The FWHM of the 1D projection of the zeroth-order "trailed image" (streak) for HETGS observations has been monitored for over two years. The width of the PSF can be broader for sources with harder spectra, so filtering on E<2keV or E<3keV has been applied in these cases in more recent analyses. From the plot here it appears the FWHM is very constant and no significant defocus trend is visible.
The 1D FWHM was measured using the "streak" events as described in Measured and Modelled zeroth-order 1D projections.
[ also available in a postscript version ]
Software and Data File
The data were processed with HAK software and ISIS as described at Measured and Modelled zeroth-order 1D projections using an updated version of the ISIS scripts. The ISIS fit results were then manually entered into an rdb file which an IDL procedure reads to create the plot. These components are given here for reference:
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Dan Dewey at email@example.com.