The total sky background, as well as the relative importance of its various
components, vary greatly with wavelength, with time, with Galactic and
ecliptic coordinates and from observatory to observatory.
[Leinert et al. 1998] give a detailed account of the values of the sky background
over a wide range of wavelengths, from the far ultraviolet to the far
infrared. Generally, zodiacal light is the greatest contribution, followed
by air glow, integrated starlight, diffuse Galactic light and diffuse
extragalactic light.
Table 3.1 gives typical values for the sky background as observed
from the ground in the bands.
22.0 | 22.7 | 21.8 | 20.9 | 19.9 |
If seen from above the atmosphere, the sky background is fainter by a factor
depending on the waveband. In the band this factor can be as large as
1.5 mag, and still larger factors can apply at longer wavelengths.
Specifically, [Gilmore 1997] has provided values of the sky background
obtained excluding stars from some low Galactic and ecliptic latitude HST
WFPC2 images. These values are listed in Table 3.2, and are
believed to overestimate the typical sky background.
22.87 | 22.06 | 21.46 |
Note, however, that the sky background as measured by GAIA will be slightly brighter, due to unresolved objects, an effect which can conservatively be taken into account by increasing the values given in Table 3.2 by 0.5 mag. Note also that the real sky can be much brighter, due to scans through the Moon, bright planets, Galactic nebulae and very crowded regions. All such special cases need to be considered individually. In the following, specifically in Chapter 5, we will accordingly assume an -band sky background of mag/arcsec. Note that, in this context, the corrections to be applied for the conversion between HST WFPC2 magnitudes and standard magnitudes, which in principle could be carried out using the HST WFPC2 calibration by [Holtzman et al. 1995b], are negligible.
The accuracy in the determination of the sky background is a critical factor in the final accuracy of galaxy surface photometry, and particularly for low-surface-brightness galaxies or in the outermost faint regions of otherwise high-surface-brightness galaxies. In these cases, one often has to follow the galaxy surface brightness distribution down to less than 1% of the sky background. Such measurements will clearly be meaningless unless the sky background can be extremely accurately determined, and small errors in the determination of the sky background can result in large errors of the derived galaxy surface brightness radial profile. Even more dramatic errors are to be expected in the radial distributions of galaxy color indices, since these have to be obtained by subtracting two derived surface brighness radial profiles.
Besides the already significant advantage of the lower sky background, space observations are relatively easy to correct for this effect, since they do not suffer from the rapidly varying (in space and time) air glow. On the other hand, the typically small field of view of space observatories does not generally allow a very accurate determination of the sky background in the surroundings of a diffuse object such as a galaxy. This is still a severe problem with CCD detectors, e.g. with HST observations. It is expected, however, that this will not be the case for GAIA, since the Astros, provided the necessary data can be accommodated into the telemetry, will be able to map the sky background over the whole sky at the desired resolution. It should also be emphasized that the dominant contribution to the measurement errors in GAIA surface photometry is the readnoise, with an essentially negligible contribution from the sky background.