The FP_TEMP keyword contains the temperature at the focal plane during the observation. The units are kelvin, so the temperature in °C is approximately FP_TEMP - 273. The keyword can be found in the level 1 event file (i.e. evt1). It is now also included in the level 2 event file (i.e. evt2) if the observation was processed with either software version DS 7.3.0 (released June 2004) or CIAO 3.1 (or later) was used to create a new level 2 event file.

unix% dmkeypar acisf01838_000N001_evt1.fits fp_temp echo+
153.446014

The temperature of this observation is therefore approximately -120°C.