From the article “Hell Yeah, Hubble!“:

The first thing we did was take a patch of sky that was relatively empty. No bright stars, no large galaxies or clusters, no planetary nebulae, just a little tiny patch of black, empty sky. And then we point Hubble at it.

And what do we do? We sit there. And wait. Collecting tiny, miniscule amounts of light. First, for minutes on end. And then the minutes turn into hours, and the hours turn into days. All the while, Hubble just patiently sits there, pointing at the same patch of empty sky. Over 10 days, Hubble took a photograph of the same exact patch on the sky 342 times. They then added up the light from all 342 of these images. The result?