Detailed Instrument Description

Science Data Store

As an event is detected and classified as valid, the video processing unit causes the science data store (SDS) to increment by one the memory location corresponding to the event centroid. The image is gradually built up over the exposure time. After stopping the exposure, the SDS can be read out without disturbing the stored image. The SDS can accept a 512 512 line image in the 16 bit word mode or a 512 1024 line image in the 8 bit word mode. The cycle time for the SDS is compatible with the camera scan rate of xb3 106 pixels per second. This rate is maintained for all formats and zoom. In this latter mode, the camera read beam scans the target twice as fast in the line direction as it does in the normal imaging mode but the pixels are twice as long. The detector generates an increment command for every pixel in which a photon event has been detected. A scan of 256K pixels occurs in less than 30 milliseconds while a scan of 4K pixels (i.e., a 64 64 pixel squared format) takes place in approximately 512 microseconds.

There are two different interface circuits in the SDS (SDS-1 and SDS-2), with unit 1 dedicated to the camera of the F/48 relay and unit 2 to the camera of the F/96 relay. The SDS memory is physically divided into 22 modules of 16K 16-bit data words each holding 32 words from each of 512 lines, but only 16 modules are active at any given time. These 16 modules are accessed in sequence to reduce the required memory cycle time. Each SDS word has 22 bits, with the extra 6 bits being used for "single-bit" error correction and "two-bit" error detection. Included in the engineering telemetry are error detection and correction bits set for each logical module. If more than 6 SDS memory modules fail, the memory can still be operated in a reduced data mode. In this case, zeros will appear in the downlink for those modules that are not available. Data loss occurs from the ``right-hand'' side, so if only 15 modules are up, words 0-479 for each of the PDA lines of 512 pixels would be obtained.

The SDS can be operated in either the normal imaging mode or the SDS dump mode so it is necessary to interrupt the pixel increment commands from the detector to read out the SDS memory to the downlink. Each readout is a dump of the 256K 16-bit words of SDS memory, and hence contains 4M data bits regardless of the image format. Readout of science data is normally done under control of the NSSC-1, which controls the gating of signals to the Remote Interface Unit (RIU) of the SI C&DH including the Science Data Formatter (SDF). From the SDF, the data is fed to the downlink or the tape recorder.

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