When Galileo pointed his telescope at the sky in 1609 he initiated an optical evolution that continues to the present and shows no evidence of termination. First came bigger lenses with reduced aberrations, then large reflectors that opened the discovery and investigation of objects other than stars and planets, and by the 1890s photographic emulsions had increased sensitivity by allowing the light to be integrated over long periods. The pace of invention has grown geometrically so that the passage of a generation brings startling changes to practitioners of the art. There was a decade and a half when a single device completely transformed observational astronomy, providing improvement that effectively increased the apertures of all telescopes by a factor of three. This device was the image tube, the development of which owes much to DTM.

By 1950 improving the sensitivity of photographic emulsions gave every impression of being in a condition of diminishing returns, but hope seemed to reside in the ratio of two numbers: whereas it required about 1000 photons to produce a single blackened grain in an emulsion, a photoelectric surface will emit an electron for about every five incident photons. Inferior sensitivity was only one of the emulsion's flaws. Its response to light was non-linear, saturation limited dynamic range, heavily blackened regions overwhelmed faint images nearby, but worst of all was an affliction called reciprocity failure, the failure of a long exposure to record a proportionately fainter image.