The comparison is illustrative rather than exhaustive, with several simplifying assumptions and arbitrary choices for parameters. Illustrate the comparison in the atmospheric ''windows'', where the effect of the atmosphere on telescope sensitivity is minimized. These atmospheric windows are assigned nominal properties based on modeling and observations. No attempt is made to model details within a window. Assume that the array pixels convert photons into photoelectrons with unity gain, and that all the photon sources obey Poisson statistics.
N r are the detector dark current and read noise per pixel.
The values for parameters used in the comparison are discussed briefly below. Calculations are based on the collecting area of one 8m diameter ground-based telescope with a small central obscuration. We also assume that the NGST has the same collecting area even though the NGST concepts range from 4m to 8m diameter. There is clear advantage in SNR for increased collecting area, and an optimistic version of NGST has been adopted for this illustration. The atmospheric transmission is taken as 0.95 and applies only to the ground-based telescope.