Ground-based astronomy has been carried out for thousands of years using the human eye and, in the last century, photographic film. The recent addition of orbiting platforms has forced the development of new image sensors, several of which have already seen use or are in advanced stages of development. In this paper, we will discuss the use of electro-optical image sensors for spaceborne astronomy. Several aspects of the spaceborne imaging problem will be examined, including the astronomical phenomenology and the reasons for the use of electro-optical devices for stellar imaging. The problem of detection and measurement of stellar objects will be discussed in terms of photo-electron statistics and image quality of the optics and image detector. Astronomical and device parameters will be used to give typical sensitivity limits within the context of the Space Telescope Program. A detailed model for the sensitivity limit is given.