La Palma Technical Note 31 discusses the atmospheric
extinction on La Palma in some detail. Briefly, it is possible to separate
the extinction into a wavelength dependent component, due to Rayleigh
scattering by air molecules and absorption by ozone, and a wavelength
independent component due to dust scattering. The wavelength dependent
component is plotted as a function of wavelength and airmass in
Figure . The extinction at the zenith on a clear
night with low aerosol content is tabulated as a function of wavelength
in Table
. Table
gives the airmass as a function of zenith distance.
It can be seen that this component of the atmospheric extinction is
much more important in the blue than
in the red, varying from 3.7 magnitudes per unit airmass at 3000 Å to 0.007
magnitudes per unit airmass at 10000 Å.
The extinction due to dust scattering
varies from night to night, but is usually less than a few tenths of
a magnitude per unit airmass (see Table ). The total vertical
extinction in V band (i.e.
the sum of the wavelength independent and wavelength dependent components)
is measured each night by the Carlsberg Automatic Meridian Circle, and
these values are placed on the ING information pages of the World-Wide Web.
Table: La Palma Standard Extinction Curve
Table: Airmass as a function of zenith distance
[ TIFF ]
Figure: Theoretical
atmospheric extinction on La Palma as a function
of wavelength and airmass