The prevailing opinion before Hubble telescope’s launch was that a very long exposure with the telescope was not likely to reveal distant new objects. Any would be too faint to be detected. One of the unique features of astronomy is that one can directly look back into the past because of the finite speed of light, and there is no better way to piece together the changing nature of the universe than to detect the most distant objects. They represent the ancestors of all that we see around us. A deep field with the Hubble simply had to be tried.  When we announced to the science community that we would attempt to take a long series of exposures for a "deep field," a number of our colleagues were very troubled by our plans. Lyman Spitzer, who along with John Bahcall was one of the essential advocates to bring about Hubble, was serious, but muted in expressing his concerns.  On several occasions he asked me at council meetings, “Are you sure you want to do this?” His colleague John Bahcall was much more vocal in working to prevent what he believed to be a much too risky venture, coming so soon after Hubble’s embarrassing spherical aberration had been fixed by the historic NASA Shuttle servicing mission. His concern, certainly understandable, was that if a large segment of time on the telescope produced little or no useful results, which would no doubt become public, the fallout could tarnish the mission of the telescope beyond repair.  Scientific progress requires risk, so we executed the observations over a two-week period in December 1995. The resulting Hubble Deep Field image yielded a wondrous display of galaxies, many of them very small, faint, and distant. The image is really a core sample of the universe.  The Hubble Deep Field has truly opened up the entire universe of galaxies to study and interpretation by simple imaging, turning the pretty faces of those galaxies into true talking heads that have helped us understand how structure formed and evolved over time in the universe.
On the eve of the 25th anniversary of the launch of the Hubble Space Telescope, we asked astronomers and others involved in the telescope’s groundbreaking story to tell us about their favorite images.