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BL Lac used a spectral calibration source during its bright state. The assumption of a featureless spectrum, and the 150 counts s brightness were the reasons it was used for recalibration of the matrix. Unfortunately this source was affected by the rate-gain and this was not known at the time of recalibration.
Also used to test the tail of the PSPC point spread function to high S/N, after these tests the Lorentzian tail of the PSF was eventually rescaled (CAL/ROS/92-001a and CAL/ROS/93-015).