Column to Column Variations of the Energy Spectra
The figure below shows the energy spectra of Cas A obtained in 1995.
Each energy spectrum was created using the data from a single
column; column number is indicated in the panel.
As seen from the figure, the apparent center energies of Si and S
lines show large difference from column to column.
Because the intrinsic line energies should be same for these columns,
large scatter of the apparent line energies means that CTE is
different from column to column.
ASCA XRT has a relatively large PSF and photons from a point source
may scatter over ~100 columns.
Therefore, data with different CTE may be combined to produce an
energy spectrum. Column to column variations of the CTE inevitably
bring the degradation of the energy resolution.