In alternating exposure mode (also referred to as interleaved mode), there is a long frame time and a short frame time, and the observation alternates between them. This is sometimes done when observing a source the may be piled. The observer gets the piled long-frame information, as well as the unpiled short-frame data.

All the long frames are joined into one event file and the short frames are joined into another event file. The resulting event files have "e1" and "e2" in the filename to indicate the two exposure lengths. Check the EXPTIME header keyword to see which exposure time is recorded in each file:

unix% dmkeypar acisf01612_000N003_e1_evt1.fits EXPTIME echo+
0.1
unix% dmkeypar acisf01612_000N003_e2_evt1.fits EXPTIME echo+
0.4

The chandra_repro script will correctly process both e1 and e2 versions of the observation. Users can also separate the long and short exposures into separate directories using the splitobs script.

Do not merge the e1 and e2 files. They should be analyzed separately.