The FWHM of the 1D projection of the zeroth-order PSF was measured for a variety of point-source HETGS observations and shows no clear trend over ~140 days; see the plot below. 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 ]
Note that for observation ids 1481 (PKS 2149-306) and 457 (Mkn 421), near the middle of the plot, there were relatively few streak events and hence the statistics are poor. These observations have PSF cores that are slightly to moderately piled-up; fits to 1D projections of these zeroth-order cores give values very close to the overall average seen here.
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 read to create the plot. These components are given here for reference:
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Dan Dewey at email@example.com.