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epevents (epevents-6.42) [xmmsas_20060628_1801-7.0.0]

Trailing Events: Classification of the patterns Algorithm Home Index

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SAS coding via PATTERN and FLAG:

Any events that have obtained a negative charge sum PI (see above) and a $\index{Columns!FLAG}{\tt FLAG}>0$ get a positive total charge PI assigned; if these have $\index{Columns!FLAG}{\tt FLAG}=0$ then they get a positive total charge PI assigned and the INVALID_PATTERN bit set in the FLAG column.

In the implementation of version 6.30.3 (and earlier) single events close to edges or bad pixels do not get PATTERN=0 but get the $8^{\rm th}$ bit set, too, and thus PATTERN=128 (Sect.7.2). All (apparent) singles at the CCD edges, readout window, or close to bad pixels get PATTERN=128. Additionally, they get the corresponding flag set, e.g. CLOSE_TO_READOUT_WINDOW. So there is a double-coding, the $8^{\rm th}$ PATTERN bit says that a FLAG bit is set and the FLAG says which one. If one is interested in ``safe" single events (as far as the location with respect to CCD edges, readout window, and bad pixels is concerned) then the selection expression ``PATTERN==0'' is already sufficient. This implys that one will not keep single-pixel photons with RAWX=1 or RAWX=64, so there will be larger gaps between the CCDs in such images. If this is not desirable then select ``(PATTERN==0)||(PATTERN==128)''. Then one has to keep in mind the effect of possible charge losses: for broad-band imaging purposes this may be fine, however, accurate spectral modelling should use ``FLAG==0''.

The first example in Sect.7.4.1.3 was considered as valid double (here PATTERN=4), the others not and therefore do not have PATTERN=1-4 but get PATTERN=205: next unused PATTERN, i.e. 13, plus the $7^{\rm th}$ and $8^{\rm th}$ bit set. Similarly, triples (and quadruples) with the main charge at the CCD egde, readout window, or bad pixel, do get PATTERN 14+64+128 = 206 (and 15+64+128 = 207, respectively), and the relevant FLAG setting.

The last invalid quadruple example in Sect.7.4.1 where the mimimum charge is not opposite to the main charge, is an invalid quadruple, so it gets a ``base'' pattern of 15 plus the $7^{\rm th}$ bit set, i.e. PATTERN=79.

Any very large pattern, with $n\geq 5$ pixels involved, gets a PATTERN = 64+32+(n-5) if no edges or bad pxiels are involved, and PATTERN = 128+64+32+(n-5) if an edge or bad pixel is touched by this event ensemble.

Note: as of version 6.30.4 PATTERN values of 128 have been changed to 0 (i.e. $8^{\rm th}$ bit is not set anymore for singles), and PATTERN values of 205 have been changed to 1-4 (i.e. $7^{\rm th}$ and $8^{\rm th}$ bit are not set anymore for doubles). All the FLAG coding and all other PATTERN bit coding is unchanged.

Note: a keyword SCR_88=``T'' in the EXPOSURE (and EVENTS) extension of the output file of epevents indicates that this new behaviour was used, if the keyword is absent or contains ``F'' then the old behaviour (PATTERN=128 or PATTERN=205 for singles and doubles close to CCD edges or bad pxiels) is present.


Trailing Events: Classification of the patterns Algorithm Home Index

XMM-Newton SOC/SSC -- 2006-06-28