What is the focal plane temperature for my observation?

The FP_TEMP keyword contains the temperature at the focal plane during the observation. The units are kelvin, so the temperature in °C is FP_TEMP - 273.15. 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 if CIAO was used to create a new level 2 event file.

unix% dmkeypar acisf01838_000N002_evt1.fits fp_temp echo+
153.446014

The temperature of this observation is therefore -119.7 °C.