A rediscussion of the atmospheric extinction and the absolute spectral-energy distribution of Vega.

For both the Lick and the Palomar calibrations of the spectral-energy distribution of Vega, the atmospheric extinction was treated incorrectly. A model is presented for extinction in the earth's atmosphere and this model is used to calculate corrections to the Lick and Palomar calibrations. A method is described that can be used to fabricate mean extinction coefficients for any mountain observatory. Selected portions of the corrected Lick and corrected Palomar calibrations are combined with the new Mount Hopkins calibration to generate an absolute spectral-energy distribution of Vega over the wavelength range 3300-10,800 A. Until better measurements become available, the use of this calibration is recommended for all practical applications.