The IPCS

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:

• For most applications, count rates are limited to about 1 Hz/pixel. Above this level coincidence errors become severe and the detector is no longer linear. At higher count rates, there is a risk of physical damage to the detector. These restrictions mean that IPCS observers must use extreme caution to avoid over-illuminating the detector.
• The IPCS offers a wide variety of formats and pixel sizes. Full details are given in AAO UM 10, which should be consulted when planning an observing run. Up to 30mm of spectrum can be recorded on the input cathode, so with 2048 frame lines the spectral pixels are 15um wide. With 1024 frames lines selected, the spectral pixels will be 30um, and so on.
• The data are accumulated in the external memory, which is updated every TV frame time (typically 50ms). An online display of the memory contents can be adjusted to view all or a selected subset of the 2D image. The maximum data window for spectroscopy is 256 spatial increments x  2048 frame lines in the spectral direction.

• The best attainable spectral resolution is ~ 2.4 pixels. The spatial resolution is a complex function of the pixel size and camera format, but is typically 2 pixels.
• The first photocathode of the EMI image intensifier used in the IPCS is a standard type S-20. A typical sensitivity curve as a function of wavelength is shown in Figure 6.2.

 
Figure 6.2: IPCS: schematic diagram

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