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Дата изменения: Tue Aug 16 03:26:38 2005 Дата индексирования: Sat Dec 22 07:59:11 2007 Кодировка: Поисковые слова: п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п п |
The offset optimization routine is similar to the routine in
used by the task eposcorr,
which optimizes
the statistic
,
where
represent the source numbers of two different sets,
is
the distance between two sources and
the distance error.
A basic difference between the two routines is that eposcorr uses sky coordinates, whereas eident uses pixel coordinates.
Unlike for the task eposcorr we do not
have to worry here about chance alignments, since most of the sources are likely to be detected by all three instruments.
Since after a number of observations the positional
offsets between the detector systems should be known,
the offsets can also be set using input parameters.
The error in the offsets is estimated using the second moment in the
distribution of the statistic
around the maximum value
and divided by
, with
the number of matching sources.
For the merging of the three input source lists (one for each instrument) the following scheme was used. The tasks uses a data structure containing the relevant parameters for each source. Additionally there are two extra fields called cross_ref and dist, both are arrays with dimension 2. The cross_ref elements are initiliazed to zero, but when two sources from different data sets are near each other the cross_ref fields will point to each other in the following way:
The pixel coordinates (called in accordance with other detection tasks X_IMA/Y_IMA) are not updated, but instead the estimated offsets between the detectors are listed in the header. The sky coordinates and other derived quantities will be averaged. The output set will be used by eboxdetect/emldetect to correct offsets between the instruments and make a combined detection run. However, if for some reason the combined detection using all three instruments together does not work satisfactory, eident can be used to merge the three individual source lists (i.e. one for each instrument) into one, which can be read by the pipeline task srcmatch to correlate with the OM source list and produce the final summary source list.
XMM-Newton SOC/SSC -- 2005-08-15