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This matrix was the result of a recalibration based strongly
on the bright source Mkn 421.
This source was chosen because of the belief that its spectrum is,
to a first approximation, featureless.
While this choice can justifiably be criticised,
there are no perfect soft X-ray spectral calibration targets,
and very few that can be considered good.
Subsequent to the analysis done to produce this matrix,
it was found that there might be an additional source of error
in using the Mkn 421 data.
There is a suggestion that the data were possibly affected
by the rate-gain effect
(see Sect. 3.2.2),
even though the count rate was only 
countss 
and the source is relatively soft.
The production of this matrix was made as a temporary solution
until the engineering model PSPC could undergo additional
ground calibration tests in the PANTER facility.
This matrix is thought to be the best available for data
taken after 1991 October 11.
This corresponds to the following matrices:
