Resolution 101

hen astronomers talk about how clearly a telescope "sees," they are referring to its resolving power, or resolution — how fine a detail it can see or how close together two objects (such as stars) can be and still be seen as two distinct objects. Astronomers measure the resolving power of a telescope in terms of degrees. Degrees are further divided into arcminutes and arcseconds.

1 degree = 60 arcminutes = 3600 arcseconds

The best telescopes on the ground can rarely see detail that is less than 1 arcsecond wide or differentiate between two stars that are less than 1 arcsecond apart in the sky. This is because turbulence in Earth's atmosphere causes images to ripple and shimmer. By comparison, Hubble can see detail down to less than 0.1 arcsecond across — more than 10 times clearer. This is one of the main reasons astronomers like to use Hubble.