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Extinction and Sky Brightness

The extinction at J, H and K was estimated from observations of standard stars at about 0.10, 0.11 and 0.07 mag/airmass, respectively, but these values are likely to vary from night to night, and the best solution is to monitor a few standards throughout the night, over the same range in air mass as your objects. A model of the sky emission together with a representation of telescope emission is shown in Figure 6, while an atmospheric transmission spectrum is shown in Figure 7. The winter and summer sky brightnesses are given in Table 5.3.

Figure: Calculated sky emission for winter (T = 278 K) under dry conditions between 1 and 2.5  tex2html_wrap_inline1109 m. The strong airglow emission lines dominate the spectrum shortwards of 2.3 tex2html_wrap_inline1109 m, where thermal emission from the sky starts to become apparent. However, emission from the telescope increases rapidly beyond 2.2 tex2html_wrap_inline1109 m and dominates the background. The dotted and dashed curves represent the emission from a 20% emissive telescope at 285 and 275 K respectively.

Figure 7: Calculated sky transmission spectrum for La Palma.


Shaun Hughes
Thu Mar 14 14:48:38 GMT 1996