Image Credit: W. M. Keck Observatory
Last night and tonight, I am on the Big Island of Hawaii using the Keck I telescope at the W. M Keck Observatory. The twin Keck telescopes are two of the biggest telescopes in the world, and have produced a lot of outstanding science in their 19 years of existence.
Many people assume we use larger telescopes because they can magnify things more than smaller telescopes. Most of the time, this is not true. Earth's atmosphere blurs images so much that just about any professional telescope can see the sharpest details the atmosphere allows (more on this later). In fact the Hubble Space Telescope, if it were on the Earth, would be considered a moderately small telescope -- it's the fact that Hubble is above Earth's blurring atmosphere that makes it such a powerful tool.
The main reason many astronomers use big telescopes is to collect more light. The larger the aperture of the telescope (in other words, the bigger the mirror), the more light the telescope collects. When you study faint objects, which most astronomers do, collecting more light often means you can do more rigorous science. There are limits, which is why astronomers aren't all trying to use the largest telescopes every night. If a planet, star or galaxy you want to study is bright enough that you can get the information you need with a smaller telescope, then you want to use the smaller telescope, saving time on the big telescopes for projects that need all of that glass.
My studies on this observing run involve white dwarf stars, the glowing embers of normal stars that have finished their life cycles. The white dwarfs I'm studying are 22nd magnitude in visible light. (Magnitudes are a way of measuring how bright a star appears to be.) The faintest star you can see with your own eye on a very dark, clear night is around 6; magnitude 22 stars are about 3 million times fainter than this. That's why I need a big telescope!
In recent years, technology has allowed astronomers to use lasers and flexible mirrors to correct for Earth's atmospheric distortion. When working, these corrective measures (called adaptive optics) actually allow larger telescopes to see finer details, or effectively magnify images more, than smaller telescopes. For that reason, some astronomers now do use the big telescopes to look at bright stars. This work has allowed astronomers to take some of the first direct pictures of planets around other stars, and to take very sharp pictures of planets and moons in our Solar System.
Still, most astronomers use big telescopes to collect more light, not to see sharper. My white dwarfs are so small that even the mighty Keck Telescope would have no hope of allowing us to see the surface of the nearest white dwarf star -- we would need a telescope with a diameter of roughly 3000 km to do that!