Editing Hubble Deep Field (section)

==Data processing==
[[File:Galaxy in each of the four wavelengths comprising the HDF.jpg|thumb|300px|A section of the HDF about 14 [[arcseconds]] across in each of the four [[wavelength]]s used to construct the final version: 300 [[Nanometre|nm]] (top left), 450 nm (top right), 606 nm (bottom left) and 814 nm (bottom right)]]
The production of a final combined image at each [[wavelength]] was a complex process. Bright [[pixel]]s caused by cosmic ray impacts during exposures were removed by comparing exposures of equal length taken one after the other, and identifying pixels that were affected by [[cosmic ray]]s in one exposure but not the other. Trails of [[space debris]] and [[artificial satellite]]s were present in the original images, and were carefully removed.<ref name="Williams1996" />

Scattered light from the Earth was evident in about a quarter of the data frames, creating a visible "X" pattern on the images. This was removed by taking an image affected by scattered light, aligning it with an unaffected image, and subtracting the unaffected image from the affected one. The resulting image was smoothed, and could then be subtracted from the bright frame. This procedure removed almost all of the scattered light from the affected images.<ref name="Williams1996" />

Once the 342 individual images were cleaned of cosmic-ray hits and corrected for scattered light, they had to be combined. Scientists involved in the HDF observations pioneered a technique called '[[Drizzle (image processing)|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 [[arcsecond]]s 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. The HDF images produced at each wavelength had final pixel sizes of 0.03985 arcseconds.<ref name="Williams1996" />

The data processing yielded four [[monochrome]] images (at 300&nbsp;nm, 450&nbsp;nm, 606&nbsp;nm and 814&nbsp;nm), one at each wavelength.<ref name=Ferguson1/> One image was designated as red (814&nbsp;nm), the second as green (606&nbsp;nm) and the third as blue (450&nbsp;nm), and the three images were combined to give a color image.<ref name=Hubble_image>{{cite web |publisher=NASA |date=1996 |title=Hubble's Deepest View of the Universe Unveils Bewildering Galaxies across Billions of Years |url=http://hubblesite.org/newscenter/archive/releases/1996/01/image/a/ |access-date=January 12, 2009}}</ref> Because the wavelengths at which the images were taken do not correspond to the wavelengths of red, green and blue light, the colors in the final image only give an approximate representation of the actual colors of the galaxies in the image; the choice of filters for the HDF (and the majority of Hubble images) was primarily designed to maximize the scientific utility of the observations rather than to create colors corresponding to what the [[human eye]] would actually perceive.<ref name=Ferguson1>Ferguson et al. (1999), p.88</ref>