Abstract
Many of the more specialised digital cameras produced for use with telescopes have no information provided to describe their sensitivity in terms of a recognised system, such as an ISO speed rating, whereas digital cameras supplied for pictorial imaging usually do have an ISO speed quoted by the manufacturer. The science of photographic sensitometry has become established over the past 110 years and, in many respects, the knowledge gained can be applied satisfactorily to non-photographic camera systems used for astronomy and also for many other scientific and technical applications. A method is described by which an ISO speed rating can be assigned to a digital camera system used for imaging in astronomy.