Astronomical data of ground based telescopes are highly influenced by several factors as moon light, zodiacal light, telescope emission, and the Earth's atmosphere, called sky background. Nowadays, we take the sky background into account during the data processing, mainly using semi-empirical methods and calibration by known sources. Part of the Austrian ESO in-kind contributions is a new model of the sky background, which is based on physical models rather than on a semiempirical ansatz. This new model will be used during both, the planning of observations and the calibration of already taken data. In particular, the planning of observations is crucial since observing time at telescopes is very expensive in terms of man power and financial ressources. Hence using a more accurate background model leads to a more efficient use of ESO telescopes.
The new sky backgorund model consists of five components:
1. Earth's atmosphere
The Earth's atmosphere acts like a filter to the light from astronomical objects. It affects the entire wavelength range by absoption and emission, which crucially depends on the chemical components of the atmosphere. We are modelling these effects using radiative transfer codes, which are able to calculate the absorption and emission effects caused by various molecules. Standard atmospheric compositions are refined by measurements taking into account local peculiarities. This leads to atmospheric profiles adapted to the special conditions at the ESO observing sites. Fig. 1 shows the transmission and the radiance of such an atmospheric profile over the wide wavelength range covered by the ESO instruments. It also shows the wavelength range of CRIRES; which is a high resolution spectrograph in the infrared.
2. Airglow (emission lines)
Radiative transfer codes are able to calculate several physical process. However, in the upper atmosphere non-thermal processes are dominating the emission caused by molecules. The physical processes behind the emission are not fully understood, and it varies on several time scales, from minutes to the 11-year solar cycle. Currently a new model is being developed within our group for estimating these variations.
3. Moon light
Although moon light is reflected sunlight only, the influence of it is highly dependent on the lunar phase, the angle between the object and the moon, and the wavelength range of interest.
4. Zodiacal light
Like with the moon light, the zodiacal light is reflected sunlight. However, the reflection is caused by gas and dust particles concentrated within the ecliptic. Honce, the influence of the zodiacal light is highly dependent on the object's position on the sky, the position of the sun, and the time of observation.
5. Scattered Starlight
The bright nearby stars and the Milky Way are adding an additional component. The scattering in the earth's athmosphere leads to an additional diffuse background.
Additionally a grey body as estimate for the telescope thermal emission is added.
The sum of the components shown in Fig. 2 gives an overview of which wavelengths the various components are dominant.
Current members of the group are: