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  Sensitivity change in Phase 3

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                                                2010/01/05,15
Dear AKARI phase 3 observers,

   As announced in the Phase 3 workshop in November 2009, we noticed 
that there is a change in the sensitivity in long exposure data of the
imaging observations after April 2008, when the detector temperature
started exceeding 42K. We found that the change strongly correlates
with the detector temperature (NPT1T). For the temperature range so
far experienced, the change is about +/-10%. We derived the following
correction formulae for the sensitivity change. They give us the
ratio relative to the absolute calibration previously given. Since
the observations used for the absolute calibration were performed
at the time when the sensitivity was changing, the ratio at the
earlier time is larger than unity. The detector temperature is given
in the NPT1T parameter in the FITS header. The temperature can change
during the observations to some extent, but the variation due to this 
change is less than 1% and will not affect the absolute calibration.
We have not confirmed the sensitivity change in the short exposure
data. The sensitivity variation in the spectroscopic data is now under
investigation.

   If we define the correction formula as

       f = a x (T - Tc) + b,

   where Tc = 41.8K, then,

       for N2 long,  b = 1.035 (+-0.050),
       for N3 long,  b = 1.052 (+-0.033),
   and 
       for N4 long,  b = 1.061 (+-0.046).

   For T <= Tc,

          a = 0 for N2, N3, and N4.

   For T > Tc,

       for N2 long  a = -3.314e-02 (+-7.331e-04) 
       for N3 long  a = -3.922e-02 (+-5.883e-04)
   and
       for N4 long  a = -4.202e-02 (+-7.617e-04).

Sincerely yours,
IRC team
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Sincerely yours,
IRC team