High Energy Transmission Grating Spectrometer Calibration

This page provides an overview of the HETGS in-flight calibration. It is linked from the HETGS Users web page and from the Chandra Science Center Calibration web page.

In most areas, the HETGS is performing according to pre-flight expectations. For recent developments regarding HETGS calibration, see the work in progress section of this web page. References that may be of interest include:

HETGS in-flight performance Summary

Work in progress (Updated 10/14/05)

  1. The effects of pileup are examined in an analysis by Dan Dewey and are apparent in this plot comparing the MEG and HEG spectra of the Crab pulsar.  (The dashed line is the MEG spectrum, which shows a deviation from the HEG spectrum near 2.1 keV, which is associated with the EA increase due to the mirror Ir M edges.)   The HETGS calibration observation of Cyg X-2 is very heavily affected by pileup, to the extent that the continuum is very difficult to measure.  From the Crab and Cyg X-2 observations, we surmise that pileup has a greater effect on HETGS spectra than would be predicted from naive models and that the spectral region that is most often affected is where the MEG EA peaks near 2 keV.

    Pileup correction may be considered to be a calibration task. At present, there is a method for correcting for pileup within ISIS and the jdpileup function in sherpa (see the pileup model thread). A simple pileup correction method seems to be sufficient for calibration tasks, which does not involve iteration or forward folding. We will be assessing the accuracy of pileup correction methods with calibration data.

  2. We will be testing the models of the high order efficiencies of the HETG using data from 3C 273 and Cyg X-2, among other sources. This work is on hold while we complete analysis of the first order efficiency, which has higher priority.

  3. There are a few stages in grating data analysis that involve data filtering or corrections which have not yet been examined as part of the calibration program. Issues that may have an effect on the reliability of derived spectra include:

Contact Herman L. Marshall (hermanm at space.mit.edu) for further information about HETGS calibration.

Last modified: 1/30/07