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: http://xmm.vilspa.esa.es/sas/6.1.0/doc/gtialign/node3.html
Дата изменения: Mon Nov 22 22:55:57 2004 Дата индексирования: Sat Dec 22 09:10:54 2007 Кодировка: |
The purpose of this task is to take GTI files, and align them with the frame readout times of a given instrument, so that they can be used to properly filter events and calculate exposure times for that instrument. The output GTI's are also useful for timing analysis.
The problem is illustrated in the figure below.
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_________________| |_____________________
time -->
The top line represents the frame readout time boundaries for a given instrument, i.e. each vertical line represents the end of one frame readout cycle and the beginning of another. The bottom line represents a GTI, as produced for example from housekeeping data. Up corresponds to a good time interval. Note that the frame integration time interval is not constant.
Looking at frame integration period 2, we see that the GTI is valid only for part of this interval. However, all events from this interval will be labeled with the same time. Thus all events from interval 2 should be excluded. Similarly, all events from interval 5 should be excluded as well. Thus the GTI should be adjusted as follows:
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____________________
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________________________| |_____________________________
time -->
This is what is meant by GTI alignment.
The gtialign task takes as input the GTI file to be aligned, and the event list to which the GTI's will eventually be applied. There should be exposure extension(s) in the event list, which will contain the frame readout times for the events in this file. This data will be used to adjust the GTI start and stop times as described above. The different CCD's in the instruments may have different frame integration start and stop times, so that the GTI file will have to be aligned separately for each CCD. Note that the frame integration time for a particular CCD can also vary within a given exposure.
Depending on the convention used, the times associated with individual events can be set to the beginning, end or middle of the frame readout intervals (or, in principle, any value in between). This is also true of the exposure extension, which stores the frame readout times as an absolute time and an interval.
In the case of the event times, this task adjusts the GTI's such that any event in a frame with a good GTI will be accepted, no matter where in the frame readout interval this time is set.
However, for the exposure information, it is necessary to know what
convention is being used for storing frame timing information. This
will be determined by referring to the OGIP keyword TIMEPIXR in the
header of the exposure extension. A value of 
corresponds to time
stamps referring to the beginning of the time interval, 
to
the end of the time interval, etc. If the keyword is not present,
the default value is 
, i.e. the time stamp refers to the middle
of the time interval.
The output of this task will be a FITSfile, with a GTI extension for each CCD in the event list. Each extension will have the name STDGTIn, where n is the CCD number for that extension. Where deemed useful, the extension will also contain CCD specific keywords such as CCDID, CCDNODE, and FRMTIME. The primary header of this file will contain all the standard XMM keywords, as copied from the original GTI file.
All GTI files will be OGIP compliant.