EDS Structure and Operation
The EDS takes the single stream of data flowing out of the PCA and processes it simultaneously in up to six different ways before it is telemetered, thereby allowing more than one instrument configuration to be run at one time. Your data tape will therefore contain the same PCA events packaged in up to six different ways.
The key elements of EDS structure and operation are as follows:
Much more detail about the EDS can be found in the Technical Appendix.
- The EDS has six EAs (Event Analyzers) devoted to the PCA (the other two are for the ASM). Each EA sees all the PCA data - just as if the original incoming data stream were duplicated five times. The EDS has two identical halves, the "A" side and the "B" side, each with three EAs for the PCA.
- Each EA can run in any of seven basic "modes" which are further defined and tuned by a set of parameters (such as energy bin boundaries). The implementation of a mode with specific parameters is known as a "configuration". Understanding how the configuration determines the format of your data is very important and is discussed below.
- Two of the EAs always run in the "Standard-1" and "Standard-2" configurations. The other four EAs run in configurations specified by the PI on his or her original proposal.
- The EDS, not the PCA, stamps events with their arrival times and performs background rejection. (Note: "background rejection" is the sifting out of non-X-ray events by on-board electronics. "Background subtraction" is the derivation of the true net signal from a cosmic source, and is covered in its own chapter.)
The EDS mode is the broad scheme for packaging PCA data, while the configuration is the specific implementation of the mode. When you work with PCA data, the configuration is more conspicuous than the mode itself - in XDF, for example, you'll specify "Standard-1" data to reduce rather than "Binned Mode" data. In fact, as a general rule, data in different configurations - even of the same mode - should not be reduced together. And of course, contemporaneous data in different configurations must never be combined since they're the same events!
Here, we've summarized the modes and provided links to detailed descriptions of their configurations listed below. These descriptions focus on the data reduction aspects of the configurations, and, where file formats and keywords are discussed, complement the more general information in the Data Files chapter.
Given that your data set will include at most half-a-dozen configurations from the much larger number of possibilities, the descriptions are written to be complete and self-contained: you only need to read about your particular configurations:
- Event Encoded mode yields data as a time-series of unevenly spaced events each described by arrival time, pulse height, PCU ID, etc. Configurations comprise:
Binned Data mode yields data in a time-series of regularly accumulated histograms of pulse height (energy), time and event type. Configurations comprise:
Single-Bit Code mode generates a stream of ones and zeros representing events and clock ticks. Generic Single-Bit configurations have names like SB_125us_0_249_1s.
Burst Catcher mode uses two EAs: one to search data and generate a trigger (which may be activated by count rate, rate of change of count rate or hardness), and the other to collect data (which may be in binned or event modes). Configurations comprise:
Delta-binned mode yields data in a time-series of regularly spaced histograms of the times between events. Generic Delta-Binned configurations have names like D_4us_0_249_1024_64s_F.
Fast Fourier Transform mode accumulates simultaneously two channels of 256 time bins (e.g., two energy bands) and averages the power density spectrum and cross spectrum. Generic FFT configurations have names like F_500us_0_12_249_64s.
Pulsar Fold mode yields data in a time-series (counted in pulse periods) of regularly spaced histograms of pulse height (energy), pulsar phase and event type. There are no pre-programmed Pulsar Fold configurations - they must be specially created for each observation.