed to be serviced, repaired and upgraded over its lifetime. There have been five service missions, where astronauts from the Space Shuttle would spacewalk to the satellite to work on it. In fact, only one of Hubble’s current instruments, a Fine Guidance Sensor, was on board the telescope when it was launched. The rest were installed once the observatory was in orbit.

That design feature turned out to be the one that may have saved Hubble.

Shortly after it initially came online, researchers discovered that there was a flaw in the main mirror: a “spherical aberration.” Though the mirror was smooth enough, it didn’t have the right curvature. And while the curvature was too flat by 0.000004 inch, a tiny fraction of the width of a human hair, it was enough to create out-of-focus images. The error was attributed to a minuscule fleck of paint on the calibration tool used to confirm the mirror’s shape during assembly. 

Hubble was still able to perform other scientific observations not possible from the planet’s surface, but the flaw turned the telescope into fodder for stand-up comedians for years.

The first maintenance mission in 1993 was used to replace one of the telescope’s instruments with a device named the Corrective Optical Space Telescope Axial Replacement (COSTAR) designed to correct the mirror’s flaw. COSTAR featured small mirrors on robotic arms that adjusted the paths of light beams entering Hubble’s scientific instruments. It was a resounding success, resulting in the crystal-clear images we’re used to seeing from Hubble.

“You could not have asked for a better proof point of the value of designing for maintainability,” said retired NASA astronaut Kathryn Sullivan—one of the astronauts who deployed the Hubble in 1990. “You have a multi-billion-dollar flagship mission that’s got extraordinary scientific potential, but for this one thing. If you cannot fix that one thing, you’re going to write the whole thing off. And if you can fix it, off you go.”

Subsequent missions have seen astronauts replace instruments with upgraded and more powerful ones while also performing routine repair work.

Hubble has revealed a more accurate age of the universe, helped discover dark energy, showed that the universe’s expansion is accelerating, discovered supermassive black holes and gamma-ray bursts, and captured images of galaxies in all stages of evolution.

But the telescope can only see so far away—constrained by the size of its mirror and the precision of its instruments. NASA is already working on Hubble’s replacement, the much larger and more powerful James Webb Space Telescope. 

In time, Hubble will be decommissioned and will be deorbited into the ocean—but until then, expect the telescope to continue expanding human understanding of the universe.

Read more about Hubble’s successors at The Future of NASA’s Space Telescopes.