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XMM-Newton Science Analysis System


emchain (emchain-11.15) [xmmsas_20070708_1801-7.1.0]

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Bad pixels detection

Figure 3: Organisation of the EPIC-MOS chain: Bad pixels search. Same conventions as in Fig 1

In most cases relying on the bad pixels registered in the CCF is not enough. This is because the CCF stores only the bright pixels of relatively high occurrence, but bright pixels at a low level may be a nuisance as well.
If badpixfindalgo=EM (default) or EP, the whole analysis of one exposure (Fig 1) is run first with emevents and emenergy in a simplified mode, and skipping badpix and attcalc. Then one of two bad pixel finding algorithms is called (Fig 3), depending on badpixfindalgo.

• If badpixfindalgo is set to 'EP', badpixfind is called. This is a conservative algorithm which will find clear-cut bright pixels or columns.
• If badpixfindalgo is not set or set to 'EM', embadpixfind is called. This is a more sensitive algorithm which will detect bad pixels, segments of columns or rows down to the statistical limit of Poisson counts.

Figure 4: Organisation of the EPIC-MOS chain: Flare screening between first and second bad pixels searches. Same conventions as in Fig 1

If part of the exposure is affected by flares, this can seriously reduce the power of the bad pixels search (flares act as noise for the bad pixels and make detecting them more difficult). Therefore an intermediate flare screening (like in Sect. 3.3) is necessary (Fig 4). The bright pixels (which can perturb the flare screening) are flagged using ebadpixupdate. The resulting files are used to generate Good Time Intervals outside flares. Those Good Time Intervals are exposure specific whatever the value of globalflare. Then the bad pixel search is run a second time on the data outside flares, in incremental mode.

For badpixfindalgo=EM (or not set), the algorithm is called a third time (incrementally) on energies below 500 eV (and after flare screening), unless lowenerbadpix=N. This sometimes detects bad pixels more easily, because most appear at low energy.

The resulting bad pixels file is then used by badpix in the main pass on the data (Fig 1). Bad pixels declared in the CCF are read as well, and merged with those found in the local search. This second pass restarts just after emframes. The stopafterbadpixfind parameter allows to stop emchain before the main pass, to investigate in detail how the bad pixels detection worked.

If badpixfindalgo is set to NO, then the first loop is not done and the bright pixels are read from the CCF.