Therefore, observations from space provide particularly favorable conditions for deep imaging and photometry since at most visual wavelengths the sky background is lower and since the absence of atmospheric seeing effects normally results in sharper images. In spite of their great importance and success the Hubble Deep Fields have some drawbacks. One obvious disadvantage is the compared to modern ground-based faint object cameras relatively small field of view of the HST WFPC which limited the Hubble Deep Fields to areas of only about 6 square arc minutes. Moreover, the compared to present large ground-based telescopes relatively modest aperture of the HST requires very long integration times to collect a sufficient number of photons from faint objects. True color image of the inner 6' times 6' of the FORS Deep Field.
North is up, East to the left .
Unfortunately, a quick look at the Paranal seeing statistics shows that periods of seeing below 0.2 arcsec are so rare that observing programs requiring such good seeing for sizable amounts of time would require years to complete, even if all the time with excellent seeing would be allocated to one such a program. On the other hand, moderately good seeing occurs relatively often. At 0.6 arcsec the well sampled VLT PSF is about 3 times larger and its surface about 10 times larger than for the HST WFPC, resulting for unresolved objects in a 10 times larger sky background. At these wavelength the VLT is competitive only at better seeing conditions or with an increased total exposure time. But with the larger FOV of the FORS instruments significantly larger statistical samples can be obtained in a single FORS field.