The FWHM of the 1D projection of the zeroth-order "trailed image" (streak) for HETGS observations is extended through much of 2012. The FWHM has remained stable.
For the plot shown here the data (event files) were taken from the repository of TGCat products. These have been nominally reprocessed without pixel randomization and with EDSER (energy-dependent subpixel event repositioning) enabled. Hence the average FWHM value seen here is less than for previous versions. Only energies below 3 keV are used.
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.
[ also available as postscript or pdf versions. ]
Software and Data File
The streak width was measured using event files in the TGCat archive; the mapping of obsid to tgcatid is recorded in the file: tgcat_findzo_HETG.txt. The streak events are extracted by using an augmented version of the TGCat findzo.sl routine. The new function get_streak_events.sl was created and appended to findzo.sl; the combination was put into findzo_plus.sl which is then evaluated instead of findzo. The actual fitting of datasets is done in do_streak_fit.sl; see its comments for some further information. The resulting FWHM and error values are manually put in a data file, focus_trends_v3_121031.dat . Finally the FWHM-vs-time plot shown here is made with make_streak_plot.sl from the focus_trends data file.
The plot below shows a recent example of streak fitting: note that the streak events' X values in pixels have been offset to 15.0 "A" for fitting purposes and the Y-axis is log scale. Since the goal is to fit the "core Gaussian", the fit range is limited to +/- 2 pixels; the residuals show a characteristic signature that is the slight difference between the real PSF and a Gaussian.