and oxygen (O2) are dissociated by solar energy in the upper reaches of the atmosphere and may encounter each other to form NO. Other chemicals that can create air glow in the atmosphere are hydroxyl (OH), atomic oxygen (O), sodium (Na), and lithium (Li). The sky brightness is typically measured in units of apparent magnitude per square arcsecond of sky. Calculation 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 Ns: (where h is Planck's constant; hν is the energy of a single photon of frequency ν). At V band, the emission from airglow is per square arc-second at a high-altitude observatory on a moonless night; in excellent seeing conditions, the image of a star will be about 0.7 arc-second across with an area of 0.4 square arc-second, and so the emission from airglow over the area of the image corresponds to about . This gives the number of photons from airglow, Na: The signal-to-noise for an ideal ground-based observation with a telescope of area A (ignoring losses and detector noise), arising from Poisson statistics, is only: If we assume a 10 m diameter ideal ground-based telescope and an unresolved star: every second, over a patch the size of the seeing-enlarged image of the star, 35 photons arrive from the star and 3500 from air-glow. So, over an hour, roughly arrive from the air-glow, and approximately arrive from the source; so the S/N ratio is about: We can compare this with "real" answers from exposure time calculators. For an 8 m unit Very Large Telescope telescope, according to the FORS exposure time calculator, 40 hours of observing time are needed to reach V = 28, while the 2.4 m Hubble only takes 4 hours according to the ACS exposure time calculator. A hypothetical 8 m Hubble telescope would take about 30 minutes. It should