The High Energy Transmission Grating (HETG) is used for high resolution spectroscopy of bright sources in the range 0.4-10 keV (31-1.2 Å). The HETG has been used to measure Doppler velocities of orbiting systems, even as low as 50 km/s, and plasma outflow velocities from a few hundred to 10's of thousands of km/s. Because the HETG can clearly resolve lines from O to Fe-K, detailed line diagnostics can be applied.
A complete description of HETG is given in Chapter 8 of the Proposers' Observatory Guide (POG). The HETG is primarily used with ACIS-S. The dispersed spectrum forms a shallow X on the ACIS-S array centered at the zeroth order position, as shown in this image. The table below summarizes common HETG science modes. The optimal mode depends primarily on source intensity.
|Source Type||Count Rate||ACIS Configuration||Frame Time||Wavelength Range||Energy Range||Resolving Power (E/ΔE)|
|Extended or Multiple Sources||Up to 10 cps1||Full ACIS-S, faint mode||3.2 s||1.2-31 Å||0.4-10 keV||1000-65|
(200 at 6.5 keV)
|Single Point Source||Up to 14 cps1||774 row subarray, faint mode||2.4 s||1.2-31 Å||0.4-10 keV|
|Up to 30 cps1||256 row subarray||1.1 s||1.2-18 Å||0.7-10 keV||1700-85|
|>30 cps||Full ACIS-S, CC mode||1.2-31 Å||0.4-10 keV||1000-65|
1. These approximate count rates refer to the zeroth-order HETG count rate, e.g. from PIMMS. We assume a 1.7 photon index power law, absorbed by a column of 3e20.