Previous: Exposure times
Up: Exposure times
Next: Telescope efficiency
Previous Page: Exposure times
Next Page: Telescope efficiency
La Palma Technical Note 31 (King, 1984) 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 component due to dust scattering which is to some extent wavelength independent because large dust particles dominate the size distribution. The wavelength dependent component is plotted as a function of wavelength and airmass in Figure 2.2. Table 2.3 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. The total vertical extinction in the 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 users with access to STARLINK may examine these results in the file ß RGVAD::SYS$SYSDEVICE:[LPINFO.EXTINCTION]CAMCEXT.DAT.. These data may be used to estimate the dust extinction on any particular night. Obviously, the use of a plane-parallel azimuth-independent geometry (the sec z law) is less accurate for dust extinction than for air.