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EDS Configurations: Generic Binned



General Description

The generic Binned configurations are designated with a character string beginning with B, e.g. B_250us_2A_0_17_Q. Files containing Binned data are in science array format. The science data occupy the XTE_SA extension in the form of histograms accumulated from good xenon events (i.e. those which survive background rejection, but sometimes including propane veto events) according to time, in two or more channel bands. The columns containing the histograms are preceded by the Time column which gives the time of the start of each accumulation.


Detailed Description

The naming convention of the configuration provides a summary of the properties of the data. For generic binned data, the format

B_ttt_ccX_0_hh_b

gives

B
- mnemonic denoting a generic binned mode
ttt
- the time bin size ("us" denotes microseconds)
cc
- the number of energy channels
X
- the code denoting the binning of the energy channels
hh
- the upper channel boundary
b
- the code denoting the number of bits used.
For example, in the B_250us_2A_0_17_Q configuration, two histograms with ~250-microsecond resolution are accumulated from channels 0 to 17, with the "A" channel distribution/binning scheme. "Q" means that there are 4 bits per bin, i.e. that up to 16 counts can be accumulated per ~250 microseconds without overflowing the telemetry. For a complete list of available Binned mode configurations, see the
RXTE PCA Configurations page. For more details about the different binned mode configurations, please consult the Technical Appendix.

While the configuration name provides a summary of the properties of the data, further information is written in the file itself. This more detailed information is accessible via the ftools fstruct, flcol, fdump and fkeyprint. We recommend that you run these tools on sample files (one, say, per configuration), starting with fstruct, the least verbose. Here are the key pieces of information, and where to find them:

  1. What is the true time resolution? The time resolution for Binned configurations is the step size of the histograms. This is not necessarily the same as the number in the name of the configuration, which is an approximation. For example, in the B_250us_2A_0_17_Q configuration, the step size is really 1/2**12 seconds, i.e. 244.140625 microseconds. To derive this number, work out the nearest inverse power of two from the configuration name.

  2. What are the channel boundaries and energy resolution? The configuration name gives the number of channel bands, but not the channel boundaries themselves, which are denoted by a code letter. To derive the channel boundaries, either look them up in the Technical Appendix, or run fdump on the data file and look at the header of the second column, i.e. the one containing the science data. The value of the 2CPIX2 keyword gives the channel boundaries. For example, in the B_250us_2A_0_17_Q configuration, the value of 2CPIX2 is '0~11,12~17' which tells you that the two bands cover channels 0-11 and 12-17. Fkeyprint can also give the value of 2CPIX2. The energy resolution depends on the configuration: the bins in any spectrum you extract will be the same as the channel bands in the configuration

  3. Are PCU combined or selectable? Is the propane layer included? A few Binned configurations with "low" time resolution (500 ms, 62 ms) separate data by PCU. This is not directly apparent from the configuration name. To find out whether your data have PCU ID, or if they include the propane layer, you can either look up the configuration in the Technical Appendix or run flcol on the data file. For example, in the B_500ms_46M_0_49 configuration, the column names are Time, XeCntPcu0, XeCntPcu1, XeCntPcu2, XeCntPcu3, XeCntPcu4, VpCntPcu0, VpCntPcu1, VpCntPcu2, VpCntPcu3 and VpCntPcu4 - clearly indicating that the data have PCU ID and include the propane veto layer. By contrast, in the B_16ms_16B_0_49_H configuration, the column names are just Time and XeCnt, indicating that all PCU are combined and that no propane data are present.

Note that the accumulation time of the histograms is not a key piece of information for most users, as it only dictates the arrangement of the data in the file, rather than their scientific properties. The RXTE ftool saextrct transparently extracts data without your having to know how many steps each histogram contains or how often the histograms are accumulated. However, if you intend to write your own data reduction software, the accumulation time is crucial. To determine it, use fkeyprint to look at the DELTAT keyword. For example, in the B_250us_2A_0_17_Q configuration, the value of DELTAT is 2 seconds.

The above information - time resolution, channel boundaries etc - is also available in one place in the data file, namely the DDL string which occupies the TTDES2 keyword. DDL - Data Description Language - is a compact way of describing the properties of the data for downstream software. Although somewhat terse, it is not difficult to read. For example, the value of TTDES2 for the B_250us_2A_0_17_Q configuration is:

D[0~4] & E[X1L^X1R^X2L^X2R^X3L^X3R] & C[0~11,12~17] & T[0.0;0.000244140625;8192]
 
which, broken down into its parts, means:

  • D[0~4] - events come from detectors 0-4, i.e. PCUs 0-4 (DDL's ~ symbol denotes a range).

  • E[X1L^X1R^X2L^X2R^X3L^X3R] - each event comes from only one of the six elements per PCU, i.e. the six Xe anodes (DDL's ^ symbol denotes EOR - exclusive or).

  • C[0~11,12~17] - two channel ranges, 0-11 and 12-17, are included.

  • T[0.0;0.000244140625;8192] - time steps with zero offset, 0.000244140625 seconds wide, 8192 per histogram


Reduction requirements and options

Spectra and light curves are extracted from generic binned data using the ftool saextrct. Binned configurations with just a few channels will, of course, result in crude spectra. Apart from adjusting screening criteria, your primary reduction options will always include:

  • Increasing the size of time bins when extracting a light curve
  • Selecting a channel range when extracting a light curve (Note that saextrct specifies channel ranges according to the 0-255 scale.)
The RXTE Cookbook recipe
Reduction and Analysis of PCA Spectra describes reduction of Standard 2 mode data, but also provides general guidelines for reducing any generic binned configuration.


Gain and offset

Gain and offset corrections are applied by the EDS to generic Binned data.


Other features

In common with other configurations in science array format, generic Binned files have two additional columns called Spillage and ModeSpecific, as well as a coda of keywords summarizing EDS status.


Return, if you like, to the PCA Issues chapter or to the Table of Contents.

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 Tuesday, 14-Sep-1999 13:45:43 EDT.