So just how good are Spitzer's infrared eyes? Well, the primary definition of that is what we call resolving power.
Up close, say at about a comfortable reading distance of around 18 inches, the average human eye can resolve 1/200 of an inch. That's why most printing is done at 300 dots per inch. Your eye can't see the individual dots because they're too small. But if you get the paper closer to your eye, you can probably start to make out the individual dots. If you move the paper farther away, the dots disappear and form the letters on the page. If you keep moving it back, soon the letters and words will start to blur together, and eventually you can't make out what they say any more. That's your resolving power.
Let's imagine a 747. It's 231 feet long, and has a wingspan of 211 feet. How close you are to that 747 makes a big difference in how much detail you can see on it. Up close, you can probably see individual joints where parts are joined together, chips in the paint, and all sorts of other fine details.
But, if you see a 747 flying 35,000 feet up in the air, you wouldn't see much detail at all. If there were no atmosphere, clouds, or other obstructions (and enough light), the average human eye could see a 747 at a whopping 150 miles up. But, you'd see it as a tiny point, and that's it. You wouldn't even know it was an airplane.
Of course, the reason we built telescopes is to see things we can't. Resolving power is where telescopes really shine. The detector on Spitzer with the best resolving power can see something almost 60 times smaller than the human eye can. It could see that 747 at a distance of over 9,000 miles!
The human eye can scan the entire sky in the amount of time it takes you to turn your head. Because of their design, telescopes only see a very small portion of the sky at once. At any one time, Spitzer can see less than 1/20,000 of the area that a human eye can see!
Spitzer will never be able to map the entire sky in its lifetime, but luckily it doesn't have to. In 1983-1984, the United States and Europe flew a space telescope called IRAS. It was a very low-resolution mission, but it mapped more than 96% of the sky during its 10-month life. From 1997-2001, the Two Micron All Sky Survey (2MASS) mapped the entire infrared sky (at one of the few infrared wavelengths that can be seen from the ground), again at fairly low resolution. And that's only two missions of many.
Spitzer actually uses these previous survey missions to help select targets. When astronomers see something interesting in a survey, they then turn more sensitive telescopes, like Spitzer, on the target to see what they can resolve.
Spitzer only has a limited lifespan, about five years. That means around 2008, we will need a new generation of infrared telescope to take over. Luckily, several missions are already in the works. The European Space Agency plans to launch Herschel Space Observatory in 2007, and NASA is scheduled to launch the James Webb Space Telescope in 2010. And since technology improves all the time, so we expect the next generation of infrared telescopes to provide even more spectacular results!