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EDS Configurations: Generic FFT



General Description

The generic FFT (Fast Fourier Transform) configurations are designated with a character string beginning with F, e.g. F_500us_0_12_249_64s. Files containing FFT data are in science array format. The science data occupy the XTE_SA extension in the form of histograms containing:

  • power density spectra (PDS) for two energy bands in the Pds column,
  • cross spectra (CS) between the two energy bands in the Xcs column, and
  • the mean count rate in the two energy bands used in the calculation of the PDS and CS.

The PDS and CS are derived from good xenon events (i.e. those which survive background rejection). The columns containing the histograms are preceded by the Time column which gives the time of the start of each accumulation, i.e. the period over which the PDS and CS are calculated.


Detailed Description

The name of the configuration provides an almost complete summary of the properties of the corresponding data. For FFT mode data, the format

F_ttt_ll_mm_hhh_rr

gives

F
- mnemonic denoting an FFT mode
ttt
- the time bin size ("us" denotes microseconds)
ll
- the lower bound of the lower channel
mm
- the lower bound of the upper channel
hhh
- the upper bound of the upper channel
rr
- the readout time (either 16 s or 64 s)

For example, in the F_500us_0_12_249_64s configuration, PDS and CS are calculated over 64-second chunks from data with 500-microsecond resolution, in two bands from channels 0-11 and 12-249.

Missing from the configuration name is the fact that the PDS and CS histograms have 129 bins, regardless of the time resolution and accumulation time. In addition, the true time resolution is not necessarily the same as the number in the configuration name, which is an approximation. For example, in the F_500us_0_12_249_64s configuration, the PDS and CS are calculated from bins 1/2**11 seconds wide, i.e. 488.28125 microseconds. To derive this number, work out the nearest inverse power of two from the configuration name.


Reduction requirements and options

The XTE GOF does not presently supply any software that can reduce FFT mode data.


Gain and offset

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


Other features

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


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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 13:45:44 EDT.