The Hubble Deep Field (HDF) is
an image of a small region in
the constellation Ursa Major,
constructed from a series of
observations by the Hubble Space
Telescope. It covers an area 2.5 arc minutes across, about one
24-millionth of the whole sky,
which is equivalent in angular
size to a 65 mm tennis ball at a
distance of 100 meters. The
image was assembled from 342
separate exposures taken with
the Space Telescope's Wide Field
and Planetary Camera 2 over ten
consecutive days between
December 18 and December 28,
Three years after the HDF observations were taken, a region in the south celestial hemisphere was imaged in a similar way and named the Hubble Deep Field South.
The similarities between the two regions strengthened the belief that the universe is uniform over large scales and that the Earth occupies a typical region in the universe (the cosmological principle).
A wider but shallower survey was also made as part of the Great Observatories Origins Deep Survey. In 2004 a deeper image, known as the Hubble Ultra-Deep Field (HUDF), was constructed from a total of eleven days of observations.
The HUDF image was at the time the most sensitive astronomical image ever made at visible wavelengths, until the Hubble Extreme Deep Field (XDF) was released in 2012.