HRC-S Time-dependent Gain Corrections and PI Filter
A new HRC-S gain map was released in CALDB 4.9.2 (6 July 2020) and a new pulse-height filter for use with LETG data was introduced in CALDB 4.9.4 (15 December 2020). Over half of Level=2 background events can be removed at wavelenghts >20 Angstroms with a loss of ~1% of X-ray events.
The primary differences from the old gain map and PI filter are:
Gain corrections are time dependent; the HRC-S gain is now roughly half what it was at the beginning of the mission.
PHA is replaced with scaled SUMAMPS, which creates a much more smoothly-varying gain map.
PI is replaced with SAMP-based PI.
The old PI filter became less effective over time because it did not account for decreasing gain. Less background is removed at short wavelengths because the filtering threshold must be higher to avoid excluding X-ray events.
- HRC-S gain curves of 18 CalDB epochs normalized to epoch 2001.5. The green curve in the top panel is for epoch 2001.22, when gains are close to those during pre-flight laboratory calibration, and the blue curve in the bottom panel is the gain around the time of this time-dependence calibration (Jan 2020). The gain calibration is valid to at least 2022.
- PI pulse height distributions for flight background between 2000 and 2019: the difference between X-ray and background distributions is the basis of the pulse-height filter.
- Comparison of old and new PI filters
Users working with HRC-S data should reprocess to improve the TGAIN calibration in their data by running the chandra_repro script or by following the Create a New Level=2 Event File thread. The PI background filter is applied to HRC-S/LETG data, as explained in the "Apply background filter" section of the LETG/HRC-S Grating Spectra thread.