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far sight [Q&A]


When it comes to telescopes, a question that astronomers get asked with some regularity is, how far does it see?

Abraham Lincoln was once supposedly asked how long his legs were, to which he is said to have answered, “long enough to reach the ground.” Same here. How far can the Hubble see? Far enough to see what it is seeing.

The thing that determines whether an object is visible is not how far it is, but how bright it is. A black cat on a dark night would be invisible a footstep away, but a supernova 6500 light years away can be seen in daytime.

This highlights some important characteristics of astronomy. One, the instruments are remote sensors, and play no role in enhancing the original source signal (quite unlike tabletop experiments in Physics). And two, the defining characteristic of an instrument is its sensitivity, the capacity to detect a signal of at least some minimal level. Better instrumentation pushes the envelope on this level, becoming capable of detecting fainter and fainter objects. This lets us pick up objects that are further away but were too faint to be detected before. So yes, the Hubble can see farther than the naked eye, but because it can see fainter, not because it is far sighted.

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