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Difference between ground calibration and on-orbit calibration made in August 2008 is shown on the plot below separately for each of 10 segments along a crystal. Each histogram entry represents difference in asymmetry calibration constants for one crystal, converted to equivalent position bias in mm, using average asymmetry slope( 0.0022 per mm).
(For C&A 21 June, look at the plot below first)
Each histogram shows gaussian spread with RMS ~2.5 mm, but the first and the last segment have also substantial systematic bias ~4 mm:
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In reality the influence of this bias in ground calibration is even more significant, because for segments 0 and 11, which cannot be calibrated directly (because of spread caused by the effect of direct light) the calibration constant was linearly extrapolated from segments 1 and 10 and the bias of 4 mm become 8 mm in segments 0 and 11.
The error mentioned above was the biggest, but not the only source of systematic errors in asymmetry calibration. When I compared the calibrations based on data collected in August 2008 and in October 2009, the difference shows systematic behaviour with position along the crystal:
(C&A 21 June please look at the following section second)
This effect looks smaller than effect of the error in ground calibration procedure, but it affects all crystals. The change of asymmetry slope demonstrate substantial variations from crystal to crystal, but strongly correlated with the time drift of energy scale
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Each curve corresponds to period of 6 Ms (~2 month)
Conclusions
Conclusion is folowing:
- we can't use one calibration file for while 2 -year period, we have to update it every 2-3 month.
- 6 files were generated for the period august 2008 - october 2009
- we really need to generate asymmetry calibration for the period starting from October 16 2009 (since this date GCR files become empty due to software bug)
- no problems for earth limb data reprocessing - we can use August 2008 calibration file.
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