Hubble's Amazing Optics

The Optical Telescope Assembly

COSTAR: Glasses for Hubble

Hubble's Amazing Optics
 Hubble's Amazing Optics

Primary mirror before installation

hat gives Hubble such remarkable eyesight? What makes its pictures of distant objects so sharp? Its position above Earth's atmosphere — although clearly advantageous — is only part of the answer. Without powerful eyesight, Hubble would not be able to take full advantage of its unique location.

Hubble's "eyes" are actually a system called the Optical Telescope Assembly. That system consists of two mirrors, support trusses, and the apertures (openings) of the instruments. Hubble's optical system is a straightforward design known as Ritchey-Chretien Cassegrain, in which two special mirrors form focused images over the largest possible field of view.


Resolution 101


What light does Hubble see?

Light Path

Incoming light travels down a tube fitted with baffles that keep out stray light. The light is collected by the concave (curved inward, like a bowl) primary mirror and reflected toward the smaller, convex (curved outward, like a dome) secondary mirror. The secondary mirror bounces the light back toward the primary mirror and through a hole in its center. The light is then focused on a small area called the focal plane, where it is picked up by the various science instruments.


Cleanroom inspection of primary mirror

Mirror Quality

Hubble's mirrors are very smooth and have precisely shaped reflecting surfaces. They were ground (shaped by removing glass with abrasives) so that their surfaces do not deviate from a perfect curve by more than 1/800,000ths of an inch. If Hubble's primary mirror were scaled up to the diameter of the Earth, the biggest bump would be only six inches tall.

Shortly after Hubble's deployment in 1990, scientists found that the curve to which the primary mirror was ground was incorrect, causing "spherical aberration." Fortunately, corrective optics were able to solve this problem.

Hubble's mirrors are made of ultra-low expansion glass and kept at a nearly constant room temperature (about 70 degrees Fahrenheit) to avoid warping. The reflecting surfaces are coated with a 3/1,000,000th-inch layer of pure aluminum and protected by a 1/1,000,000th-inch layer of magnesium fluoride. The magnesium fluoride makes the mirrors more reflective of ultraviolet light.