Theoretical Background Spectrum

A theoretical spectrum is the last possibility for choosing a sky background. This spectrum is shown in Figure 7.

Figure 7: A synthetic spectrum of the sky background

The fundamental parts of this calculation are the OH-line database [OH-LIN,ROUS00] and the transmission data of the atmosphere [AIRTRA]. The ratio of intensities for two lines may change during the night or from observation to observation. This is the reason why it is possibile to change the relative intensities via an ini-file. The predefined values are adjusted to Mauna Kea's night sky spectrum [SKYBA]. For modeling the sky background, we assume:

• OH-line absorption due to the light travel through the atmosphere, scaled with T
• thermal emission of a blackbody with a temperature of 250K, scaled with (1-T)
• zodiacal emission (a blackbody spectrum; T=5800K)
• increasing intensity with larger zenith distances (described by the van-Rhijn's function)

Then the sky background can be described as:

$${\frac{{ \mathrm{T} \cdot OH+(1-\mathrm{T}) \cdot BB_{\mathrm{250... ...mathrm{R}}{\mathrm{R}+\mathrm{h}}\right )^2 \cdot \sin^2\mathrm{z} \right )}}}}$$ (16)

Nsky	:	number of photons from sky background
T	:	transmission of the atmosphere
OH	:	intensity of the OH-line
BBx	:	blackbody with a temperature x
R	:	earth radius ( $\sim$ 6378 km)
h	:	hight of emitting layer ( $\sim$ 100 km)
z	:	zenith distance