Hubble Deep Field

It covers an area about 2.6 arcminutes on a side, about one 24-millionth of the whole sky, which is equivalent in angular size to a tennis ball at a distance of 100 metres.

[1] 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 28, 1995.

[2][3] The field is so small that only a few foreground stars in the Milky Way lie within it; thus, almost all of the 3,000 objects in the image are galaxies, some of which are among the youngest and most distant known.

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 and ultraviolet light images than can be obtained with seeing-limited ground-based telescopes (when good adaptive optics correction at visible wavelengths becomes possible, 10 m ground-based telescopes may become competitive).

[2] After the spherical aberration was corrected during Space Shuttle mission STS-61 in 1993,[4] the improved imaging capabilities of the telescope were used to study increasingly distant and faint galaxies.

Once Hubble's corrective optics were shown to be performing well, 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 filters.

The choice of filters to be used for the HDF depended on the throughput of each filter—the total proportion of light that it allows through—and the spectral coverage available.

Scientists involved in the HDF observations pioneered a technique called 'drizzling', in which the pointing of the telescope was varied minutely between sets of exposures.

Each pixel on the WFPC2 CCD chips recorded an area of sky 0.09 arcseconds across, but by changing the direction in which the telescope was pointing by less than that between exposures, the resulting images were combined using sophisticated image-processing techniques to yield a final angular resolution better than this value.

[9] The final images were released at a meeting of the American Astronomical Society in January 1996,[10] and revealed a plethora of distant, faint galaxies.

In all, the HDF is thought to contain fewer than twenty galactic foreground stars; by far the majority of objects in the field are distant galaxies.

[13] The HDF data provided extremely rich material for cosmologists to analyse and by late 2014 the associated scientific paper for the image had received over 900 citations.

The wealth of galaxies at different stages of their evolution also allowed astronomers to estimate the variation in the rate of star formation over the lifetime of the Universe.

While estimates of the redshifts of HDF galaxies are somewhat crude, astronomers believe that star formation was occurring at its maximum rate 8–10 billion years ago, and has decreased by a factor of about 10 since then.

[10][12] Very-high redshift objects (Lyman-break galaxies) cannot be seen in visible light and generally are detected in infrared or submillimetre wavelength surveys of the HDF instead.

[23] Submillimeter observations of the field have been made with SCUBA on the James Clerk Maxwell Telescope, initially detecting 5 sources, although with very low resolution.

[11] Radio images of some individual sources in the field have been made with the European VLBI Network at 1.6 GHz with a higher resolution than the Hubble maps.

The Hubble Deep Field
The dramatic improvement in Hubble's imaging capabilities after corrective optics were installed encouraged attempts to obtain very deep images of distant galaxies .
The HDF is at the centre of this image of one degree of sky. The Moon as seen from Earth would fill roughly one quarter of this image.
The HDF was in Hubble's northern Continuous Viewing Zone, as shown by this diagram.
Diagram illustrating comparative sampling distance of the HDF and the 2004 Hubble Ultra-Deep Field
A section of the HDF about 14 arcseconds across in each of the four wavelengths used to construct the final version: 300 nm (top left), 450 nm (top right), 606 nm (bottom left) and 814 nm (bottom right)
Details from the HDF illustrate the wide variety of galaxy shapes, sizes and colors found in the distant universe.
Deep field image taken by ALMA and Hubble. [ 14 ]
On 11 October 2022, the James Webb Space Telescope spent over 20 hours observing the long-studied Ultra Deep Field of the NASA/ESA Hubble Space Telescope for the first time. [ 18 ]
The HDF imaged by the Spitzer Space Telescope . The top segment shows the foreground objects in the field; the bottom shows the background with the foreground objects removed.