The XDL detectors are 2-dimensional photon counting devices; there are 1024 pixels in the spectral (X) direction and 360 pixels in the spatial (Y) direction. The Ly- and O VI subsystems each have an associated Image Processor (IP). The IPs process detector data in real time.
Each IP has two Image Table RAMs, one for the X direction on the XDL array and one for the Y direction. Together these Image Table RAMs contain the ``detector mask'' which defines the active area on the detector and specifies how events from different pixels on the detector are to be binned. Each IP also contains one Image RAM, or Accumulator Array, which contains the event count for each bin, i.e., the number of times an event is detected in a certain pixel or group of pixels.
An image is accumulated as follows. Each time a photon is detected, the X and Y coordinates of the pixel in which the event occurred are passed to the corresponding IP. The IP electronics either rejects the event or converts the (X,Y) coordinates into an address A in the Accumulator Arrays. The conversion is done as follows. Coordinate X (a number between 1 and 1024) is fed into the address side of the X Image Table RAM, and the contents AX at this address is read out. Similarly, the Y coordinate (a number between 1 and 360) is fed into the Y Image Table RAM, producing a number AY. If AX and AY are both nonzero, AX and AY are concatenated into a single address A, and the contents of this address in the Accumulator Array is incremented by one, causing the event to be counted. On the other hand, if either AX or AY is zero, the event is rejected (not counted). These functions are performed electronically (i.e., not by software) at a rate of up to times per second.
A detector mask consists of the 1024 AX values and 360 AY values contained in the Image Table RAMs. The active area on the detector consists of those rows and columns with nonzero values of both AX and AY. Events in pixels with the same (AX,AY) combination will be counted in the same ``bin'' of the Accumulator Array. Hence, neighboring rows and/or columns on the detector can be combined into an array of super-pixels.
Before an image can be accumulated, a detector mask must be loaded into Image Table RAM form the REU. Detector masks must be generated on the ground and uplinked prior to reading into the IP. The REU can store up to six detector masks per XDL detector. Each mask will consist of up to five rectangular areas (panels) placed at arbitrary locations on the XDL array, and with uniform size of the super-pixels.
Instrument configuration data and exposure timing data are added to the science data prior to telemetry.
The photomultiplier (PMT), which forms the core of the visible light detection system, can be operated either in photon-counting or photo-diode mode. In the latter case high voltage is applied only to the first stage of the dynode. Data from the Visible Light subsystem is accumulated into two Visible Data Buffers, which are also operated in ping-pong mode. Since the amount of visible light data is rather small, a number of visible light measurements are collected in one of the buffers before the data is transmitted.