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Bit flips in the MOS offset tables

This algorithm detects and corrects bit flips in the MOS offset tables, resulting in improved data quality and event rejection. It is implemented in SAS 6.

josephinehaley
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Bit flips in the MOS offset tables

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  1. First detected in 0640_0160160101 MOS 1, CCD 6 by M. Ceballos in routine screening. RAWY=303 had its offset increased by 256. All low energy events on that row did not pass the EDU. All events on that row and rows just next had ENERGYE3 decreased by 512 or 768, so that they were flagged as UNDERSHOOT and rejected by emevents. All events 2 rows off (301 and 305) had ENERGYE4 decreased by 1280, so that many were flagged as BAD_E3E4 and rejected by emenergy. Lasted for two revolutions, disappeared by itself (?) Other occurrence: 0576-0577 (MOS 2, CCD 4, RAWY=384, +2048). 23 March 2004 Jean Ballet, CEA/SAp Bit flips in the MOS offset tables

  2. At the last Cal/Ops meeting (09/03), I was asked to think of an algorithm to detect that feature automatically. The idea is to use E3 (sum of charges in pixels surrounding the maximum) for that purpose. For each row/column, compute median E3, keeping only single events > 500 eV (mostly real). Detect rows/columns where that value is unusually large (negative). Invert (locally) the linear equation relating median E3 and additional offset. E3i = 3 Δoffi-1 + 2 Δoffi + 3 Δoffi+1 Weight each equation (each row) by number of events in that row. Assume that Δoff is 0 where the median is compatible with 0. Allows to recover flips for bits ≥ 64. Bit 32 is set in normal conditions (offsets ~ 50) so a flip would result in a too small offset. Events would be lost (entire row/column > threshold) over three rows/columns. Not seen yet ? Lower bits are more difficult. Detection algorithm

  3. MOS offset flips • Charged particle induces bit flip in a single onboard offset value (here +256 at RAWY=303) • Results in very negative ENERGYE3/4 values over two rows on each side, rejected by emevents • Unknown to eexpmap • Can last a few orbits • Detected automatically in SAS 6. Only one row is lost, known to eexpmap 0640_0160160101, MOS 2, CCD 6 RAWY = 301 to 305 very low

  4. emevents 8.2.1 Detects the wrong offsets Writes them to an OFFSETS extension Flags the events on that row/column (ON_BADOFFSET) emchain 11.4.3 ON_BADOFFSET bit (13) in XMMEA_EM Merges OFFSETS extensions (for exposure calculation) Implementation in SAS 6

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