Calibration Update

Larry David

Since the launch of the Chandra X-ray Observatory over 20 years ago, no degradation in performance has been detected in the HRMA, or the LETG and HETG gratings. However, all of the detectors have experienced some degradation in performance due to the harsh environment of space, the overall warming of the telescope, and the build-up of molecular contaminant on the ACIS filters. These changes require periodic releases of calibration products to correct for the changes in gain and QE of all four focal plane detectors.

The calibration team continues to release updated ACIS gain tables (tgain) every six months by co-adding observations of the ACIS external calibration source (ECS). ACIS is exposed to the ECS whenever it is in the stowed position, which occurs during each radiation belt passage. In April 2020, the calibration team released a higher resolution version of the “det_gain” file, which is the ACIS detector gain measured during the first three months after cooling to -120C. This file corrects the gain droop near the central node boundary of the FI chips. All subsequent gain corrections are applied relative to the det_gain file. The calibration team is presently re-calibrating all I0, I1, I3, I4, and S2 tgain files with the new det_gain file. Preliminary results indicate that the new tgain files will significantly reduce the present gain uncertainties of 0.3%.

An updated version of the ACIS contamination model, which included a slight modification to the build-up rate of contamination, was released in November 2019. Since that time we have executed ACIS calibration observations of A1795 in December 2019, LETG/ACIS-S “Big Dither” observations of Mkn 421 in January 2020, and ACIS observations of E0102 in February 2020. All of these observations are consistent with the current ACIS contamination model and show that the depth of the contaminant is still increasing at a linear rate.

A significant fraction of the present ACIS calibration effort is dedicated to calibrating the response of the CCDs when operating at temperatures above the nominal operating temperature of -120C. The three main calibration products (i.e., gain, QE, and spectral response) all degrade with increasing temperature. The FI chips are much more temperature sensitive than the BI chips. The primary reason for the degradation with temperature is that CTI increases with temperature, so that more good grades morph into bad grades which reduces the QE, especially near the top of the chips (i.e. the aim-point on I3). The additional CTI also broadens the spectral response. To complicate matters further, the CTI increases at a non-linear rate with temperature at warmer temperatures. The calibration team recently approved the execution of ACIS-S3 only observations up to -109C (an increase from the prior limit of -111C), since the BI chips show little degradation with increasing temperature. However, we cannot execute observations with the FI chips at temperatures exceeding the present limit of -112C, until we have the appropriate calibration products for such observations. To improve the calibration of existing observations with the FI chips at temperatures above the nominal operating temperature and to permit the execution of observations with the FI chips at temperatures above the present limiting temperature, the calibration team is working on developing the following calibration products: 1) a non-linear temperature-dependent CTI correction, QE maps in a range of temperature bins, and spectral resolution files in a range of temperature bins.

Both the HRC-I and HRC-S continue to undergo a steady decline in both detector gain and QE. The calibration team continues to release annual updates to the detector gain and QE for each detector to correct for these trends. The HRC-I detector gain is monitored with annual raster scans of ARLac and the HRC-S detector gain is monitored primarily with annual LETG observations of HZ43. In addition, the calibration team carries out an annual set of contiguous Mkn 421 observations with all possible combinations of the gratings/detectors for cross-calibration purposes. These observations have shown that the high energy HRC-S QE has not been decreasing as fast as that predicted by the previous version of the HRC-S QE maps in the CALDB over the past few years. A new set of HRC-S QE maps was released in May 2020 to correct this problem. A similar adjustment to the HRC-I QE is under development to maintain cross-calibration consistency.

The calibration team will be releasing an empirical ACIS PSF in the near term which can be used to establish whether a source is extended, carry out deconvolutions to investigate small scale substructure, and serve as a touchstone for upgrades to the HRMA raytrace model. The latest version of the Chandra Source Catalog 2.0 was used to identify useful sources. Only on-axis point sources with no other sources within 2 arcmins, no pile-up, and with sufficient counts such that stacking would not introduce a bias in the radial profile were used. The empirical ACIS PSF will soon be posted as an event list fits file on the calibration web pages. By releasing an event list fits file, the user can filter the data to whatever energy band is needed to compare to a given target.