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The UCL image photon counting system (IPCS) detects individual photon events in a two-dimensional image by means of a high-gain EMI four-stage image intensifier (S20 photocathode) optically coupled to a continuously scanning television camera (Philips Plumbicon). It then records their central positions in a 512k-pixel digital electronic memory. An important feature of the system is the event centre detection (ECD) logic which identifies the centroid of each photon event despite the degradations imposed by the electron optics, thus providing increased spatial resolution. The usable photocathode is ~40mm in diameter, but severe spatial distortions and reduced resolution are encountered beyond 30mm. Figure 6.1 shows a schematic diagram of the detector unit.

The IPCS is now rarely used as a detector on the AAT &endash; only for very high dispersion instruments (like UCLES, and then only for very faint objects) does the lack of read-noise counteract their much lower quantum efficiency enough to make them competetive with CCDs.

The IPCS is described in detail in AAO UM 10: IPCS users' manual, but some of the most important features are as follows:

Figure 6.2: IPCS: schematic diagram

Figure 6.3: Detective quantum efficiency of a typical IPCS image tube

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