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The extinction as a function of wavelength has been tabulated by D.L. King ( ING Tech Note No 31); a data file and interpolation program are stored in the [LPINFO] directories at Cambridge and La Palma. These data are valid only for a very clear night with low aerosol content, and correspond to an extinction of 0.10mag/airmass in the `V' band. (A value of 0.15 is more typical, and values above 0.18 will usually include a contribution from dust). The actual extinction for any (clear) night can be obtained from the Carlsberg Meridian Circle data (also stored in [LPINFO]), and for values at `V' between 0.08 & 0.16, the extinction curve usually has the same shape as King's curve.
The King extinction curve is repeated here. is the value in magnitudes per airmass. Note the low value at 5500Å.
Sahara dust can extinguish starlight and reflect light from the Moon and streetlamps. Although it is thought normally to be grey in extinction at optical wavelengths (D. Jones, Tech Note No 10; D. Stickland et al., Observatory, 107, 74, 1987), very non-grey extinction has been reported once (Andrews & Williams, ibid, 109, 15, 1989).