I've looked at the difference between CAL asymmetry calibration made on the ground with cosmic muons and on orbit with using CNO.

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:

The reason for this bias was the error in ground asymmetry calibration procedure, which was later fixed in the on-orbit code: the 10-bin histogram of longitudinal position (each bin corresponds to a longitudinal segment) for each crystal was filled using asymmetry value for each event as a weight. The average value in each bin was used as a calibration constant, but it works properly only if events are uniformly distributed within a bin. This is not the case for the segments 1 and 10, because the event selection required that particle track crosses both top and bottom crystal surfaces at more then 30 mm from the crystal ends, while bin size is 27 mm. Together with angular distribution of the particle tracks this selection makes part of the segments 1 and 10 empty and thus introduces systematic bias in average coordinate of events in these segments. For on-orbit code this problem was fixed by histogramming only the difference between actual asymmetry value and average linear function with slope 0.0022 per mm - this way the systematic bias was decrease by factor of 10.

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

for trhe same crystal, as shown on the following plot:

This correlation suggests that the drop of light yield is caused by increase of light absorption in the crystal due to radiation damage. In the crystal most sensitive to the radiation damage the systematic bias in longitudinal position is ~3 mm per segment.The evolution of asymmetry calibration in this crystal during first 1 year is shown on the following plot:

Each curve corresponds to period of 6 Ms (~2 month)


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.



  1. As Sasha pointed out, if the CAL xtal response maps are responsible for the CTBCORE discrepancy and the HE PSF discrepancy between Real Data and Monte Carlo, then as the xtal response maps deviate increasingly with time from the ground calibration, the HE PSF should degrade with time.  I asked Toby (and thus Marshall) to look for this.

    As Marshall shows in this confluence page , there is no evidence for significant evolution of the HE PSF with time; thus I conclude that the slope of the xtal response maps is not responsible for the HE PSF problem.  There may be some other problem from the CAL or even xtal response maps, but I don't see that it can be the slope.

  2. I'm not sure that Eric's conclusion follows. Since the pointing is dominated by the tracker (with a bit of "input" from the CAL in the form of the "neutral vertex") it's not obvious that errors in the CAL position will necessarily lead to increases in the PSF. Instead, it could be that the main effect is to remove the offending events from the sample I would submit that the peak in CTBCORE *below* the cut is evidence of this effect. All to be checked, of course!

    1. I agree that a position error might remove offending events from the sample, but I think the conclusion that the slope of the xtal response maps is not responsible for the HE PSF problem must still be true.  There is no time evolution of the PSF, so the increasing deviation in the xtal maps must not be the cause of the PSF deviation.

      If Leon's supposition is correct, we should see the effective area (or the g-ray rate from constant sources like pulsars and the galactic diffuse) decreasing monotonically in time.

      1. As it happens, Aous has fit the flux from the CTA1 pulsar with a linear model from datasets binned on a range of timescales.  See CTA1 Flux Variability confluence page.  In a 19-month dataset, he found that the uncertainty in the slope of a linear model was 5e-11 ph cm-2 s-1 per day over ~550 days, with an average flux of 4e-7 ph cm-2 s-1 above 100 MeV.  That works out to be 6.9% in 550 days, or 4.6% per year.  Let's be generous and call that 5% per year.

        So, CTA1 shows us that the Aeff above 100 MeV, averaged over the spectrum of CTA1, is constant to within 5% per year.  The change in the slope of the xtal response maps can't cause more than a 5% change per year in effective area.

        More to come on other sources....

        1. Referring to Eric's C&A presentation ( https://confluence.slac.stanford.edu/display/SCIGRPS/2010/05/07/Mitigating+issues+with+CTBCORE ), the MC/data difference is insignificant below ~1GeV and becomes pronounced above ~3GeV. So the Aeff above 100MeV probably masks any effect.

          1. OK, perhaps so.  So we should look at Galactic diffuse and at CTBCORE distributions themselves.