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XMM-Newton Science Analysis System


emchain (emchain-11.4.3) [xmmsas_20040318_1831-6.0.0]

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Bad pixels detection

Figure 3: Organisation of the EPIC-MOS chain: Looking for bad pixels. 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. The very bright pixels (which perturb the flare screening) are detected and flagged using ebadpixupdate. The resulting files are used to generate Good Time Intervals outside flares (flares act as noise for the bad pixels and make detecting them more difficult). Those Good Time Intervals are exposure specific whatever the value of globalflare.
Then one of two bad pixel finding algorithm 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. If lowenerbadpix=Y (default), the algorithm is called a second time (incrementally) on energies below 500 eV.

The resulting bad pixels file is then used by badpix in the main pass on the data (Fig 1). Dead pixels are read in the CCF, but bright pixels written in the CCF but not uplinked (and not found by the local bad pixel finder) are ignored. 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.