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 | ||
| Notes: 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. | ||||||