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An observer uses the spectroscopy set of CCDs in conjunction with the High Energy Transmission Gratings (HETG) to obtain very high resolution energy information from the observed source(s). When used with the HETG, an "X" is formed on the six spectroscopy CCDs. The distance from the center of the "X" reflects the energy of the detected X-rays. The central pixels of the "X" will receive many more events than the "arms" of the "X." In order to use the pixels at the center of the "X" for relatively bright sources, the instrument must provide the capability of interleaving short exposure times with longer ones. ACIS provides this capability within the Timed Exposure Science Mode.
An observer performs a Spectroscopy Science Mode by commanding the hardware (not described in this document) to move the gratings into place, and by configuring and running a Timed Exposure Science Mode, as described in Section 3.2.2.2 on page 48. The observer uses the Timed Exposure Parameter Block settings to select the Spectroscopy CCD Array, and uses the primary exposure times to acquire data from the "arms" of the "X". The observer uses the secondary exposure time to provide data from the central pixels, minimizing event pile-up, and the duty-cycle parameter to control how often to acquire these short exposures. In all cases, all active CCDs are clocked in unison.
FIGURE 27. Representative HETG projection onto ACIS
Since Timed Exposure Science Mode covers the needs for Spectroscopy observations, refer to Timed Exposure Science Mode, Section 3.2.2 on page 48, for the functional requirements.
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