In order to calculate the relative intensity of airglow, we need to convert apparent magnitudes into fluxes of photons; this clearly depends on the spectrum of the source, but we will ignore that initially. At visible wavelengths, we need the parameter S0(V), the power per square centimetre of aperture and per micrometre of wavelength produced by a zeroth-magnitude star, to convert apparent magnitudes into fluxes – . If we take the example of a "V"=28 star observed through a normal "V" band filter ( bandpass, frequency ), the number of photons we receive per square centimeter of telescope aperture per second from the source is "N""s":