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Last modified: 15 December 2011


CalDB Public Release Notes

Public Release Date: 01 DEC 2011

Public Release Only


CalDB is an upgrade to the Chandra CalDB, which includes the following item:

  • HETG Grating Efficiency (GREFF) version N0007


A. HETG Grating Efficiency (GREFF) version N0007

Location: $CALDB/data/chandra/default/greff/
Filename: hetgD1996-11-01greffpr001N0007.fits

The ratio of HEG and MEG first order grating efficiencies has been corrected, and an updated HETG grating efficiency file (GREFF) is being released as a result.

Please see the HETG GREFF Why? page for more information regarding the usage of the HETG GREFF.


CIAO tool "fullgarf", when analyzing data from the HETG, with any detector.




A. HETG Grating Efficiency (GREFF) version N0007

The cross-calibration of the first-order HEG and MEG portions of the HETG has been adjusted via analysis of 30 Blazar observations. The procedure used was to fit ratio of the MEG and HEG fluxes from the observed data to a curve, and derive suitable corrections for each of the MEG and HEG while maintaining the consistency of the LEG-MEG cross-calibration, previously determined. The sense of the correction is that MEG efficiencies are to be corrected by multiplying by the derived fit while HEG efficiencies are corrected by dividing by the fit. Hence, dividing MEG corrections by HEG corrections gives the curve.

Furthermore, the correction is split between the HEG and MEG at 1  keV (12.4Å), with the MEG corrected above 1 keV (wavelengths < 12.4Å), and the HEG getting the remaining corrections. The derived corrections are shown in Fig. 1. We note that the observation data are poor or non-existent beyond 17Å. Therefore, we require that the HEG correction goes to unity smoothly beyond that point.

In Fig. 2, we present the actual corrections in HEG, MEG, and in their ratio, for the new HETG file relative to the previous version (N0006). Note that the modified GREFFs are within 10% of their previous values.

Fig. 1: The MEG/HEG correction curve derived from blazar data data versus wavelength (solid curve).
The dashed curves give the individual corrections for HEG and MEG, so that their ratio agrees with the solid curve below 17Å.

Fig. 2: Ratios for the HEG and MEG curves from the N0007 and N0006 HETG GREFF files, confirming that the corrections in Fig. 1 have been applied.

A more thorough presentation of the derivation of the corrections to the HEG and MEG GREFFs is given in "Updating the HETGS Grating Efficiencies" by Herman Marshall, 07 October 2011.