Update: February 14, 2007

Mirror Mirror

By Pam Eastlick for THE DEEP on line

Welcome to The Deep science and technology column where we cover topics from the deep sea to deep space and beyond.

Although many of us think of telescopes as gigantic glasses peering into the depths of space complete with big lenses; that’s not the way it usually happens.  Telescopes come in two flavors, refractors and reflectors. 

The first telescope that was turned to the heavens was built and operated by Galileo Galilei in the early 1600’s.  Galileo's telescope was a refractor.  It used lenses to gather and focus the light.  Unfortunately, lenses not only bend light, they break it into its component colors.  Objects viewed through uncorrected refracting telescopes have bright rainbow rings of light around them, making the object difficult to see.
Sir Isaac Newton discovered that you could use a curved mirror as a light gathering device.  The curved mirror focuses the light to a single point where it is reflected by a flat mirror into an eyepiece.  All the world's great telescopes are Newtonian reflectors.  They are giant light buckets that capture the incoming light of distant stars.

Over the last hundred years, telescopes grew larger as astronomers tried to capture the light of fainter and fainter objects.  The largest and most famous single mirror telescope is the Hale Telescope on Palomar Mountain near San Diego, CA.  The mirror on the Hale telescope is 5 meters or about 16 feet in diameter.  For comparison, the dome in the UOG Planetarium is about 7 meters in diameter.

The Hale Telescope on Palomar Mountain

The Palomar mirror is about as big as a single mirror can be.  Casting and cooling difficulties make it virtually impossible to make a larger mirror.  However, several new telescopes have recently come on-line that use multiple mirrors whose images are computer merged. 

The Keck telescope on Mauna Kea in Hawaii uses the combined images of 36 separate mirrors for an effective size of 10 meters.  The largest combined telescope is the Very Large Telescope in the Andes Mountains in Chile.  The light of four 8-meter multiple mirror telescopes is combined into a single image.

The Very Large Telescope Array in the Chilean Andes

When we took telescopes to space, weight was a very definite factor and the mirror for the premier space telescope; the Hubble Space Telescope is just under 8 feet across.  Until Hubble, we’d never looked at the sky with any kind of telescope without Earth’s air in the way and the Hubble Space Telescope has taught us more about the universe around us in the last 15 years than we learned in the 50 years before it was launched.

But times do move on and as soon as Hubble was launched; astronomers began to plan for larger space telescopes with much bigger mirrors.  But they ran headlong into the problem that keeps you and me earthbound.  It still costs $10,000 a POUND to put anything into space and glass is very heavy.  It was time for thinking outside the glass box.

Multiple mirrors were considered as an option but how do you assemble a multiple-mirror telescope in space?  Enter the James Webb Telescope (named for a NASA administrator).  This telescope will see mostly heat since vast dust and gas clouds block our view in visible light of some of space’s most interesting corners.  It’s scheduled for launch in 2013 and its multiple mirrors take advantage of the lightest metal, beryllium.  Beryllium has been used in other space telescopes and has worked well at the super-frigid temperatures of space in which the telescope will operate.

The telescope's mirror is made up of 18 mirror segments that form a total area of 25 square-meters (almost 30 square yards) when they all come together.  If the mirror were assembled completely and fully opened on the ground, there would be no way to fit it into a rocket.  So the Webb Telescope's 18 mirror segments will be set into place when the telescope is in space.  Engineers solved this problem by allowing the segmented mirror to fold, like the leaves of a drop-leaf table.

Each of the 18 mirrors can be moved individually, and they will be aligned together to act as a single large mirror.  Scientists and engineers can also correct for any imperfections after the telescope opens in space, or if any changes occur in the mirrors during the life of the mission. 

Each of the hexagonal-shaped mirror segments is 1.3 meters (4.26 feet) in diameter, and weighs approximately 20 kilograms or 46 pounds. The completed primary mirror will be over 2.5 times larger than the diameter of the Hubble Space Telescope's primary mirror, but will weigh roughly half as much.

JWST will have a 6.6 meter (21.65 feet) diameter primary mirror, which would give it a significant larger collecting area than the mirrors available on the current generation of space telescopes. Hubble Space Telescope's mirror is a much smaller 2.4 meters (7.8 feet) in diameter. (Credit: NASA)
The James Webb Space Telescope will collect light approximately 9 times faster than the Hubble Space Telescope.  This increased sensitivity will allow scientists to see back to when the first galaxies formed just after the Big Bang.  The larger telescope will have advantages for all aspects of astronomy and will revolutionize astronomical studies of how stars and planetary systems form and evolve.

Construction of the 18 mirrors has been completed and they will now be shipped to California where they will be ground and polished.  After the grinding and polishing, the mirror segments will be delivered to Ball Aerospace for assembly. Once the mirrors are completed, they’ll go to NASA's Goddard Space Flight Center, Greenbelt, Md., for final assembly on the telescope.

An artist’s concept of the James Webb telescope in space.

Not only will the telescope have folding mirrors; it will also have a gigantic sunshade that will protect its sensitive optics.  One of its instruments must operate at 6 degrees Kelvin, which is approximately –450 degrees.  It’s that cold in space, but not if the instrument is in sunlight.  The highly reflective shutters will keep the Webb telescope at its proper operating temperature.

We expect the James Webb telescope to peer back to the first days of the universe and we expect it to unlock a whole new array of answers and, most importantly, a whole new array of questions for it is by questions we learn.