The ABC of XTE
PCA Issues
PCA Issues
Postscript version of this chapter
This chapter is not a substitute for the parts of the Technical Appendix that deal with the PCA (Proportional Counter Array) and the EDS (Experiment Data System). Rather, it covers those aspects of the PCA and EDS that pertain directly to data reduction, namely:
The essential aspects of the PCA and how they determine the nature and overall structure of PCA data are as follows:
The PCA team monitors the gain and offset values of the
PCA energy-to-channel conversion by analyzing Standard-2 data which contain tagged events from the on-board calibration source. Updated values are sent to the EDS and applied to data in all configurations except the following:
Tools for generating the response matrices take account of whether the
gain and offset have been applied by the EDS.
The instrumental properties of the PCA are:
Before reading about the EDS and its modes and configurations, it's a good idea to review briefly the nature of PCA data before they are packaged by the EDS.
PCA events are passed to the EDS as a series of 19-bit words (with additional start and stop bits) at a rate of ~4 MHz (i.e. every 1/2**22 seconds). The PHA information occupies 8 bits (i.e 256 channels), the remaining 11 bits being devoted to various discriminator and veto flags. More details in the PCA chapter of the Technical Appendix.
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:
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:
Introduction
Note that although the EDS and PCA are separate subsystems, it is often helpful to consider them as parts of the same thing. For data reduction purposes, PCA data means PCA data that have been processed by the EDS.
PCA & EDS: Structure and Properties
PCA Structure
VP - All propane anodes connected together
XL1 & XR1 - 1st xenon layer of two interleaved sets of anodes
XL2 & XR2 - 2nd xenon layer of two interleaved sets of anodes
XL3 & XR3 - 3rd xenon layer of two interleaved sets of anodes
VX - xenon veto layer
ALPHA - events from the calibration source detector
Depending on the configuration, the science data from each PCU may comprise signals from up to six signal chains (XL1-3, XR1-3).
PCA Properties
Energy range: 2 - 100 keV
Energy resolution: < 18% at 6 keV
Time resolution: 1 microsec
Spatial resolution: collimator with 1 degree FWHM
Collecting area: 7000 square cm
Sensitivity: 0.1 mCrab
Background: 2 mCrab
What PCA Data Are Like Before Being Packaged By The EDS
EDS Structure and Operation
Much more detail about the EDS can be found in the Technical Appendix.
EDS Configurations
EDS Modes
The ABC of XTE is written and maintained by the RXTE GOF. Please email xtehelp@athena.gsfc.nasa.gov if you have any questions or comments. This particular page was last modified on Thursday, 16-Sep-1999 08:53:40 EDT.