In this section we give the minimum diffuse sky brightness to be expected (values for an arbitrary field-of-view have to be estimated as a sum of the components of the night sky brightness). For the ultraviolet and the infrared, extraterrestrial values are given. For the visual spectral region we give the values as seen from ground. Here, the extraterrestrial values would closely correspond to the minimum brightness of the zodiacal light, stars being resolved by optical space telescopes like the HST. For the near-infrared, sky brightness as seen from ground is also included.

In the infrared, total brightnesses as observed by the DIRBE experiment onboard COBE are conveniently available in the form of weekly averages of the brightness seen in different viewing directions from the heliocentric position taken by COBE during the respective week. The data, covering the 10 photometric DIRBE bands from 1.25

m to 240

m (see Sect. 8.5. (click here)), including Stokes Parameters Q and U for the 1.25

m, 2.2

m and 3.5

m bands, are available on CD-ROM or tape. Under http://www.gsfc.nasa.gov/aas/cobe/cobe_-home.html on the World Wide Web one finds the information necessary to actually receive those data.

4.1. Ultraviolet

4.1.1. Far UV (91.2 nm - 180 nm)

The sky brightness over most of this band is the sum of starlight and starlight scattered by interstellar dust. The Sun's flux is sufficiently low that zodiacal light is virtually non-existent. An intense diffuse emission in this band is emission from hydrogen Lyman-alpha at 121.6 nm. This flux is produced by scattering of solar radiation by neutral hydrogen in the Earth's geocorona, and by scattering from neutral interstellar hydrogen entering the heliosphere. The geocoronal flux varies by more than a factor of 10 between day and night; typical fluxes range from 3 kR (night) to 34 kR (day). This flux varies with distance from the Earth's geocorona. An excellent exposition of the variation of this flux as a function of these variables is given by Raurden et al. (1986). See also Sect. 6 (click here).

4.1.2. Near UV (180 nm - 300 nm)

The sky brightness in this range is primarily the sum of zodiacal light, starlight, and starlight scattered by interstellar dust. The zodiacal light in this range has not yet been well characterized, the presently available information is shown in Sects. 8.4. (click here) and 8.6. (click here). The integrated starlight is discussed in Sect. 10.2. (click here). Scattering by dust near early type stars is a major contributor to the diffuse flux in this range, and is highly variable from place to place in the Galaxy (see also Sect. 11.5. (click here)).

4.2. Visual

Table 4 (click here), adapted from a recent paper (Leinert et al. 1995), gives minimum and maximum values of broadband sky brightness as observed in moonless nights at several observatories in suitable "dark regions'' of the sky. The main constituents of this diffuse brightness are airglow, zodiacal light and tropospherically scattered light, in this order, but in roughly comparable quantities. The variation between minimum and maximum is mostly due to solar activity, which leads to increased airglow emission. The individual entries in Table 4 (click here) are not stricly comparable. Some of the measurements were performed with small telescopes and excluded stars only down to about 10 mag (San Benito Mt.), about 13 mag (Kitt Peak, 90 cm telescope, diaphragm 50'') and about 12 mag (Hawaii, 15 cm telescope, 6.5

'). The residual contributions of individual stars to their observed zenith brightnesses then can be estimated (Roach & Megill 1961) to be still

, and

, respectively, both at B and V. In clear nights therefore the sky appears to be more or less equally dark at all major observatories.

Figure 8 shows the observed variation of sky brightness in a starless spot for a typical night, both for intermediate-band and broad-band observations. The central wavelengths of the intermediate bands have been selected to coincide with minima of the night sky spectrum. Figure 9 indicates what emission may be expected outside those bands. Brightness variations usually are well correlated between different wavelength bands (see Leinert et al. 1995 and Fig. 28 in Sect. 6.3. (click here)). An example for the variation of sky brightness with solar activity is given in Fig. 10.

Figure 8: Variation of the night sky brightness at Calar Alto during the course of one night. Left: Observations in medium band filters, including Strömgren u and b on June 23, 1990. L134 is a dark cloud in Ophiuchus at ecliptic latitude 15

, the Draco field is at high ecliptic latitude, hence the lower brightness level. Right: Observations in broad band filters on June 27, 1990 near the ecliptic pole. - The effect of dawn and dusk can be seen in the data around 15 h and 21 h siderial time

Figure 9: A low resolution night sky spectrum at Palomar Observatory, taken on November 28, 1972 (Turnrose 1974), compared to medium band measurements on Calar Alto (CA)

Figure 10: Correlation between the night sky brightness observed at Calar Alto at 525 nm with the solar activity, measured by the 10.7 cm radio flux density (in units of 10⁴ Jy)

4.3. Near-infrared from the ground

The near-infrared sky brightness seen from ground at a typical observing site is shown in Fig. 11. Below 2

m the night sky emission is dominated by OH airglow emission (see also Sect. 6 (click here)). Above 2

m thermal emission by the atmosphere is dominating. Between 2

m and 4

m emission from the telescope also adds a considerable fraction to the total radiation.

The situation is quite different for observations from Antarctica. The much reduced thermal emission in an environment with winter temperatures below

leads to a substantial reduction of sky background particularly in the K photometric band (Ashley et al. 1996; Nguyen et al. 1996, see Fig. 12 (click here) and Table 5 (click here)). Because of the absence of strong airglow emission between 2.3

m and 2.5

m (see Fig. 27, Sect. 6.1.c (click here)), in this spectral region values of zenith sky brightness as low as 50

Jy arcsec^-2 (

) have been measured. The dependence on zenith distance is normal: proportional to sec z down to z

. In the L band, between 2.9

m and 4.1

m, still an improvement by a factor of 40 - 20 was found (Ashley et al. 1996).

Figure 11: Near-infrared spectrum of the night sky brightness, measured just inside the cryostat window of the UKIRT IRCAM camera (McCaughrean 1988). Note that 10⁴ photons m^-2 s^-1

''^-1

m^-1 correspond to 4.23 Wm^-2 sr^-1

m^-1. From Beckwith 1994

Figure 12: Near-infrared sky brightness around 2.3

m as observed in Antarctica on May 31, 1994 with an ambient temperature of

. The dip around 2.4

m is due to the lack of airglow emission in this region. The South Pole data are compared to observations obtained at the Siding Spring observatory (Australia) with an ambient temperature of +10

. From Ashley et al. (1996)

**Table 5:** Comparison of K band sky brightnesses^a
				I
Site	(m)	(m)	(Jy/'')	(mag/'')	Ref.
Mauna Kea	2.22	0.39	4000	13	1
Mauna Kea	2.11	0.35	2700	13.4	1
Balloon	2.4	0.1	< 26	< 18.4	2
Balloon	2.38	0.08	13019	16.7	3
South Pole	2.36	0.14	16267	16.5	4
South Pole	2.40	0.04	50	17.7	5
^a adapted from Nguyen et al. (1996).

4.4. Infrared

Table 6 (click here) shows the darkest spots on the sky from 1

m to 240

m as measured by the infrared photometric experiment DIRBE on the COBE satellite in an

wide field-of-view (adapted from Hauser 1996). These are conservative upper limits to the cosmic infrared background light. For wavelengths of

m, where the zodiacal light (thermal emission) dominates, the darkest fields are close to the ecliptic poles. For longer wavelengths, the thermal emission of interstellar dust is dominating, and the darkest fields are found in regions around the galactic poles with particularly low HI 21 cm emission (Lockman et al. 1986).