The detector is an intensified photodiode array and is described in detail by Long et al. (1985). The image intensifier portion of the detector consists of two 25 mm 80:1, 10 m pore microchannel plates (MCP's) and an aluminum-overcoated P 20 phosphor deposited on a 6 m pore fiber optic, all mounted into a modified Varian Conflat flange. The photocathode material is cesium iodide (CsI) deposited, in a thin layer, directly to the front surface of the top MCP. Mounted against the back of the intensifier fiber optic is a 1024 channel Reticon photodiode array. The individual photodiodes are .Individual photon events produce a detectable signal in about 12 photodiodes. The array is scanned and digitized in 1 msec by the Reticon control electronics, which is mounted close to the detector on one of the spider arms. The Spectrometer Processor (SP) then centroids these events to an accuracy of 1/2 diode and provides a 2048 pixel histogram of the location of each photon on the detector. This gives a wavelength accuracy of 0.5 Å. In high time-resolution mode the SP also outputs a list of events with their locations and a time tag accurate to msec. Characteristics of the detector system are listed in Table 1-1 and a schematic is shown in Figure 2-5.