HETGS Resolving Power

HETGS Resolving Power

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Results and Discussion

Using the resolving power error buget described in Section 2.2 of the HETG Ground Calibration report (1.7 Mb.ps.gz download), predictions for the HETGS flight E/dE are made, plots below (.ps version). Both "optimistic" and "conservative" curves have been prepared (details in the report.) The results are available as rdb files of E/dE vs energy.

Predictions based on MARX 2.04 Flight-HRMA:

[8/3/98:] Note that the Flight-HRMA performance is the chief non-HETG input required for the E/dE prediction; in particular the 1-dimensional projection of the HRMA point spread function onto the dispersion (Y) axis. The flight HRMA model used for the predictions above is from MARX, Version 2.04 (1st NRA release, Oct. '97). This MARX version is now known to differ from the SAOSAC on-orbit predictions, especially at high energies -- thus the HEG E/dE values given here, especially above 4 keV, are too low. The resolving power curves and data files for the gratings will be updated pre-flight based on MARX Version 2.20 (TBR).

For example, at 8.0 keV the present HEG optimistic [conservative] E/dE estimate is 83 [79], shown by the solid [dashed] line in the HRMA-HEG plot above. This E/dE value is based on the HRMA-HEG having an effective 1-D projection sigma of 32.66 microns rms, see HETG Report Figure 2.4. This effective sigma value is RSS combined with blurs due to other terms of 10.6 micron rms [14.5 um rms] and therefore produces a line response with a FWHM of 2.35*SQRT(32.66^2 + 10.6^2) = 81 [84] microns dispersed to a distance of 6690 microns, that is 6690/81 = E/dE = 83.

If, however, SAOSAC flight-predicted rays at 8.04 keV are used (ray file 8.04778_000_000.fits from MST's On-Orbit Simulations), then the total FWHM of the simulated line response function, plotted below, is predicted to be 44 [52] microns corresponding to a predicted on-orbit resolving power of 152 [129] at 8 keV. The HRMA effective 1-D projected sigma in this case is ~ 17 microns.

(plots from ~/idl/xrcf/ heg_level1_spec.pro)

This web page is: http://space.mit.edu/HETG/res_power/res_power.html.

Please send any comments and updates to Dan Dewey at dd@space.mit.edu.