Zeroth-order 1D FWHM vs Time, 28 November 2007

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The FWHM of the 1D projection of the zeroth-order "trailed image" (streak) for HETGS observations has been monitored for about eight years. Recent calibration and TOO observations (PKS 2155; GRS1915+105, filtered for energies < 3keV) are in good agreement with a stable FWHM.

Note: The FWHM value plotted here is for ACIS data with spatial-pixel "randomization" turned ON.
The recent EX Hy data was also analyzed with randomization OFF and gave a value of FWHM = 34.5 um; this change is about as expected.

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.

The 1D FWHM was measured using the "streak" events as described in Measured and Modelled zeroth-order 1D projections.

[ also available as postscript or pdf versions. ]

Software and Data File
See earlier focus trends updates for info on software; the latest focus_trends.rdb file is available here.

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