Up: Optical detectors
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.
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
avoid over-illuminating the detector.
- The IPCS offers a wide variety of formats and pixel sizes. Full details
given in AAO UM 10, which should be consulted when planning an observing
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
frame time (typically 50ms). An online display of the memory contents can
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
the spectral direction.
- The best attainable spectral resolution is ~ 2.4 pixels. The
resolution is a complex function of the pixel size and camera format, but
typically 2 pixels.
- The first photocathode of the EMI image intensifier used in the IPCS
a standard type S-20. A typical sensitivity curve as a function of wavelength
is shown in Figure 6.2.
6.2: IPCS: schematic diagram
Figure 6.3: Detective quantum efficiency of a typical
a detector Up: Optical
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