The GALEX telescope
Posted: April 24, 2003

The GALEX spacecraft. Credit: NASA
At the heart of the Galaxy Evolution Explorer is a telescope designed to look out into space from Earth orbit. In certain ways the telescope is like a smaller version of the Hubble Space Telescope. It is quite a bit smaller than Hubble, however, and it has fewer instruments to record the light gathered by its main mirror. In addition, it is optimized for one specialty -- surveying galaxies in ultraviolet light.

Like Hubble, the telescope is of a Cassegrain design, named for the French sculptor Guillaume Cassegrain, who invented it in 1672. In this design, light from distant objects in space enters the telescope and is reflected by a primary mirror at the telescope's rear. The light is then gathered onto a smaller mirror suspended in the middle of the telescope near the front end. The light in turn reflects back toward the rear of the telescope, where it passes through a hole in the middle of the primary mirror. At the rear, behind the primary mirror, is the sensor that records the image. Three centuries ago, this "sensor" would have been the eye of the astronomer peering into the telescope. Later, the living eye was replaced by photographic film. Modern space telescopes record images with electronic devices which turn them into digital data that can be transmitted to Earth.

The primary mirror in the Galaxy Evolution Explorer's telescope is 50 centimeters (19.7 inches) in diameter. Like mirrors in many amateur telescopes, it is made of a kind of glass called fused silica, with a thin coating of aluminum. The aluminum must be protected by another coating of a clear material, however, to keep it from oxidizing and degrading. The coatings used in conventional telescopes would not work for this mission because they absorb too much of the ultraviolet light that the instrument is designed to gather. The optics in the telescope are therefore coated with a material called magnesium fluoride, which is transparent to ultraviolet light.

Scientists want to hunt for star birthing galaxies, in both the near-ultraviolet and the farultraviolet, so the telescope must deliver the light it gathers to two separate detectors. It accomplishes this in an ingenious way. Instead of swapping out different detectors at different times, the telescope directs the light through an unusually shaped lens that scientists call a "dichroic beam-splitter." This lens is coated with many extremely thin layers of special materials that cause the far-ultraviolet light to reflect off the lens surface, while allowing the near-ultraviolet light to proceed unimpeded through the lens. The reflected light proceeds to the far-ultraviolet detector, while the light that passes through the lens proceeds to the near-ultraviolet detector. This allows both detectors to perform science observations at the same time.

Compared to most telescopes either on the ground or in space, the Galaxy Evolution Explorer's instrument has a very wide field of view -- about 1-1/4 degree, or nearly three times the diameter of the Moon. With a field of view this large, objects around the edges of the visible area become distorted. The beam-splitter's unusual shape corrects this distortion.

After going their separate ways at the beam-splitter, near-ultraviolet and far-ultraviolet light are each filtered one more time to remove unwanted wavelengths. The detector for near-ultraviolet light is also sensitive to visible light, which must be removed. This is accomplished by reflecting the light off a mirror with a sophisticated coating that cuts out visible light but refelcts the near-ultraviolet.

The far-ultraviolet light is passed through a filter that cuts off very short wavelength light resulting from the ultraviolet glow produced by Earth's tenuous upper atmosphere. This glow would otherwise ruin the instrument's view of distant galaxies.

Eventually the light, both near- and far-ultraviolet, reaches its ultimate destination, the pair of detectors. These are devices somewhat similar to detectors in night vision goggles in the sense that they can react to very low levels of light, but they are optimized for ultraviolet rather than visible light. The detectors are contained in sealed tubes and are the largest detectors of their kind ever flown in space.

In addition to gathering basic ultraviolet images, scientists are also interested in analyzing the spectral signatures of light from distant galaxies to measure their "red shift." Because of the Big Bang, all of the galaxies in the universe are moving away from each other. Like dots taped to an expanding balloon, galaxies that are farther apart move away faster from each other.

The light from a galaxy moving away from us is shifted to the red end of the spectrum. The farther from us it is, the greater the "red shift." Scientists use this red shift to determine the galaxy's distance. Because these distances are enormous, the light from distant galaxies takes a significant fraction of the age of the universe to reach us. Scientists thus can view galaxies of any age, back to the time when they first formed.

The Galaxy Evolution Explorer's instrument is therefore equipped with a "grism" lens mounted so that it can be rotated into the beam of light coming from the telescope. The grism gets its name from the fact that it is a prism with a grating on one surface. This lens breaks down light into its various color wavelengths, which reveal telltale lines caused when light is absorbed or emitted by various elements. The grism on the Galaxy Evolution Explorer is made from calcium fluoride crystal, the first such optic to be flown in space.

The telescope is shielded by a cover that is opened shortly after launch. Once opened the cover cannot be closed, so spacecraft controllers must exercise caution never to let the telescope point too close to the Sun or the bright Earth. In addition, the satellite itself automatically prevents the telescope from being pointed too near to the Sun.

Cleanliness is always important in building spacecraft, but it was especially so in the case of assembling the instrument for the Galaxy Evolution Explorer. Organic contamination only two molecules thick would be devastating to the ultraviolet performance of the optics. Elaborate measures are therefore taken to assure that all materials, processes and environments that the instrument is exposed to are carefully controlled.