Editing Hubble Deep Field (section)

==Conception==
[[File:Improvement in Hubble images after SMM1.jpg|thumb|300px|left|The dramatic improvement in Hubble's imaging capabilities after corrective [[optics]] were installed encouraged attempts to obtain very deep images of distant [[Galaxy|galaxies]].]]

One of the key aims of the astronomers who designed the Hubble Space Telescope was to use its high [[optical resolution]] to study distant galaxies to a level of detail that was not possible from the ground. Positioned above the [[atmosphere]], Hubble avoids atmospheric [[airglow]] allowing it to take more sensitive [[Visible light|visible]] and [[ultraviolet light]] images than can be obtained with [[Astronomical seeing|seeing-limited]] ground-based telescopes (when good [[adaptive optics]] correction at visible wavelengths becomes possible, 10&nbsp;m ground-based telescopes may become competitive). Although the telescope's mirror suffered from [[spherical aberration]] when the telescope was launched in 1990, it could still be used to take images of more distant galaxies than had previously been obtainable. Because [[Speed of light|light takes billions of years]] to reach Earth from very distant galaxies, we see them as they were billions of years ago; thus, extending the scope of such research to increasingly distant galaxies allows a better understanding of how they evolve.<ref name=Ferguson1998>Ferguson et al. (1999), p.84</ref>

After the spherical aberration was corrected during [[Space Shuttle]] mission [[STS-61]] in 1993,<ref name="Trauger1994">Trauger et al. (1994)</ref> the improved imaging capabilities of the telescope were used to study increasingly distant and faint galaxies. The [[Hubble Medium Deep Survey|Medium Deep Survey]] (MDS) used the Wide Field and Planetary Camera 2 (WFPC2) to take deep images of random fields while other instruments were being used for scheduled observations. At the same time, other dedicated programs focused on galaxies that were already known through ground-based observation. All of these studies revealed substantial differences between the properties of galaxies today and those that existed several billion years ago.<ref>Abraham et al. (1996)</ref>

Up to 10% of the HST's observation time is designated as Director's Discretionary (DD) Time, and is typically awarded to astronomers who wish to study unexpected transient phenomena, such as [[supernova]]e. Once Hubble's corrective optics were shown to be performing well, [[Robert Williams (astronomer)|Robert Williams]], the then-director of the [[Space Telescope Science Institute]], decided to devote a substantial fraction of his DD time during 1995 to the study of distant galaxies. A special Institute Advisory Committee recommended that the WFPC2 be used to image a "typical" patch of sky at a high [[galactic latitude]], using several [[optical filter]]s. A [[working group]] was set up to develop and implement the project.<ref name="Williams1996">Williams et al. (1996)</ref>