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Image Photon Counting Devices
The most popular of the photon counting
devices was the IPCS. The Image Photon Counting System (IPCS)
is an imaging detector which counts individual photon events with zero
read-out noise. This detector is based on the principle
of event centering: individual photon events are detected by means of an
image intensifier (made by Thorn-EMI Ltd.), on the front of which is mounted a photocathode. Photons
incident on the photocathode result in the emission of an electron. Each of
these electrons triggers a cascade of electrons through the image
intensifier, producing a signal of about 107 electrons at the
output. This splash of electrons is detected by a Vidicon or CCD detector,
and passed to an image processing unit which calculates the centroid of each
spalsh, and hence the position on the photocathode for each photon event.
The (x,y) coordinates of each photon event are then passed to the Detector
Memory System, which counts the number of photons detected in each pixel by
incrementing an appropiate memory location. During the course of an
integration, a two-dimensional image or spectrum is built up in the Detector
The sensitivity of the IPCS is determined by the characteristics of the Image Tube phosphorus, i.e. the S20 photocathode. The IPCS is sensitive up to about 7000 angstrom, with an overall quantum efficiency of about 20%. The response is significantly better than that of the uncoated GEC CCD detectors and is more extended into the blue than the RCA CCD detector (both CCDs were on offer at the ING when the IPCS was in use). In practice the IPCS is only used for observations where the signal-to-noise is limited by the detector, i.e. high dispersion spectroscopy on faint objects (in dark time).
A first-generation IPCS was in use at the IDS spectrograph (INT) since 1984 until 1992. This detector made use of a Vidicon tube for event detection, and therefore required elaborate set-up procedures for alignment and minimizing of image distortion. Image size was maximum 2048×256 pixels, but for spectroscopic use a format of 2048×32 was more typical. Pixel size was usually 15µ in the dispersion direction.
A second-generation IPCS with CCD readout was in use at the WHT instruments (ISIS and UES) since 1991 until July 1996. Maximum format was about 2400×3200; pixel size 12µ.
The Imperial College of Science and Technology, RGO, and Instrument Technology Ltd at Hastings developed a small ligh-weight high gain microchannel plate (MCP) intensifier specifically designed for photon counting applications. In particular, the intensifier characteristics of event width and gain were designed to macth the requirements of the CCD-IPCS electronics. The Microchannel plate Intensified CCD Photon Counting System (MIC) was used with the Queens University of Belfast Echelle Spectrograph at the JKT.
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