ACIS Focal Plane Temperature
The temperature of the ACIS detectors is regulated using a passive radiator and active heaters. The focal plane temperature is measured periodically using a resistance thermometer and recorded, as a function of time, in an observation-specific mission timeline file (acisf*_mtl1.fits). The median focal plane temperature during an observation (in units of Kelvin) is recorded by the keyword FP_TEMP of the event-data file.
A typical observation
![[The temperature has a small range, with hard upper and lower bounds.]](imgs/obs25274_fptemp.png)
A typical observation
This distribution of temperatures is typical for observations in 2023-2024. The FP_TEMP values are read out at arbitrary times but have highly quantized or discrete values. These values are linearly interpolated to the time grid used in the mission timeline which leads to the scattering of data points between discrete readout vaules.
Temperature variation at perigee
![[The temperature varies strongly with time, taking 18 ks to settle.]](imgs/acis_not_typical_fptemp.png)
Temperature variation at perigee
This plot shows the temperature increase at perigee, and the long time it requires to cool to its set-point temperature.
Three set-point temperatures have been used for guest observations:
| FP_TEMP [°C] | FP_TEMP [K] | Dates |
|---|---|---|
| -99.5 | 173.7 | 1999 Aug 14 - 1999 Sep 08 |
| -109.2 | 164.0 | 1999 Sep 17 - 2000 Jan 29 |
| -119.7 | 153.5 | 2000 Jan 30 - present |
The lower temperature of -119.7 °C yields better spectral resolution for the front-illuminated (FI) ACIS detectors. However, due in part to an increasingly restrictive set of constraints on the orientation of the spacecraft, it has become more difficult to maintain a stable temperature at this setting. Deviations from the set-point temperature can occur during an observation, particularly for observations that occur shortly after perigee. While most deviations since 2023 are approximately 3 °C, they can be as large as 10 °C (e.g. Figure 2). Since the FI ACIS charge-transfer inefficiencies (CTIs) are temperature sensitive, deviations from the set-point temperature lead to changes in the effective gains and energy resolutions of the ACIS detectors.
The effective gain is calibrated using data with temperatures near each set-point temperature. Furthermore, a temperature-dependent adjustment to the gain is calibrated for deviations from -119.7 °C, over the range from about -120 to -105 °C. This adjustment is built into the CTI adjustment applied by the tool acis_process_events.
The ACIS response matrices are calibrated at temperatures of -119.7 and -109.2 °C, but not at temperatures in between. Therefore, analyses of ACIS spectral data can be affected by changes in the focal plane temperature during an observation. ACIS response matrices calibrated at temperatures between -105 and -119.7 °C are planned for release in the future.
While deviations in the focal plane temperature can also influence the effective area of grating spectral analyses through the order sorting performed by the tool tg_resolve_events, analyses of imaging and timing data are insensitive to deviations in the focal plane temperature.
Users who are concerned that their data may be adversely affected by changes in the focal plane temperature should contact the CXC Helpdesk.
References
Grant, C, & Posson-Brown, J., 2009, Proceedings of the 2009 Chandra Calibration Review