They obtained accurate distances by measuring the velocities of galaxies with ground-based telescopes. But many galaxies were beyond the range of ground-based observatories. That s when astronomers turned to the Deep Field exposures. By analyzing the colors in these exposures, astronomers were able to estimate distances. Astronomers express distance in terms of redshift.
Redshift is a measure of how fast galaxies are moving away from Earth.
Since the universe is expanding, the more distant galaxies are receding at a faster rate and thus have a higher redshift. Once astronomers establish a galaxy s redshift, they can make a rough estimate of its distance. The distance isn t exact because the universe s expansion rate is not yet precisely known. Using the 10-meter W.M. Keck atop Hawaii s Mauna Kea to measure the velocities of objects in the Deep Field, astronomers have obtained redshifts for 125 galaxies and roughly 500 more in the surrounding area. They did so by comparing Deep Field exposures taken through different color filters.