Sky background Model - ESO in-kind projects

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-empirically 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 rather on physical models 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 as 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 ESO instrumentation. It also shows the wavelenght range of CRIRES; which is a high resolution spectrograph in the infrared.



Fig.1: Radiation and transmission of the earth's atmosphere, calculated with a radiative transfer code incorporating 12 molecules. The upper two panels show the entire wavelength range covered by the ESO isntrumentation, the lower two panel are zooms into the wavelength range of CRIRES, an infrared high resolution spectrograph as example.


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 are not fully clear yet, the emission varies on several time scales, from minutes to the 11-year cycle of the sun. 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 moon cycle, the angle between the object and the moon, and the wavelength range of interest.

4. Zodiacal light

As 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. Telescope/Instrument emission

As the infrared is one of the major wavelength regime the emission of the telescope and the instrument is heavily influencing astronomical observations by thermal radiation. We model this radiation using a grey body approach, which is the same as a black body incorporating a certain emission factor.





Fig.2: The five componenst of the sky background model (beginning on the left side): the sky emission lines (airglow), the mon light, the zodiacal light, the atmospheric model, and the grey body emssion.




Fig.3: Direct comparison of all components, the black line gives the total sum.



The project is a research contract in the context of the joining of Austria the European South Observatory ESO.


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