Hail to the Victors (CCDs): The history of optical detectors in astronomy from the 1960’s to the present.

Abstract

The last three decades have seen a sea change in optical detectors used for ground-based astronomy, from the monopoly of the photographic plate in the 1960s, to a bewildering array of furiously competing intensifier tubes in the 1970s, to the swift victory of the CCDs. This progression is illustrated in figure 1, which shows the rapidly varying demography of the different “animal species” in use at ESO. Up to the late 1960s, the photographic plate was the only panoramic detector available. It was also a very large detector, actually bigger than the largest CCD mosaics of today. However, the quantum efficiency (QE) of photographic plates was very small. I remember measuring their detective quantum efficiency (DQE) for long exposures. The procedure for getting the best performance from photographic plates was labourintensive: plates were first hyper-sensitized with forming gas, pre-flashed, exposed to the astronomical source, post-flashed, processed and finally measured. The result of this whole cooking process turned out to be a “whopping” 0.75% QE peak! Although this sounds feeble by today’s standards, it was nevertheless a truly great achievement at the time. Even worse, this now dismal value could only be obtained around some optimal flux. Outside of a narrow dynamic range — no more than a factor 3 in light intensity — DQE dropped below 0.1%.