Contrast and signal-to-noise ratio in long-distance starlight imaging

A small telescope on an airplane or in low Earth orbit can, in principle, resolve ground objects under starlight with useful resolution. For an ~50-cm aperture and ~100-s exposure, one can obtain a resolution of tens of centimeters from an aircraft and a few meters from orbit. Such starlight images are photon poor, and feature detection depends on photon statistics. Scattered light, atmospheric absorption, and foreground airglow all degrade image contrast. I report an investigation into image signal-to-noise ratio using first-order analytical approximations. We find that, for a given angular resolution, the signal-to-noise ratio for spaceborne images is degraded approximately a factor of 1.7, compared with airborne images, by foreground airglow. Image signal-to-noise ratio improves as the passband moves to the red and as skies become brighter from artificial illumination.