10. Southern African Large Telescope
Gianluigi Guercia/AFP/Getty Images
SALT's large mirrors allow it to peer out at distant galactic sights.
In an optical telescope, performance depends primarily on two characteristics: the ability to collect light bouncing off objects and the ability to focus that light to produce a clear image of those objects. The ability to collect light is directly related to aperture size, or the area available to capture light. In the huge telescopes of modern astronomy, one or more mirrors serve to collect light. The size of the mirror is the single most important determinant in how far a telescope can see, since the farther away an object is, the fainter the light bouncing off it.
It's mirror size that makes the Southern African Large Telescope (SALT), located just outside Sutherland, South Africa, stand out among the most amazing telescopes in the world. With an 11.1-meter (36.6-ft) aperture, SALT can see distant galaxies and quasars that are one billion times fainter than what can be seen with the human eye [source: GTC]. No one can manufacture a single mirror of that size, which is about the height of a three-story building, so SALT uses a collection of hexagonal mirrors pieced together into one perfectly smooth mirror. A single imperfection would distort the image it captures.
If we could see a candle flame on the moon with the naked eye, we'd have the optical acuity of SALT. However, while SALT comes in first with its 11-meter aperture, part of the telescope design blocks the edges of the mirror. Its actual aperture is more like 9.2 meters (30.18 ft) [source: GTC]. The Keck telescope system in Hawaii has a larger actual aperture, which makes it the second largest telescope system in the world.
