The FWHM of the 1D projection of the zeroth-order "trailed image" (streak) for HETGS observations has been monitored for over ten years. The FWHM has remains stable.
For the plot shown here the data were reprocessed with NO PIXEL RANDOMIZATION by the nominal TGCat routines, using energies below 4 keV.
The measured FWHM values, black diamond symbols, are plotted
here with 1-sigma error bars indicated (light blue.)
The Chandra observation id for the points are listed on the plot in green.
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Software and Data File
The streak width was measured using the streak events as selected in the beginning portion of the TGCat findzo.sl routine. The new routine get_streak_events.sl was created and appended to findzo.sl and put into findzo_plus.sl . The actual fitting of datasets is done in do_streak_fit.sl; see its comments for some information. The resulting FWHM and error values are put in a data file, focus_trends_v2_100105.dat . Finally the plot shown here is made with make_streak_plot.sl from the data file.
The plot below shows an example of the streak fitting: note that the streak events' X values in pixels have been offset to 15.0 A for fitting purposes. Since the goal is to fit the "core Gaussian", the fit range is limited to +/- 2 pixels; the residuals show a characteristic signature of the slight difference of the real PSF and a Gaussian.